What you will learn: You will classify electrical charge and analyze how charge interacts with matter You will infer the rules of how charge pushes

What you will learn: You will classify electrical charge and analyze how charge interacts with matter You will infer the rules of how charge pushes and pulls on the world

Why it s important: In this age of microprocessors and sensitive circuitry, a knowledge of static electrical charge may save your electronic components from damage

Chapter 32: Electrostatics Historical Background Early Experimenters The Greeks (600 B.C.) Thales of Miletus puzzled about the static charge that he got when rubbed amber with wool

The Chinese (376 B.C.) discovered that piece of Magnetite, when suspended by thread, would align itself with direction of Earth s North and South. Early form of compass for navigation

The Europeans- William Gilbert in 1660 described the electrification of many substances and coined the term electricity from Greek word for amber.

Benjamin Franklin (1752) Credited with being the first to discover that lightning and thunder are the result of electrical charges

Early Electric Power (1800s-) Electromagnetic Induction and Batteries Michael Faraday (1831) showed that moving magnet through a coil of wire caused an electric current to flow in wire

Samuel Morse s Telegraph (1837) first practical use for electricity

Gramme (1871) produced the first electrical generator

Thomas Edison and Electric light (1879)

Nikola Tesla (1883) discovered alternating current (AC)

20 th Century- Electricity sparked a new technological revolution

I. Electrical Forces and Charges (32.1) A. Electrostatics- electricity at rest (Involves electric charges, forces between them, and their behavior in materials)

B. Electrical forces 1. arise from particles in atoms 2. Occur as pair of forces acting on you at all times a. Attracting and repelling forces b. This force attributed to property called charge

1). Electrons- negative charge 2). Protons- positive charge 3). Neutrons- neutral charge

3. Much stronger than gravitational force

C. Atoms 1. Every atom has positively charged nucleus surrounded by negatively charged electrons 2. All electrons are identical (same mass and quantity of negative charge)

3. Nucleus composed of protons and neutrons. a. all protons are identical b. all neutrons identical c. Proton has mass 2000 times greater than electron d. positive charge of proton equal in magnitude to negative charge of electron. e. neutron has mass slightly greater than proton and has no charge

4. Atoms usually have as many electrons as protons, so atom has a zero net charge 5. Fundamental rule at the base of all electrical phenomena is: Like charges repel; opposite charges attract

II. Conservation of Charge (32.2) A. Electrons and protons have electric charge 1. Neutral atom- electrons equal protons (no net charge) 2. If electron removed atom no longer neutral- would have one extra proton and be positively charged

3. Ion- a charged atom a. positive ion- has net positive charge (it has lost one or more electrons) b. negative ion- has net negative charge (it has gained one or more extra electrons)

B. Electrical charge 1. Matter made of atoms 2. imbalance in numbers cause object to be electrically charged

C. Electrons 1. Inner electrons bound tightly to oppositely charged nucleus 2. Outermost electrons- loosely bound and can be easily dislodged. 3. Different materials require varying amounts of energy to tear an electron away from an atom 4. An object with unequal numbers of electrons and protons is electrically charged (either negatively or positively)

D. Conservation of charge 1. Electrons are neither created nor destroyed a. They are simply transferred from one material to another 2. Charge is conserved (cornerstone of physics along with conservation of energy and momentum)

F d 1 2 III. Coulomb s Law (32.3) A. Explains the electrical force between any two objects 1. Similar to Newton s Law of Gravitation 2. Obeys inverse-square relationship with distance 3. Discovered by French physicist Charles Coulomb (1736-1806)

B. Coulomb s Law- states that for charged particles or objects that are small compared to the distances between them, the force between the charges varies directly as the product of the charges and inversely as the square of the distance between them F k q 1 d q 2 2 d = distance between charged particles q 1 = quantity of charge of one particle q 2 = quantity of charge of other particle k = proportionality constant

1. SI unit of charge is the coulomb (C) a. One coulomb = charge of 6.24 billion billion electrons (6.24 X 10 18 electrons) b. Amount of charge that passes through common 1-W light bulb in about one second

2. Proportionality constant (k) in Coulomb s law is similar to G in Newton s law of gravitation. a. Unlike (G) in gravitation equation, (k) is a very large number k 2 / 9,000,000,000N m C 2 k 9 9.0x10 N m / C 2 2

b. Biggest difference between gravitation and electrical forces is that while gravity only attracts, electrical forces may either attract or repel.

C. Electrical forces usually balance out. 1. Weak gravitational force (attractive only) is predominant force between astronomical bodies 2. Atomic level- explains the bonding of atoms to form molecules

IV. Conductors and Insulators (32.4) A. Conductor-materials that have more loosely bound outer electrons that can roam in the material 1. Metals are good conductors of electricity 2. Also good conductors of heat Metals Non Metals

B. Insulator- Materials whose electrons are not free to wander 1. Also poor conductors of heat 2. Rubber and glass good insulators

C. Semiconductors- materials that can be made to behave as either conductor or insulator (thin layers of semi-conducting materials sandwiched together make up transistors)

D. Superconductors- materials that acquire infinite conductivity (At temperature near absolute zero, certain metals become superconductors)

V. Charging by Friction and Contact (32.5) A. Charging by Friction- can transfer electrons when one material rubs against another

B. Charging by Contact- can transfer charge by touching charged object to neutral object

VI. Charging by Induction (32.6) A. Electrons are caused to gather or disperse by the presence of a nearby charge (even without physical contact)

1. Charging by induction occurs during thunderstorms

2. Demonstrated by Benjamin Franklins kite experiment 3. Most lightning is an electrical discharge between oppositely charged parts of a cloud.

B. An object can be charged when touched when the charges are separated by induction.

C. Grounding- when we allow charges to move off (or onto) a conductor by touching it, it is common to say we are grounding it. 1. allow path to practically infinite reservoir for electric charge (the ground) 2. Important when we talk about electrical currents

3. Lightning rod- designed by Franklin to prevent large buildup of charge that would otherwise lead to a sudden discharge between cloud and building.

VII. Charge Polarization (32.7) A. When charged rod brought near an insulator, there are no free electrons to migrate throughout the insulating material. Instead there is a rearrangement of the positions of charges within the atoms and molecules.

1. One side is induced to be slightly more positive or negative than the opposite side 2. The atom or molecule is said to be electrically polarized.

3. Many molecules are electrically polarized (water)

Chapter 33: Electric Fields and Potential I. Electric Fields (33.1) A. Gravitational Field- the force field that surrounds a mass 1. Idea that things not in contact could exert forces bothered Isaac Newton and many others 2. Concept of force field eliminates the distance factor

B. Space around every mass is filled with gravitational field

C. Space around every electric charge filled with an electrical field 1. Electric field has both magnitude and direction (vector) 2. Magnitude (strength) measured by its effect on charges located in the field 3. Direction of electric field at any point, by convention, is the direction of the electrical force on a small positive test charge placed as that point.

