Electricity and Why it Moves

Transcription

1 Electricity and Why it Moves Unit 9: 1 Electricity Electrons can move, but protons are held together in the nucleus by the strong nuclear force, the strongest force in nature. Electricity is moving electrons; Protons can t move. Negative Electrons Positive Protons Electricity comes from electrons moving between atoms. Charge A neutral object has an equal amount of protons and electrons. Objects can be positive, negative, or neutral. The unit of charge is the coulomb (C). A positive object has lost electrons, so it has more protons than electrons. A negative object has gained electrons, so it has more electrons than protons. Electric Force Any two charges feel a force between them. Electric force depends on the types of charges, the distance between the charges, and the amounts of the two charges. Attracting Force Opposites attract Repelling Force Likes repel Just like gravity, electric force increases as distance decreases. Closer charges: more force. Less force More force Positive hydrogen atoms in water are attracted to the negatively charged balloon, causing the water stream to bend slightly toward the balloon. Two charged balloons repel each other, so they must be the same charge (negative). Less force More force Electric force increases if either of the charges increases. More charge: more force. Separating Charges When two insulators rub, charges move between them, causing a separation of charge (static electricity) and an electric force. rubber rod fur or hair Afterwards the rod is charged and it can exert an electric force. Naturally, objects are neutral. Work must be done to separate charges. Separating charges cause a charge difference and the electric force tries to move the charges back to neutral. If there is a difference of charge electricity can move between objects. A big enough difference can cause electricity to arc (jump a gap). Bigger differences of charge allow bigger arcs. No difference in charge electricity can not move. arcing neutral A Van de Graaff generator produces a charge difference. A big charge difference can push thru big gaps, like lightening arcing all the way to the ground. Batteries Batteries cause electrons to move by separating charges in chemicals. When connected by wires the charge can flow, eventually neutralizing the battery.. Ground Ground (the earth) can take or give an infinite number of electrons. Ground is electrically neutral. Both positive and negative charges will neutralize when grounded. To ground something you can often touch it to a pipe. Metal pipes are good electrical conductors and usually connected to ground somewhere in the building. A positive object will take electrons from ground. e s Symbol for ground A negative object will give electrons to ground. e s

2 Unit 9: 1 1. Positive A. A push or pull caused by charges. 1. Ground A. The symbol for ground. 2. Negative 3. Neutral 4. Coulombs B. The units for charge. C. When an object has more protons than electrons. D. When an object has an equal number of electrons and protons. 2. Arcing 3. Charge Difference 4. Van de Graff B. Moving electrons. C. When a spark jumps between two objects. D. Can accept or give an infinite amount of electrons. Will neutralize charge. 5. Electric force 6. Strong Nuclear Force E. What keeps protons bound in the nucleus of an atom. F. When an object has more electrons than protons. 5. Electricity 6. E. A machine that separates charge. F. Causes an electric force and charges to move. _ Two positive charges. _ A positive and negative charge. _ Two balloons on a string pushing apart. _ A 3 C charge and a 4C charge. Attract or Repel? What are the charges of the second objects? What Charge: Positive (), Negative (), or Neutral (0)? 2 protons and 4 electrons 18 protons and 16 electrons A piece of rubber after rubbing it with fur. A balloon is rubbed against hair. Afterwards it sticks to a wall. A) Is the balloon attracted or repelled by the wall? B) Are the balloon and wall oppositely charged or like charged? Does the Electric Force increase or decrease? If the distance between the charges increases? If one of the charges is bigger (increases)? If both of the charges decrease (gets smaller)? If the charges get closer? A negatively charged rubber rod is brought close to the metal top of an electroscope. A) Will the electrons in the metal stay near the rod or move away from the rod? B) Why? C) On the diagram, draw where the electrons will go. D) What will the metal leaves at the bottom do? Metal Rubber rod leaves Electroscope You walk across a carpet. When you try to touch a door knob a spark jumps between you and the door knob. Why? Two objects are charged, but do not arc. Give two ways to make them arc. An object has a charge of 4.5 C. A) Is the object positive or negative? B) Did it gain or lose electrons? C) If you touch it to ground, will it lose electrons to ground or gain electrons from ground? D) What will its charge be after it is grounded? Using the object at the right answer the following questions. A) Did it gain or lose electrons? B) When grounded, will it gain or lose electrons from ground? C) Draw a wire grounding it. D) What will its charge be after grounding? 8C

