Material World Electricity and Magnetism

Transcription

1 Material World Electricity and Magnetism Electrical Charge An atom is composed of small particles of matter: protons, neutrons and electrons. The table below describes the charge and distribution of these elementary particles inside the atom. Particle Charge and Location Particle Charge Location Proton Positive (+) Nucleus Electron Negative (-) Orbitals Nucleus No charge Nucleus Electrical charge is a property of protons and electrons. Attraction and Repulsion of Charges Like charges Repel Opposite charges attract + -

2 Static Electricity - Objects that are electrically neutral have an equal number of protons and electrons - Insulators that become charged have either gained or lost electrons o A positively charged object has lost electrons o A negatively charge object has gained electrons

3 3 different ways of charging an insulator 1) Charging by Friction Before Charging During After - 2 neutral objects (# of protons = # of electrons) - Example 1: - Both are neutral - The two objects are rubbed together - There is a transfer of electrons from one object to the other - Which gains/loses electrons is determined by its electron affinity - Polythene has a higher affinity for electrons than the duster so it gains electrons - The object that gained electrons becomes negatively charged (more electrons than protons) - The object that loses electrons becomes positively charged (more protons than electrons) - Only electrons can move, the number of protons always stays the same - Polythene has become negative while the duster is positively charged Example 2 - The duster has a higher affinity for electrons than the acetate so it gains electrons - The duster has become negative while the acetate is positively charged

4 Triboelectric Series - Used to determine if an object is more likely to gain electrons from another object (become negative) or lose electrons (become positive) - You must always compare the relative positions of the two objects you are rubbing together.

5 2) Charging by Conduction Before Charging During After - 1 neutral objects (# of protons = # of electrons) - 1 charged object (can be positive or negative) - The two objects touch - There is a transfer of electrons from the object that has more electrons to the object that has less electrons - Example 1: - A positively charged object touches the neutral electroscope - Electrons are transferred from the electroscope to the object - The objects have the same charge but not as strong - They repel one another - Both are positive

6 3) Charging by Induction Before Charging During After - 1 neutral objects (# of protons = # of electrons) - 1 charged object (can be positive or negative) Example 1: - The charged object is brought close to but does not touch the neutral object - This closeness causes a separation in charge of the neutral object where the oppositely charged particle moves close - Once the charged object is moved away the neutral object s particles return to their original position

7 Dynamic Electricity - The flow of electric current through a closed circuit Definitions - current intensity (I): the amount of charge that flows through a point of an electrical circuit in one second. - potential difference (V) :the amount of energy provided by the power supply (battery). - resistance (R) : the ability of a material to resist the flow of electric charges. Ohm s Law - There is a proportional relationship between potential difference and current intensity for a circuit of a given resistance. For a circuit where the resistance is held constant, If V then I If V then I For a circuit where the potential difference is held constant, If R then I If R then I For a circuit where the current intensity must be held constant, If V then R must If V then R must If R then V must If R then V must

8 Resistance - Ability to slow/limit the flow of current - Opposite of conductance - Even wires have resistance Factors that affect resistance/conductance of a wire How they affect resistance/conductance in a wire 1- Nature of the material - Conductors have lower resistance than insulators - Copper is the best conductor and therefore the worst insulator 2- Length of the wire - A longer wire has more resistance - A shorter wire conducts electricity better 3- Thickness of the wire - A thinner wire has more resistance - A thicker wire has a higher conductance 4- Temperature - The warmer the wire, the higher the resistance - Colder wire is better able to conduct electricity

9 3 formulas used in calculations V = IR V = potential difference (V) I = current intensity (A) R = resistance (Ω) I V R P = VI P = power (W) V = potential difference (V) I = current intensity (A) V V P I E = Pt E = energy (J or kwh) P = power (W or kw) t = time (s or h) I V R Remember: 1000 J = 1 kj In questions where the answer is in Joules, power is in watts and time is measured in seconds In questions where the answer is in kwh, power is in kw and time is measured in hours In question where the answer is in Wh, power is in W and time is measured in hours Minutes is never an acceptable measure of time in these problems you must convert seconds 60 minutes 60 hours hours x 60 minutes x 60 seconds seconds seconds 3600 x 3600 hours hours

10 Example: The graph and table below show the relationship between the potential difference and the current intensity for the circuits of two different appliances. Relationship between Potential Difference and Current a) Which appliance has the greatest resistance? b) If both appliances are used for 20 minutes, which has consumed the most energy? a) Appliance A V= 20 V I = 10 A R=? Appliance B V= 10 V I = 10 A R=? V = IR R = V I 20 V R = 10A R = 2 Ω V = IR R = V I 10 V R = 10A R = 1 Ω Appliance A has the greatest Resistance

11 b) Calculating in Joules (power in watts, time in seconds) Appliance A Appliance B V= 20 V I = 10 A P=? E=? T = 20 min x 60 = 1200 s P = VI P = (20V)(10A) P = 200 W E = Pt E = (200W)(1200s) E = J V= 10 V I = 10 A P=? E=? T = 20 min x 60 = 1200 s P = VI P = (10V)(10A) P = 100 W E = Pt E = (100W)(1200s) E = J b) Calculating in kwh Appliance A V= 20 V I = 10 A P=? E=? T = 20 min 60 = 0.33 h P = VI P = (20V)(10A) P = 200 W 1000 P = kw E = Pt E = (0. 2 kw)(0. 33 h) E = kwh Appliance B V= 10 V I = 10 A P=? E=? T = 20 min 60 = 0.33 h P = VI P = (10V)(10A) P = 100 W 1000 P = kw E = Pt E = (0. 1 kw)(0. 33 h) E = kwh