II. Electric Field Line (33.2) A. Vector quantity- Electric field has both magnitude and direction 1. Negatively charged particle is surrounded by vectors that point toward the particle 2. Positive charged particle- vectors point away

B. Electric Field Lines- used to describe an electric field 1. Field lines (lines of force) farther apart when field is weaker

2. For isolated charge- lines extend to infinity 3. For two or more charges- lines emanate from positive charge and terminate on negative charge 4. Electric field is storehouse of energy

III. Electric Shielding (33.3) A. Electric charges distribute themselves on the surface of all conductors is such a way that the electric field inside the conductors is zero. B. Electrical components often encased in metal boxes to shield them from all outside electrical activity

IV. Electric Potential Energy (33.4) A. Relationship between work and force 1. Work is done when a force moves something in the direction of the force. 2. Object has potential energy by virtue of its location

B. Charged object can have potential energy by virtue of its location in an electric field.

1. Work is equal to the energy gained by the charge 2. Energy charge has called electrical potential energy 3. If charge released, will accelerate in direction according to charge (+ or -) and turn into kinetic energy

V. Electric Potential (33.5) A. Electric Potential Energy per Charge- total electrical potential energy divided by the amount of charge

2. SI unit of electric potential is a volt (V) 3. Since potential energy measured in joules and charge measured in coulombs, 4. Since electric potential measured in volts, commonly called voltage

B. Can have large voltage with small amount of energy associated with the charged object (rub balloon and becomes negatively charged, perhaps to several thousand volts). 1. Only small amount of charge involved 2. Amount of energy also very small

VI. Electrical Energy Stroage (33.6) A. Capacitor- device capable of storing electrical energy 1. Found in nearly all electronic circuits 2. Made by pair of conducting plates separated by a small distance (but not touching)

3. Energy stored in a capacitor comes from the work required to charge it. 4. Energy is in the form of the electric field between its plates

B. Charged capacitor is discharged when conducting path is provided between the plates

VII. The Van de Graaff Generator (33.7) A. Common laboratory device that can develop high voltages

1. motor driven belt moves past comblike set of metal needles that are maintained at a high electric potential 2. electrons deposited on the belt and carried up into the hollow metal sphere 3. electrons leak onto metal points attached to the inner surface of the sphere 4. Electrons move to outer surface of the conducting sphere 5. Charge builds up to a very high electric potential (millions

33 Electric Fields and Potential An electric field is a storehouse of energy.

33 Electric Fields and Potential The space around a concentration of electric charge is different from how it would be if the charge were not there. If you walk by the charged dome of an electrostatic machine a Van de Graaff generator, for example you can sense the charge. Hair on your body stands out just a tiny bit if you re more than a meter away, and more if you re closer. The space is said to contain a force field.

33 Electric Fields and Potential 33.1 Electric Fields The magnitude (strength) of an electric field can be measured by its effect on charges located in the field. The direction of an electric field at any point, by convention, is the direction of the electrical force on a small positive test charge placed at that point.

33 Electric Fields and Potential 33.1 Electric Fields If you throw a ball upward, it follows a curved path due to interaction between the centers of gravity of the ball and Earth. The centers of gravity are far apart, so this is action at a distance. The concept of a force field explains how Earth can exert a force on things without touching them. The ball is in contact with the field all the time.

33 Electric Fields and Potential 33.1 Electric Fields You can sense the force field that surrounds a charged Van de Graaff generator.

33 Electric Fields and Potential 33.1 Electric Fields An electric field is a force field that surrounds an electric charge or group of charges.

33 Electric Fields and Potential 33.1 Electric Fields An electric field is a force field that surrounds an electric charge or group of charges. A gravitational force holds a satellite in orbit about a planet, and an electrical force holds an electron in orbit about a proton. The force that one electric charge exerts on another is the interaction between one charge and the electric field of the other.

33 Electric Fields and Potential 33.1 Electric Fields An electric field has both magnitude and direction. The magnitude can be measured by its effect on charges located in the field. Imagine a small positive test charge placed in an electric field. Where the force is greatest on the test charge, the field is strongest. Where the force on the test charge is weak, the field is small.

33 Electric Fields and Potential 33.1 Electric Fields The direction of an electric field at any point, by convention, is the direction of the electrical force on a small positive test charge. If the charge that sets up the field is positive, the field points away from that charge. If the charge that sets up the field is negative, the field points toward that charge.

33 Electric Fields and Potential 33.1 Electric Fields How are the magnitude and direction of an electric field determined?

33 Electric Fields and Potential 33.2 Electric Field Lines You can use electric field lines (also called lines of force) to represent an electric field. Where the lines are farther apart, the field is weaker.

33 Electric Fields and Potential 33.2 Electric Field Lines Since an electric field has both magnitude and direction, it is a vector quantity and can be represented by vectors. A negatively charged particle is surrounded by vectors that point toward the particle. For a positively charged particle, the vectors point away. Magnitude of the field is indicated by the vector length. The electric field is greater where the vectors are longer.

33 Electric Fields and Potential 33.2 Electric Field Lines You can use electric field lines to represent an electric field. Where the lines are farther apart, the field is weaker. For an isolated charge, the lines extend to infinity. For two or more opposite charges, the lines emanate from a positive charge and terminate on a negative charge.

33 Electric Fields and Potential 33.2 Electric Field Lines a. In a vector representation of an electric field, the length of the vectors indicates the magnitude of the field.

33 Electric Fields and Potential 33.2 Electric Field Lines a. In a vector representation of an electric field, the length of the vectors indicates the magnitude of the field. b. In a lines-of-force representation, the distance between field lines indicates magnitudes.

33 Electric Fields and Potential 33.2 Electric Field Lines a. The field lines around a single positive charge extend to infinity.

33 Electric Fields and Potential 33.2 Electric Field Lines a. The field lines around a single positive charge extend to infinity. b. For a pair of equal but opposite charges, the field lines emanate from the positive charge and terminate on the negative charge.

33 Electric Fields and Potential 33.2 Electric Field Lines You can demonstrate electric field patterns by suspending fine thread in an oil bath with charged conductors. The photos show patterns for a. equal and opposite charges;

33 Electric Fields and Potential 33.2 Electric Field Lines Bits of thread suspended in an oil bath surrounding charged conductors line up end-to-end with the field lines. Oppositely charged parallel plates produce nearly parallel field lines between the plates. Except near the ends, the field between the plates has a constant strength. There is no electric field inside a charged cylinder. The conductor shields the space from the field outside.

33 Electric Fields and Potential 33.2 Electric Field Lines think! A beam of electrons is produced at one end of a glass tube and lights up a phosphor screen at the other end. If the beam passes through the electric field of a pair of oppositely charged plates, it is deflected upward as shown. If the charges on the plates are reversed, in what direction will the beam deflect?