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4 Circuits and Symbols Unit 9: 2 Electricity works a lot like water. Often imagining how water would work in an circuit will tell you how electricity will work as well. Electricity flows through closed circuits: paths of conductors (usually wires). Any break in the circuit will cause the circuit to stop, just like a break in a pipe lets water leak out. A break in a circuit is anywhere an insulator is in the way of electricity s flow. Paper, plastic, or even an air gap can keep electrons from flowing. A closed circuit has no break: electricity can flow. An open circuit has a break somewhere: electricity cannot flow. Circuit diagrams Circuit diagrams are a short-cut method of drawing circuits. They don t need to be perfect, but they can be drawn wrong. Electrical Device Electrical Symbols Symbol Function Water Equivalent battery capacitor These components look similar, but are very different and have different functions. wire battery path for electricity to flow pushes electricity through circuit pipes pump battery light bulb switch resistor light bulb switch makes lights; resists electricity turns electricity on and off water wheel valve The diagram on the right is a faster way of drawing the circuit on the left. (Notice the direction of the battery, which is important.) resistor resists flow of electricity. restriction in a pipe Short Circuits Electricity always chooses the path of least resistance. Since wires have virtually no resistance, electricity will go thru a wire instead of a device. This is known as a short-circuit. A short-circuit (also called a short ) is a wire that by-passes a device in a circuit. Short-circuit of battery This light turns off: it is shorted Shortcircuit out of the circuit. When a device is short-circuited the current by-passes it. It is easier for the current to go thru the wire than the resistor. Both lights stay off. Short-circuiting a battery drains the battery and can be dangerous. Wires could overheat, melting the insulation, and even cause a fire. Conventional Current What s really happening. Conventional current. When studying electricity, early scientists guessed that protons ( charges) were flowing. We know now that it is the electrons that move, but it is most common to use conventional current, which follows the movement of positive charges.

5 1. Open circuit 2. Closed circuit 3. Circuit diagram 4. Voltage 5. Current 6. Resistance Which of the following are correct? A. Label the diagram: B. A. Slows down the flow of electricity. B. A short-hand way of drawing electrical circuits. C. A circuit with a break in it; no electricity will flow. D. Pushes electricity through a circuit. E. Electricity can flow through this. F. The flow of electricity through a circuit. B. C. A. D. C. 1. Valve 2. Pipes 3. Water wheel 4. Resists flow 5. Pump 1. Wires 2. Battery 3. Resistor 4. Light bulb 5. Switch Do all of the light bulbs light up? Why or why not? Match the electrical component with the water component and diagram symbol A. Resistor B. Battery C. Switch D. Wire E. Light bulb A. Used to create radiant energy. B. Pushes electricity through the circuit. C. Can turn the electricity on and off. D. Allows electricity to flow. E. Slows down the flow of electricity. 1 d. a. c. b. e. Unit 9: 2 2 D. E. 3 In the Lab 1) Build the following circuit, being sure to connect all components (parts) with wires. Make sure it works. Using only a battery, two wires, and a light bulb (no holder), make the light bulb light. Which of the following diagrams will light up the light bulb? A. B. C. D. 2) Reverse the direction of the battery. Does the light still work? 3) Remove the switch. Touch the two wires between the light bulb and battery together to be sure the light still turns on. 4) Use these two wires as probes to test which of the following are conductors (C) or insulators (I). Paper A Penny _ Plastic Cloth Water Salt Water What two parts of a light bulb must be touched to make the light bulb light up? When the light bulb lights up, is this a closed or open circuit? What does this mean about the inside of the light bulb? Paperclip Glass Wood A Dime _ Sugar Water _ Rubber When the light bulb lights up, what types of energy are used and created?