12 Circuit diagrams, components, symbols and electrical functions *** You must know the different electrical functions Component Symbol Electrical Function Description Electrical outlet, battery, power source Wires Power Supply - Provides the energy needed to move electric current - Allows electrons to Conduction flow through the circuit Resistors Resistance - Slows/limits the flow of electrons Loads - Light - Motor - Sound device etc. Transformation of energy - transform electrical energy into other forms of energy (light, heat, sound, motor etc.) - Switch Control - Opens/closes the circuit Fuse/circuit breaker Protection - Protects the circuits from surges/increases in electricity Rubber on the wire N/A Insulation - Stops electrical current from leaving the wires Ammeter N/A - Reads the current intensity in a component - Connected in series (beside the component it is reading) Voltmeter N/A - Reads the potential difference/voltage in a component - Connected in parallel (over/across a component)

13 Connection of an Ammeter in series Connection of a Voltmeter in parallel 2 types of Circuits 1) Series circuits 2 or more resistors/loads connected on the same pathway (only 1 loop) If a part of the circuit is open or an element is defective, the current stops flowing through the entire circuit

14 2) Parallel circuits 2 or more branches/pathways/loops each path new path has one resistor or load to use energy The charge splits up into different pathways If part of one pathway or branch in a parallel circuit is open or an element is defective, the current continues to flow through the other branches

15 Energy Efficiency Law of conservation of energy - energy can neither be created nor destroyed, but it can be transferred (same type of energy moves from one place to another) or transformed (changed from one type of energy to another) - Example of energy transfer: heat energy from the stoves element is transferred to the pot of water on the stove - Example of energy transformation: electrical energy is transformed into heat energy when you turn on the stove - Energy may have the appearance of being lost but in reality the energy is transformed to heat, light, or other forms of energy. - The transformed energy that is not considered useful in a system is known as the dissipated or lost energy Energy Efficiency - The energy efficiency of a machine is the percentage of energy consumed (energy input into the system) by the machine or device that is transformed into useful energy (energy output). Useful energy efficiency Consumed energy Example: A light bulb in a lamp uses 1560 J of electrical energy to give 62 J of light energy. What is the energy efficiency of the light bulb? Useful energy =62 J Consumed = 1560 J % efficiency = useful energy consumed energy x 100% % efficiency =? % efficiency = 62 J 1560 x 100% % efficiency = 4 %

16 Examplem2: A particular chemical process has an energy efficiency of only 30%. To complete this largescale chemical process, 140,000 J of energy is used. What is the energy output of this process? Useful energy =? J Consumed = J % efficiency = useful energy consumed energy x 100% % efficiency = 30 % 100 = 0.3 (change to a decimal to use in the formula by dividing by 100) useful energy = efficiency x consumed useful energy = (0.3)( J) useful energy = J

17 Electromagnetism: Forces of Attraction / Repulsion Every magnet has two poles: North (N) and South (S) All magnets have a magnetic field. A magnetic field is the space around a magnet where magnetic forces are felt (both attraction and repulsion). Lines of Force show you the shape, direction, and strength of the magnetic field around a magnet. Shape is shown by lines of force which can be straight, curved, circular, etc. Direction is shown by arrowheads. The direction is always from North to South. Strength is shown by how close the lines are to each other. The closer the lines of force are, the stronger the magnetic field. A compass needle is a free moving magnet.

18 The North pole of the compass needle is attracted to the South pole of a magnet. The compass needle will position itself parallel to the field lines that are beneath it. The behaviour of a compass in the magnetic field of a bar magnet is shown below. Forces of attraction/repulsion

19 Magnetic Field in a straight Wire A straight wire with a current flowing through it has a circular magnetic field around it. The magnetic field is represented by circular lines around the wire. The magnetic field of a straight conductor can be determined using the Right Hand Rule: Using your RIGHT hand, point your thumb towards the negative end of the wire (the direction of the current). Your fingers wrap around the wire and the curl of your fingers show the direction of the magnetic field. When a compass is placed in the magnetic field, the north end of the compass will point in the direction of the magnetic field (the direction that your fingernails are pointing)

20 To Increase the Magnetic Field in a straight wire: Increase the current intensity. Use a better conductor o Remember: Metals are conductors. Some metals are better conductors than others o Examples of good conductors: gold, silver, copper o Examples of poor conductors: nichrome Magnetic Field of a Solenoid A solenoid is a wire that is coiled around a ferromagnetic core (usually iron) The wire has current flowing through it. A solenoid has a magnetic field when the current travels through the coiled wire. o The magnetic field around a solenoid looks like the magnetic field around a bar magnet. Iron bar Magnetic field of a solenoid

21 The direction of the field lines (magnetic North and South) is determined using the Right Hand Rule: Starting at the positive end of the power supply, wrap your fingers around the coil by following the wires (go over or under the core depending on the relative position of the wire to the power supply) Your thumb points north. Increasing the magnetic field of a solenoid The Nature of the Core o A solenoid with an iron core will have a stronger magnetic field than an equivalent solenoid with an aluminum core. The Current Intensity, I, in the Coil of the Solenoid o As the current intensity increases, the intensity of the magnetic field increases. o As the current intensity decreases, the intensity of the magnetic field decreases. The Number of Turns (Loops) in the Solenoid o When the number of loops on a solenoid is increased, the intensity of the magnetic field increases. o When the number of loops on a solenoid is decreased, the intensity of the magnetic field decreases. Total strength due to current intensity and loops = I x # of loops

22 Electromagnetic Induction - Creating an electric current with a magnetic field - There are two ways in which an electric current can be generated from a magnetic field: 1) Move a Conductive Material within a Magnetic Field 2) Move a Magnet inside a Coiled Conductive Material