33 Electric Fields and Potential 33.2 Electric Field Lines How can you represent an electric field?

33 Electric Fields and Potential 33.3 Electric Shielding If the charge on a conductor is not moving, the electric field inside the conductor is exactly zero.

33 Electric Fields and Potential 33.3 Electric Shielding When a car is struck by lightning, the occupant inside the car is completely safe. The electrons that shower down upon the car are mutually repelled and spread over the outer metal surface. It discharges when additional sparks jump to the ground. The electric fields inside the car practically cancel to zero.

33 Electric Fields and Potential 33.3 Electric Shielding Charged Conductors The absence of electric field within a conductor holding static charge is not an inability of an electric field to penetrate metals. Free electrons within the conductor can settle down and stop moving only when the electric field is zero. The charges arrange to ensure a zero field with the material.

33 Electric Fields and Potential 33.3 Electric Shielding Consider a charged metal sphere. Because of repulsion, electrons spread as far apart as possible, uniformly over the surface. A positive test charge located exactly in the middle of the sphere would feel no force. The net force on a test charge would be zero. The electric field is also zero. Complete cancellation will occur anywhere inside the sphere.

33 Electric Fields and Potential 33.3 Electric Shielding If the conductor is not spherical, the charge distribution will not be uniform but the electric field inside the conductor is zero. If there were an electric field inside a conductor, then free electrons inside the conductor would be set in motion. They would move to establish equilibrium, that is, all the electrons produce a zero field inside the conductor.

33 Electric Fields and Potential 33.3 Electric Shielding How to Shield an Electric Field There is no way to shield gravity, because gravity only attracts. Shielding electric fields, however, is quite simple. Surround yourself or whatever you wish to shield with a conducting surface. Put this surface in an electric field of whatever field strength. The free charges in the conducting surface will arrange on the surface of the conductor so that fields inside cancel.

33 Electric Fields and Potential 33.3 Electric Shielding The metal-lined cover shields the internal electrical components from external electric fields. A metal cover shields the cable.

33 Electric Fields and Potential 33.3 Electric Shielding think! It is said that a gravitational field, unlike an electric field, cannot be shielded. But the gravitational field at the center of Earth cancels to zero. Isn t this evidence that a gravitational field can be shielded? Answer: No. Gravity can be canceled inside a planet or between planets, but it cannot be shielded. Shielding requires a combination of repelling and attracting forces, and gravity only attracts.

33 Electric Fields and Potential 33.3 Electric Shielding How can you describe the electric field within a conductor holding static charge?

33 Electric Fields and Potential 33.4 Electrical Potential Energy The electrical potential energy of a charged particle is increased when work is done to push it against the electric field of something else that is charged.

33 Electric Fields and Potential 33.4 Electrical Potential Energy Work is done when a force moves something in the direction of the force. An object has potential energy by virtue of its location, say in a force field. For example, doing work by lifting an object increases its gravitational potential energy.

33 Electric Fields and Potential 33.4 Electrical Potential Energy a. In an elevated position, the ram has gravitational potential energy. When released, this energy is transferred to the pile below.

33 Electric Fields and Potential 33.4 Electrical Potential Energy a. In an elevated position, the ram has gravitational potential energy. When released, this energy is transferred to the pile below. b. Similar energy transfer occurs for electric charges.

33 Electric Fields and Potential 33.4 Electrical Potential Energy A charged object can have potential energy by virtue of its location in an electric field. Work is required to push a charged particle against the electric field of a charged body.

33 Electric Fields and Potential 33.4 Electrical Potential Energy To push a positive test charge closer to a positively charged sphere, we will expend energy to overcome electrical repulsion. Work is done in pushing the charge against the electric field. This work is equal to the energy gained by the charge. The energy a charge has due to its location in an electric field is called electrical potential energy. If the charge is released, it will accelerate away from the sphere and electrical potential energy transforms into kinetic energy.

33 Electric Fields and Potential 33.4 Electrical Potential Energy How can you increase the electrical potential energy of a charged particle?

33 Electric Fields and Potential 33.5 Electric Potential Electric potential is not the same as electrical potential energy. Electric potential is electrical potential energy per charge.

33 Electric Fields and Potential 33.5 Electric Potential If we push a single charge against an electric field, we do a certain amount of work. If we push two charges against the same field, we do twice as much work. Two charges in the same location in an electric field will have twice the electrical potential energy as one; ten charges will have ten times the potential energy. It is convenient when working with electricity to consider the electrical potential energy per charge.

33 Electric Fields and Potential 33.5 Electric Potential The electrical potential energy per charge is the total electrical potential energy divided by the amount of charge. At any location the potential energy per charge whatever the amount of charge will be the same. The concept of electrical potential energy per charge has the name, electric potential.

33 Electric Fields and Potential 33.5 Electric Potential An object of greater charge has more electrical potential energy in the field of the charged dome than an object of less charge, but the electric potential of any charge at the same location is the same.

33 Electric Fields and Potential 33.5 Electric Potential The SI unit of measurement for electric potential is the volt, named after the Italian physicist Allesandro Volta. The symbol for volt is V. Potential energy is measured in joules and charge is measured in coulombs,

33 Electric Fields and Potential 33.5 Electric Potential A potential of 1 volt equals 1 joule of energy per coulomb of charge. A potential of 1000 V means that 1000 joules of energy per coulomb is needed to bring a small charge from very far away and add it to the charge on the conductor. The small charge would be much less than one coulomb, so the energy required would be much less than 1000 joules. To add one proton to the conductor would take only 1.6 10 16 J.

33 Electric Fields and Potential 33.5 Electric Potential Since electric potential is measured in volts, it is commonly called voltage. Once the location of zero voltage has been specified, a definite value for it can be assigned to a location whether or not a charge exists at that location. We can speak about the voltages at different locations in an electric field whether or not any charges occupy those locations.

33 Electric Fields and Potential 33.5 Electric Potential Rub a balloon on your hair and the balloon becomes negatively charged, perhaps to several thousand volts! The charge on a balloon rubbed on hair is typically much less than a millionth of a coulomb. Therefore, the energy is very small about a thousandth of a joule. A high voltage requires great energy only if a great amount of charge is involved.

33 Electric Fields and Potential 33.5 Electric Potential think! If there were twice as much charge on one of the objects, would the electrical potential energy be the same or would it be twice as great? Would the electric potential be the same or would it be twice as great? Answer: Twice as much charge would cause the object to have twice as much electrical potential energy, because it would have taken twice as much work to bring the object to that location. The electric potential would be the same, because the electric potential is total electrical potential energy divided by total charge.

33 Electric Fields and Potential 33.5 Electric Potential What is the difference between electric potential and electrical potential energy?

33 Electric Fields and Potential 33.6 Electrical Energy Storage The energy stored in a capacitor comes from the work done to charge it.