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7 Current, Voltage, and Resistance Unit 9: 3 Current Current flows through closed circuits. Current is the amount charges that flow each second. In a wire current never changes. Current can only change if there is a junction: a split or a join. A lot of current. Very little current. More current means more electrons flowing, which is more electricity flowing (like more water flowing). Electrical current is measured in Amps, which is coulombs/sec (just like gallons per sec). More current Devices that use more energy, use more current. A light bulb is brighter when it has more current going thru it. A device that uses more energy uses more current. A louder radio pulls (uses) more amps. Less current Voltage Voltage pushes electricity. More voltage = more current. Resistance Resistance slows down electricity. More resistance = less current. Water falls due to gravitational potential energy (Ep). Likewise, current moves because of electrical potential energy given to electrons by voltage. There must be a change (difference) of voltage for current to move. Dams hold back water. Resistors hold back electrons. Both reduce current. Voltage pumps electricity, doing work to give potential electric energy (Ep) to the electrons. Batteries give voltage. The resistors use all of the electrical potential energy. This is why the voltage always equals 0 volts just before it reenters the battery. Adding Energy (Work) 1.5V High potential energy Current flows High voltage No voltage (0 V) Zero potential energy Using Energy Just as a waterwheel slows down the falling water, resistors use the electrical potential energy to do work (something useful). Anything that uses electricity has resistance: light bulbs, speakers, etc. Regardless of the amount of resistance, all the voltage is always used up in any circuit. 1.5V 1.5V 3 V 1.5 V 0 V Batteries can add together, (increasing voltage) or subtract (canceling each other out) if they are put in the circuit wrong. Light is off 0 volts total No current With the same resistance (1 bulb), more voltage (2 batteries) causes more current (brighter). Increasing voltage increases current. Increasing resistance decreases current. Decreasing voltage decreases current. Decreasing resistance increases current. Resistors use up voltage. With the same voltage (2 batteries), more resistance (2 bulbs) = less current (dimmer bulbs). Ohm s Law Ohm s Law can tell us the current, voltage, or resistance if the other two of them are known. Current (in amps [A]) I = V R Voltage (in volts [V]) Current equals the voltage divided by the resistance. Resistance (in ohms [Ω]) Current is dependent on voltage and resistance. Current can never change voltage or current, but both voltage and resistance can change current. Ex. How much current does a 12 V battery push through a 3 Ω resistor? V = 12 v R = 3 Ω I =? I = V R = 12 v = 4A 3 Ω

8 1. Voltage 2. Current 3. Resistance 4. Amps 5. Ohms (Ω) 6. Volts A. Units for voltage. B. Restricts electricity flowing in a circuit. C. Units for current. D. Pushes electrons thru a circuit. E. Units for Resistance. F. Amount of electricity flowing in a circuit. Which has more current flowing thru it? A loud radio or a quiet radio? A dim light bulb or a bright light bulb? A fast toy car or a slow toy car? A cold wire or a hot wire? Voltage (V), Current (I), or Resistance (R)? A If you increase resistance what decreases? B. If you increases voltage what increases? C. If the current decreased what increased? D. If current increased what increased? E. If current increased what decreased? F. If resistance is decreased, what increases? G. More batteries will increase these two quantities. Voltage (V), Current (I), or Resistance (R)? A) Flowing electrons. B) Pushes electricity in circuits. C) Like a water pump. D) Measured in Ohms. E) Measured in Amps. F) A battery gives this. G) Measured in volts. Which will have more resistance: an insulator or a conductor? Which resistor is the better conductor: 150Ω or 600Ω? Which resistor is the better insulator: 24Ω or 6Ω? How much current does a 9V battery push thru a 3Ω resistor? Variables Equation Solve Unit 9: 3 H) _ Restricts the flow of current. I) _ Does work in an electric circuit. J) _ Gives electric energy. K) _ 12 ohms L) _ 36 volts. M) _ 5 amps. A 4 Ω resistor has 3A running thru it. Find the battery s voltage. Variables Equation Solve Total voltage (V T ) = Total voltage (V T ) = An 18V battery produces 2A in the circuit. How much resistance is in the circuit? Variables Equation Solve When the switch is closed will the light turn on? Why or why not? Which light bulbs will light? (All are in closed circuits.) If it does light, draw an arrow to show the direction of current. With the same resistance, which battery will cause more current to flow: a 12V or a 24V battery? Voltage give what kind of energy to electricity? Which of these two water towers would give the most voltage to a water circuit? 3V 3V 3V 9V 0V 9V 9V How much potential energy does water have after it falls to the ground? Add (A) or reduce (R) voltage? Resistors? Wires? Batteries? A B C H G F D E High, Medium, or Low voltage? Point A Point D Point F Point C Point E Point H Point B Point G A bird lands on a 20,000 volt wire. Both feet are touching. A) What voltage is the bird s left foot? B) What voltage is the bird s right foot? C) What is the difference of voltage between the bird s feet? D) So, why can a bird land on an electrical wire and not get electrocuted? How much voltage does a circuit have just before going back thru the batteries?

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10 Types of Circuits Unit 9: 4 Wires and Voltage Since wires are conductors, they have no resistance, so they have no voltage difference. The voltage change over any wire is zero. OR The voltage is the same everywhere in the same wire. 12V 12V 0V 0 V 12V 0V 12V 0V Batteries add voltage, resistors subtract voltage, and wires don t change voltage at all. Junctions Current can split apart or join together. Just like in a river, current can only change if there is a junction: a split or a join. Current joins Junction Junction Current splits 2A 2A 6A 4A 4A Total current in and out of a junction is equal. Series circuits have only one path for the electricity to flow. There are no junctions. I T I T I T The current is the same everywhere in a series circuit. If any part of a series circuit is broken, the circuit fails. The lights are dependent on one another: if either light is unscrewed both lights will turn off. Series versus Parallel The branches (paths) of a parallel circuit are independent: if one light is unscrewed, the other will stay on. Your house is wired in parallel, so that each light and appliance can be turned on and off independently. Parallel circuits have multiple paths for the electricity to flow. Junction I T (current splits) I 1 I 2 Junction I T = I 1 I 2 (current joins) The total current equals the sum of the two branch currents. Multiple Devices When adding together batteries or resistors, it depends whether they are in series or in parallel. Objects in series have the same current because there is only one path. Objects in parallel have the same voltage because they share wires (same wire: same voltage). 8 V 4 Ω 4 V 4 Ω 2A 2A 3 V 1.5 V 1.5 V 1.5 V 2A 2A 0 V 2Ω 3 V 3Ω 0 V R total = 8 Ω 0 V V total = 3 V Resistors and batteries in series add together. V total = Batteries in parallel share the same voltage (they just last longer). 9 V R total = 1 Ω Resistors in parallel reduce total resistance because there are more paths for the current to flow. Example 1: Find the total current in this series circuit. Step 1: V total = 24 volts 12 V 12 V R 1 = 1 Ω R 2 = 2 Ω R 3 = 3 Ω Step 2. R total = = 6 Ω (in series) I Step 3: V = IR total I total V = R total total 24 = = 4 amps 6 Step 1: V total = 12 volts Example 2: Find the total current in this circuit. 0 volts 12 volts 6Ω 0 volts 3Ω 12v Step 2 : I branch1 = = 2a 6Ω Step 2 : 12v I branch 2 = = 4a 3Ω Step 3 : I = I I total 1 2 = 2 4 = 6a

11 1. Series 2. Short Circuit 3. Parallel 4. Branch 5. Junction 6. Zero A. The voltage across any wire. B. A circuit with multiple paths for current to flow. C. Where current splits or joins. D. An independent path for electricity in a parallel circuit. E. When a wire by-passes a part of a circuit. F. A circuit with only one path for the electricity. Only one path for the electricity to flow. Paths are dependent on each other (one affects the other). How your house is wired. Series or Parallel Circuit? Paths are independent of each other. If one light turns off, the others stay on. If you turn off one light, all the lights turn off. Has more than one path for the electricity to flow. Two devices have the same current. Two devices have the same voltage. Unit 9: 4 1. R 1 A. Voltage between A and B. 2. V T B. The total voltage in the circuit. 3. I T C. The resistance of resistor V 2 D. The current in branch 2 of the circuit. 5. I 2 E. Total current in the circuit. 6. R T F. The voltage of battery V AB G. The total resistance of the circuit. Are these devices in Series or Parallel? A. B. C. D. 2V 2V 9V 6Ω 6Ω 5Ω 4Ω 9V V T = R T = V T = R T = A B C Understanding current. Fill in the missing information on the following graphics. D 4 Ω 4 Ω 3 A 4 A I T = 18 A E F G 6 A V T = _ V FG = _ V ED = _ I Total = V AB = _ V FB = _ V AC = _ I 2 = V EA = _ V CE = _ V DC = _ I = I = A) What is the total voltage above? B) What is the voltage from B to F? C) What is the resistance from B to F? D) Find the current flowing from B to F (label it on the diagram). 1A 1A 3A E) How much current flows from C to G (label it)? I Total = Which resistor will have more current running thru it? Why? 3 V 3 V 20 Ω 5 Ω F) So, using D and E above, what is the total current going thru point E (this is the total current [I T ])? G) Using V T and I T, find the total resistance of the circuit [R T ]. Which light bulb will be brighter?