33 Electric Fields and Potential 33.6 Electrical Energy Storage Electrical energy can be stored in a device called a capacitor. Computer memories use very tiny capacitors to store the 1 s and 0 s of the binary code. Capacitors in photoflash units store larger amounts of energy slowly and release it rapidly during the flash. Enormous amounts of energy are stored in banks of capacitors that power giant lasers in national laboratories.

33 Electric Fields and Potential 33.6 Electrical Energy Storage The simplest capacitor is a pair of conducting plates separated by a small distance, but not touching each other. Charge is transferred from one plate to the other. The capacitor plates then have equal and opposite charges. The charging process is complete when the potential difference between the plates equals the potential difference between the battery terminals the battery voltage. The greater the battery voltage and the larger and closer the plates, the greater the charge that is stored.

33 Electric Fields and Potential 33.6 Electrical Energy Storage In practice, the plates may be thin metallic foils separated by a thin sheet of paper. This paper sandwich is then rolled up to save space and may be inserted into a cylinder.

33 Electric Fields and Potential 33.6 Electrical Energy Storage A charged capacitor is discharged when a conducing path is provided between the plates. Discharging a capacitor can be a shocking experience if you happen to be the conducting path. The energy transfer can be fatal where voltages are high, such as the power supply in a TV set even if the set has been turned off.

33 Electric Fields and Potential 33.6 Electrical Energy Storage The energy stored in a capacitor comes from the work done to charge it. The energy is in the form of the electric field between its plates. Electric fields are storehouses of energy.

33 Electric Fields and Potential 33.6 Electrical Energy Storage Where does the energy stored in a capacitor come from?

33 Electric Fields and Potential 33.7 The Van de Graaff Generator The voltage of a Van de Graaff generator can be increased by increasing the radius of the sphere or by placing the entire system in a container filled with high-pressure gas.

33 Electric Fields and Potential 33.7 The Van de Graaff Generator A common laboratory device for building up high voltages is the Van de Graaff generator. This is the lightning machine often used by evil scientists in old science fiction movies.

33 Electric Fields and Potential 33.7 The Van de Graaff Generator In a Van de Graaff generator, a moving rubber belt carries electrons from the voltage source to a conducting sphere.

33 Electric Fields and Potential 33.7 The Van de Graaff Generator A large hollow metal sphere is supported by a cylindrical insulating stand. A rubber belt inside the support stand moves past metal needles that are maintained at a high electric potential. A continuous supply of electrons is deposited on the belt through electric discharge by the points of the needles. The electrons are carried up into the hollow metal sphere.

33 Electric Fields and Potential 33.7 The Van de Graaff Generator The electrons leak onto metal points attached to the inner surface of the sphere. Because of mutual repulsion, the electrons move to the outer surface of the conducting sphere. This leaves the inside surface uncharged and able to receive more electrons. The process is continuous, and the charge builds up to a very high electric potential on the order of millions of volts.

33 Electric Fields and Potential 33.7 The Van de Graaff Generator The physics enthusiast and the dome of the Van de Graaff generator are charged to a high voltage.

33 Electric Fields and Potential 33.7 The Van de Graaff Generator A sphere with a radius of 1 m can be raised to a potential of 3 million volts before electric discharge occurs through the air. The voltage of a Van de Graaff generator can be increased by increasing the radius of the sphere or by placing the entire system in a container filled with highpressure gas. Van de Graaff generators in pressurized gas can produce voltages as high as 20 million volts. These devices accelerate charged particles used as projectiles for penetrating the nuclei of atoms.

33 Electric Fields and Potential 33.7 The Van de Graaff Generator How can the voltage of a Van de Graaff generator be increased?

33 Electric Fields and Potential Assessment Questions 1. An electric field has a. no direction. b. only magnitude. c. both magnitude and direction. d. a uniformed strength throughout.

33 Electric Fields and Potential Assessment Questions 1. An electric field has a. no direction. b. only magnitude. c. both magnitude and direction. d. a uniformed strength throughout. Answer: C

33 Electric Fields and Potential Assessment Questions 2. In the electric field surrounding a group of charged particles, field strength is greater where field lines are a. thickest. b. longest. c. farthest apart. d. closest.

33 Electric Fields and Potential Assessment Questions 3. Electrons on the surface of a conductor will arrange themselves such that the electric field a. inside cancels to zero. b. follows the inverse-square law. c. tends toward a state of minimum energy. d. is shielded from external charges.

33 Electric Fields and Potential Assessment Questions 4. The potential energy of a compressed spring and the potential energy of a charged object both depend a. only on the work done on them. b. only on their locations in their respective fields. c. on their locations in their respective fields and on the work done on them. d. on their kinetic energies exceeding their potential energies.

33 Electric Fields and Potential Assessment Questions 5. Electric potential is related to electrical potential energy as a. the two terms are different names for the same concept. b. electric potential is the ratio of electrical potential energy per charge. c. both are measured using the units of coulomb. d. both are measured using only the units of joules.

33 Electric Fields and Potential Assessment Questions 6. A capacitor a. cannot store charge. b. cannot store energy. c. can only store energy. d. can store energy and charge.

33 Electric Fields and Potential Assessment Questions 6. A capacitor a. cannot store charge. b. cannot store energy. c. can only store energy. d. can store energy and charge. Answer: D

33 Electric Fields and Potential Assessment Questions 7. What happens to the electric field inside the conducting sphere of a Van de Graaff generator as it charges? a. The field increases in magnitude as the amount of charge increases. b. The field decreases in magnitude as the amount of charge increases. c. The field will have a net force of one. d. Nothing; the field is always zero.

Chapter 34: Electric Current I. The Flow of Charge (34.1) A. Potential Difference- When there is a difference in potential (voltage), between the ends of a conductor, charge will flow until both ends reach a common potential

1. Much like temperature flows from hot to cold object until they are the same temperature 2. When there is no potential difference then no longer a flow of charge through the conductor

B. To maintain a flow of charge in a conductor, a difference in potential must be maintained 1. Analogous to the flow of water from higher reservoir to a lower one 2. A suitable pump must be supplied to maintain difference. (whether we talk about water flowing or flow of charge)

II. Electric Current (34.2) A. Electric current- the flow of electric charge. 1. Solid conductors- electrons carry the charge through the circuit (electrons are free to move throughout the atomic network) a. called conduction electrons b. Protons are in fixed position in nucleus of atom and cannot move about

3. In fluids such as electrolytes (car battery)- positive and negative ions as well as electrons may compose the flow of electric charge

B. Electric current measured in amperes 1. SI unit is (A) amperes 2. 1A = flow of 1 coulomb of charge per second a. Coulomb is standard unit of charge b. 1 coulomb = 6.24 x 1018 electrons

III. Voltage Source (34.3) A. Charges do not flow without a potential difference 1. A sustained current requires a suitable electric pump to provide potential difference