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13 Power and Voltage Drops Unit 9: 5 Electrical Power Electrical power tells us how fast electricity is being used in a circuit or resistor. A more powerful device uses the same amount of electricity, just faster. Electrical Power: Power (in watts) P = VI Voltage (in volts) Current (in amps) Electrical Power equals the voltage times the current. Ex. A 12 V battery pushes 3 A thru a light bulb. How much power does the light bulb use? V = 12 v I = 3 A P = _ P = VI = P = 12(3) = 36 w 100 W 40 W A brighter bulb uses more power. Since P= VI, a bulb could be brighter because it uses more voltage with the same current OR because it uses more current with the same voltage. Voltage Drops Batteries add voltage, resistors subtract voltage, and wire don t change voltage at all. To find the voltage used by a particular resistor, you have to know the current running thru it. 1. Find the total current (I T ). 8 V 4 Ω 8 V 4A 8 V 2 Ω V total =24V R total = 6 Ω VT 24 IT = = = 4A R 6 T 24 V 2. Use V = IR for each resistor. 8 V 4A 0 V 4 Ω 2 Ω 1 8V V 1 = I 1 R 1 = 4(4) = 1 R 1 uses 1, leaving 8 V. V 2 = I 2 R 2 = 4(2) = 8 V R 2 uses 8 V, leaving 0 V. Notice V = 8 V between the resistors. Also, the total voltage used by the resistors must equal the total voltage given by the batteries: 24 volts. For objects in series the biggest resistor uses the most voltage and the most power. 24 V P T = V T I T P T = 24(4) P T = 96 w 4 A 4 Ω 1 2 Ω 8 V P 1 = V 1 I 1 P 1 = 16(4) P 1 = 64 w P 2 = V 2 I 2 P 2 = 8(4) P 2 = 32 w Both light bulbs have the same current, but the one with more resistance is brighter because it uses more voltage and uses more power. Simplifying Parallel Circuits Semi-complicated 6Ω Remember that devices in parallel have the same voltage, but they have different current. 3Ω Finding the total current and power in a parallel circuit is much easier if you think of it as two independent series circuits. Simplified = 2A 6Ω V total = 12V I Branch 1 V = = = R1 6 R 1 = 6 Ω 2A I T = 2 4 = 6A V total = 12V I Branch 2 V 4A R 2 = 3 Ω 12 4A 2 2 = = = R2 3 3Ω 12V For objects in parallel, the one smallest resistor uses the most current and the most power. 2Α 6Ω P 1 = V 1 I 1 = 12(2) = 24 w 4A 3Ω P 2 = V 2 I 2 = 12(4) = 48 w Both light bulbs have the same voltage, but the one with less resistance is brighter because it has more current and uses more power (this is how your house works). Fuses Electricity causes heat. Fuse melt (or break) when too much current passes through them, protecting expensive electronic equipment. Like fuses, circuit breakers protect against too much current, also, but can be reset, instead of replaced. 25A Too much current causes the thin strip of metal to melt. Fuse Electrons The electrons that move to make electricity do not come from the battery: they are already in the wires of the circuit. Metals are good conductors because their electrons move easily. 1 electron out 1 electron in 1. Battery pushes 3. One electron out one electron enters the battery 2. Electrons repel each other throughout the wire

14 Unit 9: 5 1. Fuse 2. Circuit breaker 3. Wire 4. Branch 5. Power 6. Junction A. An independent path in a parallel circuit. B. A device that breaks to protect against excessive current. Must be replaced. C. Where branches joint or split. D. Protects against high current, but can be reset. E. Where most of the electrons in a circuit come from. F. The product of voltage and current. Which is brighter: a 60 W or a 100 W bulb? Which one uses more power? A 4 Ω and a 10 Ω light bulb are in series. Which one is brighter? Why? A 4 Ω and a 10 Ω light bulb are in parallel. Which is brighter? Why? Label the voltages at the letters. A B 1 Ω 1 V 1 V 1 Ω 1 V 1 Ω D C V at A = V at B = V at C = V at D = The electricity in your house is 120 volts. How many amps does a 60 watt light bulb use? How much power is used by a 120 V circuit using 6 amps? A 240 volt circuit (like your dryer) has 20 amps flowing through it. How much power is it using? 9 V 3 V R 1 = 1 Ω Split the following parallel circuit into two independent series circuits. 3 V 3 V R 2 = 2 Ω R 3 = 3 Ω Series or parallel? Which resistor uses the most power? Why? V T = R T = I T = 3 Ω 6 Ω _ V over R 1 = V over R 2 = V over R 3 = P 2Ω = _ P 3Ω = _ P T = 35 amps of current goes through a 40 amp fuse. What happens? 25 amps of current goes through a 15 amp fuse. What happens? R 1 = R 2 = 6 Ω 3 Ω I T B 1 B 2 I 1 I 2 Series or parallel? Which resistor will use more power? V T = V Branches = I 1 = _ I 2 = _ I T = _ R T = P 6Ω = P 3Ω = P T = _ Find the current in each circuit. Find the total current. Find the total resistance. 12v 12v R 1 = R 2 = R 3 = 12 Ω 8 Ω 8 Ω I T I 1 I 2 Which resistor uses the most power? I 3 V T = V Branches = I 1 = _ I 2 = _ I 3 = _ I T = _ R T = P T =

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16 Meters Unit 9: 6 Multimeters As the name suggests, a multimeter can act like a multitude of meters. It can be a voltmeter, ammeter, or ohmmeter, but you have to know how to use them in a circuit. A multimeter allows you to diagnose troubleshoot) circuits or broken equipment quickly. Many people throw out items that can be easily fixed. 1 V A Ω By moving the dial you select which meter it is. Use the probes to test the circuit. Circuit Symbols for Meters Ω V A Ohmmeter Voltmeter Ammeter Ohmmeters An ohmmeter measures resistance (ohms). Measure resistors outside of the circuit. 4Ω Measure resistance outside a circuit: with no battery at all or your measurements will be wrong. 2Ω 4 Ω 4 Ω 4 Ω 2 Ω Ω An ohmmeter can measure individual resistors or multiple resistors. 6 Ω - - Must have a break If a wire or resistor is broken (bad) the ohmmeter will read infinite resistance or error. Test each part of a circuit to find which part is broken. Circuit symbol 12V Voltmeters V A voltmeter measures voltage (volts). A voltmeter must be in parallel like an ohmmeter, but with the circuit on. Must be a voltmeter because it is 12V 0V in a circuit. 4 Ω 2 Ω closed switch V 8 v A voltmeter needs the circuit on. A voltmeter can read the voltage given by a battery or used by a resistor. 4V A car battery only provides 12V, but a lot of amps. drained AA, AAA, and D cell batteries read 1.5 V when new. After time the voltage drops some. Batteries also read lower when on. A voltmeter can measure a battery in the circuit or out of the circuit, while a resistor has no voltage outside of a circuit. Ammeters An ammeter measures current (amps). An ammeter must be in series with the circuit on. A Incorrect CAUTION! An ammeter is a very delicate device. Incorrect usage can badly damage the ammeter. Correct A 3 V 3 V 6 Ω To correctly hook up an ammeter, break the circuit where you want to read the current and place the ammeter into the break. 1A 1.5 V 1.5 V 4Α A 3 1 Ω 3 Ω 3Α A 2 1Α A 1 In this diagram there is only one ammeter. It has been moved to different parts of the circuit.

17 Unit 9: 6 1. Voltmeter 2. Multimeter 3. Probes 4. Ammeter 5. Ohmmeter A. Used to measure current. B. Used to measure voltage. C. The parts of the meter that touches the metal of the circuit device. D. A device that can measure voltage, current, or resistance. E. Used to measure a resistor. How do you hook up an ohmmeter? How do you hook up a voltmeter? How do you hook up an ammeter? Ohmmeter, Voltmeter, or Ammeter _ Must be in series. _ The circuit cannot connected. _ Must be in parallel with the device being measured. _ Used to tell the amount of current in the circuit. _ The circuit must be on. _ Delicate. Can be damaged if hooked up wrong. _ Can measure if a battery is worn out. _ Can measure a resistor only in a circuit. _ Can measure a resistor out of the circuit. What is the resistance for a good wire? What is the resistance for a broken wire? 3 V Ω 2 Ω Ω Identify the meters as voltmeters or ammeters. M1: M2: M3: M4: M5: 9 V 9 V R 1 = 4 Ω R 2 = 2 Ω R 3 = 6 Ω Draw meters that will measure the following: M1: Total voltage M2: Total current M3: Voltage over R 2 Figure out what each meter reads. Figure out what each meter reads. In Lab: Measure the three resistors you are given: R 1 = ; R 2 = ; R 3 =. Put the above resistors in series. R T =. How does R T compare with the individual resistors? Put the above resistors in parallel. R T =. How does R T compare with the individual resistors? Build the following circuit: V AB = V BC = V CD = V DE = V EF = V EA = 1.5V 1.5V A F E R 1 = _ 0 R 2 = _ R 3 = _ B 0 C D R 1 = R 2 = V FA = V BE = V CD = V ED = 1.5 V 1.5 V G R 1 = R 2 = WITH THE MULTIMETER OFF: set up your circuit to read the current in the first branch (at B or E). Have the teacher check your setup before you turn it on. I 1 = _ I 2 = _ I T = _ A F Using V T and I T, calculate R T. Check it with your ohmmeter. B E C D