2. Something that provides potential difference is called a voltage source. 3. Dry cells, wet cells, and generators supply energy that allows charges to move a. Dry and wet cells- energy released in chemical reactions occurring inside that is converted into electrical energy

b. Generator-convert mechanical energy into electrical energy c. This electrical potential energy is available at the terminals of the cell or generator

B. The potential energy per coulomb of charge available to electrons moving between terminals is the voltage 1. Sometimes called electromotive force (emf) 2. The voltage provides the electric pressure to move electrons betweens terminals in circuit

C. Power utilities use large electric generators to provide 120 volts delivered to home outlets D. Voltage causes current 1. Voltage does not flow- creates the pressure 2. Charges flow through circuit (called current)

IV. Electric Resistance (34.4) A. Electrical resistance- the current depends on the resistance that the conductor offers to the flow of charge 1. Resistance or wire depends on conductivity of material

2. Also on thickness and length of wire a. Thick wires- less resistance b. Thin wires- more resistance c. Short wires -less resistance d. Long wires -more resistance

3. Resistance also depends on temperature a. High temperature- for most conductors, increased temp means increased resistance b. resistance of some conductors becomes zero at very low temperatures (superconductors)

B. Electrical resistance measured in units called ohms

V. Ohm s Law (34.5) A. Ohm discovered that current is directly proportional to voltage impressed across circuit and inversely proportional to the resistance V Voltage Current = I Resistance 1. Called Ohm s Law 2. Units for these are: 1 Amperes = 1 Volts Ohms R

a. For a given circuit of constant resistance, current and voltage are proportional b. The greater the resistance, the less the current (inversely proportional) I V R

B. Resistance of materials 1. Typical lamp cord- much less than 1 ohm 2. An iron or toaster- resistance of 15 to 20 ohms (low resistance permits large current- which produces considerable heat 3. Inside electrical devices- current regulated by circuit elements called resistors (range from a few ohms to millions of ohms

VI. Ohm s Law and Electric Shock (34.6) A. It is current not voltage that causes effect of electrical shock 1. Human body varies in resistance (100 ohms if wet, and up to 500,000 ohms if very dry)

2. If a pathway is provided for current (grounding) can be dangerous a. Home appliances are grounded with three-prong plugs to prevent electrocution b. If live wire comes into contact with metal surface of appliance, the current will be directed to the ground rather than shocking you.

B. Effect of electric shock- overheats tissues in body or disrupt normal nerve functions

VII. Direct Current and Alternating Current (34.7) A. Electric current can be DC or AC 1. Direct Current (DC)- Flow of charge in one direction a. Batteries produce DC b. Electrons move from repelling negative terminal towards attracting positive terminal

2. Alternating Current (AC)- Electrons move first in one direction and then in the opposite direction a. In North America nearly all AC circuits alternate back and forth at a frequency of 60 cycles per second. (60 hertz)

b. Voltage is normally 120 volts c. Europe adopted 220 volts (power transmission more efficient at higher voltages)

B. Popularity of AC arises from fact that electrical energy can be transmitted great distances with easy voltage step-ups that result in lower heat losses in the wires

VIII. Converting AC to DC (34.8) A. The current in you home is AC and current in battery operated device is DC 1. Can operate device with an AC-DC converter 2. Uses transformer to lower the voltage as well

B. Diode- tiny electronic device that acts as one-way valve to allow electron flow in only one direction 1. Only half of each cycle will pass through the diode 2. Capacitor used to store energy and maintain continuous current

IX. The Speed of Electrons in a Circuit (34.9) A. The signal (turning on light bulb or telephone signal) travels nearly the speed of light 1. It is not the electrons that move at this speed but the signal 2. At room temperature, electrons move inside wire at an average speed of a few million kilometers per hour due to their internal motion

B. It is the pulsating electric field that can travel 1. Conducting wire acts as a guide or pipe for electric field lines 2. Conduction electrons are accelerated by the field in a direction parallel to the field lines a. Before they gain appreciable speed, they bump into anchored metallic ions in their paths and transfer some of their kinetic energy to them b. This is why current carrying wires become hot

C. In AC circuits, conduction electrons don t make any net progress in any direction 1. Electrons oscillate rhythmically to and fro about relatively fixed positions 2. The electrons already in the wires vibrate to the rhythm of the traveling pattern.

X. The Source of Electrons in a Circuit (34.10) A. The source of electrons in a circuit is the conducting circuit material itself 1. Electrons do not flow through power lines into the wall outlets of your house. (outlets are AC) 2. When plug in AC, energy flows from outlet into appliance, not electrons

B. Energy is carried by electric field and causes vibratory motion of the electrons that already exist. 1. When you are jolted by AC electric shock, electrons making up the current in your body originate in your body 2. Electrons do not come out of the wire and through your body and into the ground; energy does

XI. Electric Power (34.11) A. Electric power- the rate at which electrical energy in converted into another form such as mechanical energy, heat, or light. 1. Equation: electric energy = current x voltage P IV 2. Units: 1 watt = (1 ampere) x (1 volt)

B. Important when consider cost of electrical energy 1. Rate varies from 1 cent to 10 cents per kilowatthour 2. kilowatt-hour represents the amount of energy consumed in 1 hour at the rate of 1 kilowatt

3. Example: a 100 watt light bulb (60W 120V) where electrical energy costs 5 cents per kilowatt-hour can be run for 10 hours at a cost of 5 cents

Assessment Questions 1. Electric charge will flow in an electric circuit when a. electrical resistance is low enough. b. a potential difference exists. c. the circuit is grounded. d. electrical devices in the circuit are not defective.

Assessment Questions 2. The electric current in a copper wire is normally composed of a. electrons. b. protons. c. ions. d. amperes.

Assessment Questions 2. The electric current in a copper wire is normally composed of a. electrons. b. protons. c. ions. d. amperes. Answer: A

Assessment Questions 3. Which statement is correct? a. Voltage flows in a circuit. b. Charge flows in a circuit. c. A battery is the source of electrons in a circuit. d. A generator is the source of electrons in a circuit.

Assessment Questions 4. Which of the following type of copper wire would you expect to have the least electric resistance? a. a thick long wire b. a thick short wire c. a thin long wire d. a thin short wire

Assessment Questions 5. When you double the voltage in a simple electric circuit, you double the a. current. b. resistance. c. ohms. d. resistors.

Assessment Questions 5. When you double the voltage in a simple electric circuit, you double the a. current. b. resistance. c. ohms. d. resistors. Answer: A

Assessment Questions 6. To receive an electric shock there must be a. current in one direction. b. moisture in an electrical device being used. c. high voltage and low body resistance. d. a difference in potential across part or all of your body.

Assessment Questions 7. The difference between DC and AC in electrical circuits is that in DC a. charges flow steadily in one direction only. b. charges flow in one direction. c. charges steadily flow to and fro. d. charges flow to and fro.

Assessment Questions 8. To convert AC to a fairly steady DC, which devices are used? a. diodes and batteries b. capacitors and diodes c. capacitors and batteries d. resistors and batteries

Assessment Questions 8. To convert AC to a fairly steady DC, which devices are used? a. diodes and batteries b. capacitors and diodes c. capacitors and batteries d. resistors and batteries Answer: B

Assessment Questions 9. What is it that travels at about the speed of light in an electric circuit? a. charges b. current c. electric field d. voltage

Assessment Questions 9. What is it that travels at about the speed of light in an electric circuit? a. charges b. current c. electric field d. voltage Answer: C

Assessment Questions 10. When you buy a water pipe in a hardware store, the water isn t included. When you buy copper wire, electrons a. must be supplied by you, just as water must be supplied for a water pipe. b. are already in the wire. c. may fall out, which is why wires are insulated. d. enter it from the electric outlet.

Assessment Questions 11. If you double both the current and the voltage in a circuit, the power a. remains unchanged if resistance remains constant. b. halves. c. doubles. d. quadruples.

Assessment Questions 11. If you double both the current and the voltage in a circuit, the power a. remains unchanged if resistance remains constant. b. halves. c. doubles. d. quadruples. Answer: D

35 Electric Circuits

35 Electric Circuits What you will learn: You will distinguish between parallel and series circuits and series-parallel combinations and solve problems dealing with them. You will explain the function of fuses, circuit breakers, and ground-fault interrupters (GFI s) and describe ammeters and voltmeters.

35 Electric Circuits Why it s important: Electrical circuits are the basis of every electrical device, from electric lights to microwave ovens to computers. Understanding circuits helps you to use them, and to use them safely.

35 Electric Circuits Any path along which electrons can flow is a circuit.

35 Electric Circuits Series and Parallel Circuits I. A Battery and a Bulb (35.1) A. Circuit- A complete pathway for electrons to flow 1. Flow of electrons like flow of water in a closed system of pipes a. The battery would be analogous to the pump b. Wires analogous to the water pipes

35 Electric Circuits B. The water flows through the pump and the electrons flow through the battery

35 Electric Circuits 35.1 A Battery and a Bulb In a flashlight, when the switch is turned on to complete an electric circuit, the mobile conduction electrons already in the wires and the filament begin to drift through the circuit.

35 Electric Circuits 35.1 A Battery and a Bulb A flashlight consists of a reflector cap, a light bulb, batteries, and a barrel-shaped housing with a switch.

35 Electric Circuits 35.1 A Battery and a Bulb There are several ways to connect the battery and bulb from a flashlight so that the bulb lights up. The important thing is that there must be a complete path, or circuit, that includes the bulb filament runs from the positive terminal at the top of the battery runs to the negative terminal at the bottom of the battery

35 Electric Circuits 35.1 A Battery and a Bulb Electrons flow from the negative part of the battery through the wire to the side (or bottom) of the bulb through the filament inside the bulb out the bottom (or side) through the wire to the positive part of the battery The current then passes through the battery to complete the circuit.

35 Electric Circuits 35.1 A Battery and a Bulb a. Unsuccessful ways to light a bulb.

35 Electric Circuits 35.1 A Battery and a Bulb a. Unsuccessful ways to light a bulb. b. Successful ways to light a bulb.

35 Electric Circuits 35.1 A Battery and a Bulb The flow of charge in a circuit is very much like the flow of water in a closed system of pipes. In a flashlight, the battery is analogous to a pump, the wires are analogous to the pipes, and the bulb is analogous to any device that operates when the water is flowing. When a valve in the line is opened and the pump is operating, water already in the pipes starts to flow.

35 Electric Circuits 35.1 A Battery and a Bulb Neither the water nor the electrons concentrate in certain places. They flow continuously around a loop, or circuit. When the switch is turned on, the mobile conduction electrons in the wires and the filament begin to drift through the circuit.

35 Electric Circuits 35.1 A Battery and a Bulb Electrons do not pile up inside a bulb, but instead flow through its filament.

35 Electric Circuits II. Electric Circuits (35.2) A. Electric Circuit- any path along which electrons can flow 1. Must be a complete circuit with no gaps 2. Gap usually provided by electric switch

35 Electric Circuits B. Most circuits have more than one device that receives electrical energy. Devices can be connected in a circuit in one of two ways, series or parallel.

35 Electric Circuits III. Series Circuits (35.2) A. Electric current has but a single pathway through a series circuit. Current is the same through each electrical device in the circuit

35 Electric Circuits B. The total resistance to current in the circuit is the sum of the individual resistances along the circuit path R = R A + R B +..

35 Electric Circuits C. The current in the circuit is equal to the voltage supplied by the source divided by the total resistance of the circuit (in accord with Ohm s law)

35 Electric Circuits D. The total voltage impressed across a series circuit divides among the electrical devices in the circuit so that the sum of the voltage drops across each device is equal to the total voltage supplied by the source.

35 Electric Circuits E. The voltage drop across each device is proportional to its resistance. This follows from the fact that more energy is wasted as heat when a current passes through a high-resistance device than when the same current passes through a device offering little resistance.

35 Electric Circuits QUESTION: What happens to current in other lamps if one lamp in a series circuit burns out? ANSWER: The path to the current will break and current will cease. All lamps will go out

35 Electric Circuits QUESTION: What happens to the light intensity of each lamp in a series circuit when more lamps are added to the circuit? ANSWER: This results in greater circuit resistance. This decreases the current in the circuit and therefore in each lamp, which causes dimming of the lamps. Energy is divided among more lamps

35 Electric Circuits QUESTION: What is the current through this series circuit? ANSWER: Use the equation for Ohm s law and solve. First calculate the total resistance. R R R A B R 9 6 15 I V R 30V I 2A 15

35 Electric Circuits QUESTION: In this simple series circuit, a small electronic component is connected before the LED light bulb. What is this and why do you think they use it? ANSWER: The component is a resistor. It produces a voltage drop in accordance to Ohm s law

35 Electric Circuits 35.2 Electric Circuits For a continuous flow of electrons, there must be a complete circuit with no gaps.

35 Electric Circuits 35.2 Electric Circuits Any path along which electrons can flow is a circuit. A gap is usually provided by an electric switch that can be opened or closed to either cut off or allow electron flow.

35 Electric Circuits 35.2 Electric Circuits The water analogy is useful but has some limitations. A break in a water pipe results in a leak, but a break in an electric circuit results in a complete stop in the flow. Opening a switch stops the flow of electricity. An electric circuit must be closed for electricity to flow. Opening a water faucet, on the other hand, starts the flow of water.