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19 Magnets Unit 9: 7 Magnet Basics Horseshoe magnet A magnet is anything that can attract or repel another magnet. Bar magnets Donut magnets attract repel Aluminum Steel 1) All magnets have two poles: a north and a south. 2) You can t separate a N pole from a S pole (you just make smaller magnets). Opposite attract Likes repel 3) Magnets exert magnetic forces of attraction and repulsion. 4) Magnets only attract ferrous metals: Iron, Cobalt, and Nickel (steel is an alloy of mostly Iron and Nickel). How do Magnets Work? N Moving or spinning electrons in atoms cause magnetism. If electrons are paired and spinning e- in opposite directions, the magnets cancel each other out. When a substance is magnetic many of the electrons are spinning in the same direction. N S Permanent magnets do not lose their magnetism. Many of the electrons in a permanent magnet are spinning the same way and the all of the little electromagnets add up. Lodestone and Magnetite are the only two natural permanent magnetic materials. Temporary magnets become magnets only when near a permanent magnet. The spinning electrons line up together when a magnet is near, but will eventually fall back after the magnet leaves. Only ferrous materials can become temporary magnets. Often bumping them (like dropping) can cause the electrons to fall back quickly. Electromagnets An electromagnet is a magnet made by moving electricity. Electromagnets are particularly useful because they allow us to create forces that we can turn on and off at will. A toaster holds the toast down with an electromagnet. When the electricity turns off, the electromagnet releases the toast. Ways to strengthen an electromagnet: 1) More electricity (more current thru more batteries, etc). An electromagnet does not have to have a core: any loops of electricity will make an electromagnet. 2) More coils (the easiest way to add electricity). 3) Add a ferrous core, especially iron, which becomes a temporary magnet. Generators and Motors Moving electricity creates magnetic fields. Moving magnets make electricity. Because electricity and magnetism are linked we can make motors and generators. Generators generate electricity. Motors use electricity. Motor or Generator? Something turns the generator (does work) causing magnets to move, which creates electricity. Dams, cars, and power plants all produce electricity in this way. Work in (air): electricity out. Electricity causes magnetic forces thru electromagnets. The electromagnets cause the object to turn (do work). Electricity in: work out (moving air). It could be either one, depending on how it is used. Any motor can create electricity and any generator will turn if electricity is applied.

20 Unit 9: 7 1. Permanent magnet 2. Temporary magnet 3. Motor A. Turns when electricity is turned on. B. Becomes a magnet near a magnet, then loses its magnetism when moved away. C. Becomes a magnet when electricity moves in wire coils. Attract or repel? Attract or repel? 4. Generator 5. Core 6. Electromagnet N S D. Uses work to make electricity. E. Does not lose its magnetism: lodestone and magnetite are only natural types. F. Center of the electromagnet coils. If the three magnets are attracting each other, label N and S on the second magnet. 1) Label the north and south poles of the nail magnet. N S If the two magnets are repelling each other, label N and S on the second magnet. The diagram show magnetic levitation (Maglev). The top bar magnet is suspended (floating) above the bottom magnet while in a graduated cylinder to keep it from falling to the side. 1) Two keep the upper magnet levitated, are they attracting or repelling each other? 2) Label N and S on the upper magnet. True or false (and why): A magnet will pick up any piece of metal. N S 2) Will the nail stay a magnet when removed from the bar magnet? 3) Is the nail a temporary or permanent magnet? What makes a magnet on the atomic level? As a magnet gets closer to another magnet, does the magnetic force increase or decrease? Draw a simple electromagnet: Creates electricity. Has loops of wire in it. Creates motion. Is turned by a force. Can make electricity. Work Electricity Motor, Generator, or Both? _ Used in a hydroelectric dam. _ Used in open or close windows in a car. _ Turns when electricity is applied to it. Electricity Work Name three ways you could increase the strength of an electromagnet: Which electromagnet is stronger? A or B? B or C? C or D? A or C? A or D? A. 9V B. 9V C. 9V D. 3V Which is the strongest of the four?