35 Electric Circuits 35.2 Electric Circuits Most circuits have more than one device that receives electrical energy. These devices are commonly connected in a circuit in one of two ways, series or parallel. When connected in series, the devices in a circuit form a single pathway for electron flow. When connected in parallel, the devices in a circuit form branches, each of which is a separate path for electron flow.

35 Electric Circuits 35.2 Electric Circuits How can a circuit achieve a continuous flow of electrons?

35 Electric Circuits 35.3 Series Circuits If one device fails in a series circuit, current in the whole circuit ceases and none of the devices will work.

35 Electric Circuits 35.3 Series Circuits If three lamps are connected in series with a battery, they form a series circuit. Charge flows through each in turn. When the switch is closed, a current exists almost immediately in all three lamps. The current does not pile up in any lamp but flows through each lamp. Electrons in all parts of the circuit begin to move at once.

35 Electric Circuits 35.3 Series Circuits Eventually the electrons move all the way around the circuit. A break anywhere in the path results in an open circuit, and the flow of electrons ceases. Burning out of one of the lamp filaments or simply opening the switch could cause such a break.

35 Electric Circuits 35.3 Series Circuits In this simple series circuit, a 9-volt battery provides 3 volts across each lamp.

35 Electric Circuits 35.3 Series Circuits For series connections: Electric current has a single pathway through the circuit. The total resistance to current in the circuit is the sum of the individual resistances along the circuit path. The current is equal to the voltage supplied by the source divided by the total resistance of the circuit. This is Ohm s law. The voltage drop, or potential difference, across each device depends directly on its resistance. The sum of the voltage drops across the individual devices is equal to the total voltage supplied by the source.

35 Electric Circuits 35.3 Series Circuits The main disadvantage of a series circuit is that when one device fails, the current in the whole circuit stops. Some cheap light strings are connected in series. When one lamp burns out, you have to replace it or no lights work.

35 Electric Circuits 35.3 Series Circuits think! What happens to the light intensity of each lamp in a series circuit when more lamps are added to the circuit?

35 Electric Circuits 35.3 Series Circuits think! What happens to the light intensity of each lamp in a series circuit when more lamps are added to the circuit? Answer: The addition of more lamps results in a greater circuit resistance. This decreases the current in the circuit (and in each lamp), which causes dimming of the lamps.

35 Electric Circuits 35.3 Series Circuits think! A series circuit has three bulbs. If the current through one of the bulbs is 1 A, can you tell what the current is through each of the other two bulbs? If the voltage across bulb 1 is 2 V, and across bulb 2 is 4 V, what is the voltage across bulb 3? Answer: The same current, 1 A, passes through every part of a series circuit. Each coulomb of charge has 9 J of electrical potential energy (9 V = 9 J/C). If it spends 2 J in one bulb and 4 in another, it must spend 3 J in the last bulb. 3 J/C = 3 V

35 Electric Circuits 35.3 Series Circuits What happens to current in other lamps if one lamp in a series circuit burns out?

35 Electric Circuits IV. Parallel Circuit (35.4) A. Each device connects the same two points A and B of the circuit. The voltage is therefore the same across each device

35 Electric Circuits B. The total current in the circuit divides among the parallel branches. Because the voltage across each branch is the same, the amount of current in each branch is inversely proportional to the resistance of the branch Remember that the voltage is the same across each of the branches I A V R A

35 Electric Circuits C. The total current in the circuit equals the sum of the currents in its parallel branches I I A Where IA, IB, IC are currents through the branches and I is the total current I B I C

35 Electric Circuits D. As the number of parallel branches is increased, the overall resistance of the circuit is decreased (just as more check-out cashiers at a supermarket decreases people-flow resistance). With each added parallel path,the overall circuit resistance is lowered. This means the overall resistance of the circuit is less than the resistance of any one of the branches. 1 1 1 1 R R A R B R C

35 Electric Circuits QUESTION: What happens to the current in other lamps if one of the lamps in a parallel circuit burns out? ANSWER: In one lamps burns out, the other lamps will be unaffected. The current in any other single branch is unchanged. The total current in the overall circuit is decreased by an amount equal to the current drawn by the lamp in question.

35 Electric Circuits QUESTION: What happens to the light intensity of each lamp in a parallel circuit when more lamps are added in parallel to the circuit ANSWER: The light intensity for each lamp is unchanged. Only the total resistance and the total current in the total circuit changes. No changes in any individual branch in the circuit occur

35 Electric Circuits 35.4 Parallel Circuits In a parallel circuit, each device operates independent of the other devices. A break in any one path does not interrupt the flow of charge in the other paths.

35 Electric Circuits 35.4 Parallel Circuits In a parallel circuit having three lamps, each electric device has its own path from one terminal of the battery to the other. There are separate pathways for current, one through each lamp. In contrast to a series circuit, the parallel circuit is completed whether all, two, or only one lamp is lit. A break in any one path does not interrupt the flow of charge in the other paths.

35 Electric Circuits 35.4 Parallel Circuits In this parallel circuit, a 9-volt battery provides 9 volts across each activated lamp. (Note the open switch in the lower branch.)

35 Electric Circuits 35.4 Parallel Circuits Major characteristics of parallel connections: Each device connects the same two points A and B of the circuit. The voltage is therefore the same across each device. The total current divides among the parallel branches. The amount of current in each branch is inversely proportional to the resistance of the branch. The total current is the sum of the currents in its branches. As the number of parallel branches is increased, the total current through the battery increases.

35 Electric Circuits 35.4 Parallel Circuits From the battery s perspective, the overall resistance of the circuit is decreased. This means the overall resistance of the circuit is less than the resistance of any one of the branches.

35 Electric Circuits 35.4 Parallel Circuits think! What happens to the light intensity of each lamp in a parallel circuit when more lamps are added in parallel to the circuit?

35 Electric Circuits 35.4 Parallel Circuits think! What happens to the light intensity of each lamp in a parallel circuit when more lamps are added in parallel to the circuit? Answer: The light intensity for each lamp is unchanged as other lamps are introduced (or removed). Although changes of resistance and current occur for the circuit as a whole, no changes occur in any individual branch in the circuit.

35 Electric Circuits 35.4 Parallel Circuits What happens if one device in a parallel circuit fails?

35 Electric Circuits V. Schematic Diagrams (35.5) A. Schematic diagram- simple diagrams to represent electrical circuits. 1. Symbols used to represent certain circuit elements 2. Circuit diagrams (schematics) show electrical connections, not the physical layout

35 Electric Circuits 3. Common Symbols:

35 Electric Circuits B. Examples of Series and Parallel circuits

35 Electric Circuits 35.5 Schematic Diagrams In a schematic diagram, resistance is shown by a zigzag line, and ideal resistance-free wires are shown with solid straight lines. A battery is represented with a set of short and long parallel lines.