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22 Electricity and Magnetism Review Unit 9: 8 1. Voltage 2. Resistance 3. Power 4. Charge 5. Current A. 8 amps B. 8 volts C. 8 coulombs D. 8 watts E. 8 ohms What moves: electrons or protons? Why? An object has a charge of 8 C. A) Is the object positive or negative? B) Did it gain or lose electrons? C) If you touch it to ground, will it lose electrons to ground or gain electrons from ground? D) What will its charge be after it is grounded? 1. Current 2. Voltage 3. Resistance 4. Insulator 5. Conductor 6. Circuit A. A path for electricity to flow. B. A material that allows electricity to flow. C. A material that resists electricity. D. Slows down electricity E. What pushes electricity in a circuit. F. The flow of electrons thru wires. A allows electricity or heat to pass through it. Electricity flows through paths called. A circuit has no breaks in it, while an circuit has a break and stops the flow of electricity. An _ will not allow electricity to pass. Electricity is made up of flowing. What are the charges of the second objects? Attract Repel Like electric charges attract/repel. Opposite charges attract/ repel. Draw a circuit of two batteries, a light bulb, a resistor, and a switch, all in series. What is electricity? Will electrons flow between the two objects? A. B. -3C -3C 5C 0C -2C -6C Why does electricity move? You have two light bulbs and a battery in a circuit. If you add another battery, do the light bulbs get brighter or dimmer? Why? What is arcing? When and why does arcing occur? You have two light bulbs and a battery in a circuit. If you add another light bulb, do the light bulbs get brighter or dimmer? Why? If a 12 v battery is connected to a 24 Ω resistor, how much current is flowing? Increases (I) Or Decreases (D) Increasing resistance _ current Decreasing resistance current Increasing voltage current Decreasing voltage current How much voltage is needed to produce 2 amps through a 4 ohm light bulb? How can you tell if two light bulbs are in parallel? How can you tell if two light bulbs are in series? If a light bulb in your house (120 V) draws 0.5 amps, how much power does it use? Is your house wired in series or in parallel? Why?

23 What is the change of voltage across a wire? Why can a bird sit on a wire and not be electrocuted? Unit 9: 8 What happens when you short circuit one light bulb in a two light bulb circuit? What happens if you short circuit a battery? Fill in the missing information on the following graphics. 3 V 5A I T = 3A R 1 = 10 Ω R 3 = 3 Ω R 2 = 5 Ω Series or parallel? 2Ω R T = V T = R T = I T = I R3 = V R2 = P T = 4Ω 3V V T = Where do most of the electrons come from that run thru a circuit? Series or parallel? A H V T = V DF = _ V HG = _ B I 1 = I 2 = I 3 = R 1 = 2 Ω G C R 2 = 3 Ω I 2 = I 3 = I T = F D R 3 = 1 Ω E N V at F = _ R T = The three magnets are repelling each other. Fill in the missing information. S The two magnets are attracting each other. Fill in the missing information. I 1 = P T = Someone asks you how what a circuit is and how it works. Tell them. What will a magnet attract? What will a magnet repel? What are fuses and circuit breakers? How are they different? What does moving electricity cause? What do moving magnets cause? What is an electromagnet? What is a motor? How does it work? How do you strengthen an electromagnet? What is a generator? How does it work?

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26 Lab: How Bright is the Light? Unit 9: S1 Purpose: To investigate how voltage and resistance affect the brightness of a light bulb in a circuit. Background: Students should know that a battery is a source of voltage. More batteries = more voltage A light bulb causes resistance in a circuit. More light bulbs = more resistance The brightness of the light bulb increases as the current through the light bulb increases. Materials and Preparation: Have 2 batteries, 2 light bulbs, wires/alligator clamps or a spring board per set-up. Electrical tape and Christmas lights offer an easy and inexpensive alternative to regular light bulbs. Effect of changing voltage on light bulb brightness: Circuit # light bulbs # batteries relative brightness Effect of changing resistance on light bulb brightness: Circuit # light bulbs # batteries relative brightness Student Outcome- Write a scientific statement to explain the effect of voltage and resistance on the brightness of a bulb.

27 Unit 9: Curriculum Notes: 1. MOST IMPORTANT DEMO. I have tried numerous ways for students to understand how voltage, resistance, and current relate. I have had them take measurements for themselves. The problem is that they take the measurement at different moments in time. They do not accept that the current is the same everywhere in a series circuit. So, make the following circuit: Variable power supply V Any resistor that allows enough current. Variable resistor By putting multiple multimeters in the circuit you can PROVE that the current is the same everywhere because they will all read same amount!!! You can change the voltage and the resistance, but all multimeters will always read the same current. It is also a good demo of how voltage and resistance affect current. 2. Concept Key Series circuits. Spend more time on series circuits before moving onto parallel circuits. For years my students have struggled with calculating voltage drops around a series circuit. This last year I used the above demo and spent more time with just series circuits and they got it. And the move to parallel circuits was easy. I had almost 100% comprehension with both circuit types and even with compound circuits.