35 Electric Circuits 35.5 Schematic Diagrams Electric circuits are frequently described by simple diagrams, called schematic diagrams. Resistance is shown by a zigzag line, and ideal resistance-free wires are shown with solid straight lines. A battery is shown by a set of short and long parallel lines, the positive terminal with a long line and the negative terminal with a short line.

35 Electric Circuits 35.5 Schematic Diagrams These schematic diagrams represent a. a circuit with three lamps in series, and

35 Electric Circuits 35.5 Schematic Diagrams These schematic diagrams represent a. a circuit with three lamps in series, and b. a circuit with three lamps in parallel.

35 Electric Circuits 35.5 Schematic Diagrams What symbols are used to represent resistance, wires, and batteries in schematic diagrams?

35 Electric Circuits VI. Combining Resistors in a Compound Circuit (35.6) A. Sometimes it is useful to know the equivalent resistance of a circuit that has several resistors in its network R 1. Equivalent resistance- value of the single resistor that would comprise the same load to the battery or power source 2. Calculate using the rules for adding resistors in series and parallel R A R B R C... 1 R 1 R A 1 R B 1 R C

35 Electric Circuits B. Series circuits- R R R A B R 8 8 16

35 Electric Circuits C. Parallel circuits- 1 1 1 1 1 1 2... R R R A B R 8 8 8 2 1 R 8 2R 8 R 4

35 Electric Circuits 35.6 Combining Resistors in a Compound Circuit The equivalent resistance of resistors connected in series is the sum of their values. The equivalent resistance for a pair of equal resistors in parallel is half the value of either resistor.

35 Electric Circuits 35.6 Combining Resistors in a Compound Circuit Sometimes it is useful to know the equivalent resistance of a circuit that has several resistors in its network. The equivalent resistance is the value of the single resistor that would comprise the same load to the battery or power source. The equivalent resistance of resistors connected in series is the sum of their values. For example, the equivalent resistance for a pair of 1-ohm resistors in series is simply 2 ohms.

35 Electric Circuits 35.6 Combining Resistors in a Compound Circuit The equivalent resistance for a pair of equal resistors in parallel is half the value of either resistor. The equivalent resistance for a pair of 1-ohm resistors in parallel is 0.5 ohm. The equivalent resistance is less because the current has twice the path width when it takes the parallel path.

35 Electric Circuits 35.6 Combining Resistors in a Compound Circuit a. The equivalent resistance of two 8-ohm resistors in series is 16 ohms.

35 Electric Circuits 35.6 Combining Resistors in a Compound Circuit a. The equivalent resistance of two 8-ohm resistors in series is 16 ohms. b. The equivalent resistance of two 8-ohm resistors in parallel is 4 ohms.

35 Electric Circuits 35.6 Combining Resistors in a Compound Circuit For the combination of three 8-ohm resistors, the two resistors in parallel are equivalent to a single 4-ohm resistor. They are in series with an 8-ohm resistor, adding to produce an equivalent resistance of 12 ohms. If a 12-volt battery were connected to these resistors, the current through the battery would be 1 ampere. (In practice it would be less, for there is resistance inside the battery as well, called the battery s internal resistance.)

35 Electric Circuits 35.6 Combining Resistors in a Compound Circuit Schematic diagrams for an arrangement of various electric devices. The equivalent resistance of the circuit is 10 ohms.

35 Electric Circuits 35.6 Combining Resistors in a Compound Circuit think! In the circuit shown below, what is the current in amperes through the pair of 10-ohm resistors? Through each of the 8- ohm resistors? Answer: The total resistance of the middle branch is 20 Ω. Since the voltage is 60 V, the current = (voltage)/(resistance) = (60V)/(2 Ω) = 3 A. The current through the pair of 8-Ω resistors is 3 A, and the current through each is therefore 1.5 A.

35 Electric Circuits 35.6 Combining Resistors in a Compound Circuit What is the equivalent resistance of resistors in series? Of equal resistors in parallel?

35 Electric Circuits VII. Parallel Circuits and Overloading (35.7) A. When add more devices (pathways) in house, the combined resistance is lowered in the circuit 1. Therefore, greater amount of current occurs 2. Can overload circuit and may result in heat (fire) B. Connect fuses in series along supply line to protect (fuse or circuit breaker prevents overloading)

35 Electric Circuits Ammeters and Voltmeters Ammeter An ammeter measures current in any branch or part of a circuit It does not change the current in the circuit

35 Electric Circuits Voltmeter Used to measure voltage drop across some part of a circuit Has very high resistance so that is causes the smallest possible change in current or voltages in the circuit

35 Electric Circuits conventional current flow theory an older theory stating that electric current flows from the more positive source to the more negative source

35 Electric Circuits

35 Electric Circuits 35.7 Parallel Circuits and Overloading To prevent overloading in circuits, fuses or circuit breakers are connected in series along the supply line.

35 Electric Circuits 35.7 Parallel Circuits and Overloading Electric current is fed into a home by two wires called lines. About 110 to 120 volts are impressed on these lines at the power utility. These lines are very low in resistance and are connected to wall outlets in each room. The voltage is applied to appliances and other devices that are connected in parallel by plugs to these lines.

35 Electric Circuits 35.7 Parallel Circuits and Overloading As more devices are connected to the lines, more pathways are provided for current. The additional pathways lower the combined resistance of the circuit. Therefore, a greater amount of current occurs in the lines. Lines that carry more than a safe amount of current are said to be overloaded, and may heat sufficiently to melt the insulation and start a fire.

35 Electric Circuits 35.7 Parallel Circuits and Overloading Consider a line connected to a toaster that draws 8 amps, a heater that draws 10 amps, and a lamp that draws 2 amps. If the toaster is operating, the total line current is 8 amperes. When the heater is also operating, the total line current increases to 18 amperes. If you turn on the lamp, the line current increases to 20 amperes.

35 Electric Circuits 35.7 Parallel Circuits and Overloading To prevent overloading in circuits, fuses or circuit breakers are connected in series along the supply line. The entire line current must pass through the fuse. If the fuse is rated at 20 amperes, it will pass up to 20 amperes. A current above 20 amperes will melt the fuse ribbon, which blows out and breaks the circuit.

35 Electric Circuits 35.7 Parallel Circuits and Overloading Before a blown fuse is replaced, the cause of overloading should be determined and remedied. Insulation that separates the wires in a circuit can wear away and allow the wires to touch. This effectively shortens the path of the circuit, and is called a short circuit. A short circuit draws a dangerously large current because it bypasses the normal circuit resistance.

35 Electric Circuits 35.7 Parallel Circuits and Overloading Circuits may also be protected by circuit breakers, which use magnets or bimetallic strips to open the switch. Utility companies use circuit breakers to protect their lines all the way back to the generators. Circuit breakers are used in modern buildings because they do not have to be replaced each time the circuit is opened.

35 Electric Circuits 35.7 Parallel Circuits and Overloading How can you prevent overloading in circuits?