1103 Period 14 Solutions: Electric Current and Capacitance

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1 Name Section 1103 Period 14 Solutions: Electric Current and Capacitance Activity 14.1: How Can Electric Charge Do Work? 1) Charge does work Your instructor will demonstrate a Wimshurst machine, which separates electric charge. a) Describe what happens to the hanging soda cans as electric charge from the Wimshurst machine flows onto the cans. Explain how the separated charge does work on the cans. b) Why do you see sparks between the cans or between the balls of the Wimshurst machine? Activity 14.2: What is an Electric Circuit? 2) Lighting a bulb Arrange one battery, one connecting wire, and one small light bulb (not in a tray), so that the bulb lights. You may need to try several different arrangements. a) Draw a diagram showing your arrangement of the battery, wire and bulb that worked. b) Explain why this arrangement worked and other arrangements you tried did not work. 85

2 3) Plumbing Analogies Your instructor will demonstrate plumbing analogies for circuits. Fill in the electrical concepts represented by the plumbing display. a) Water d) Pump b) Flowing water e) Water pressure c) Plastic tubes f) Narrow plastic tubes g) Group Discussion Question: Are charges used up to make a bulb light? If not, what happens to make it light? Activity 14.3: What is Electric Current? 4) Electric current is created by a flow of charge Connect a 1.5 volt D cell battery and a single-bulb tray with connecting wires so that the bulb lights. a) Your instructor will show you how to use a digital multimeter to measure the amount of current flowing through the circuit. b) How many coulombs of charge move through this circuit per second? Activity 14.4: How is Charge Stored in Capacitors? 4) Capacitance Your instructor will discuss capacitance. Capacitance is a measure of how easily an object stores electric charge. a) In Activity 13.4 we found that charges on the Leyden jar have more voltage when the inner cylinder and plastic cup were removed. Next, your instructor will measure the capacitance of the charged Leyden jar when it is assembled and when the inner cylinder and plastic cup are removed. In which case will the Leyden jar have greater capacitance? Prediction: Answer: b) Why does the capacitance of the Leyden jar change? 5) Foil Capacitors: Examine a torn apart foil capacitor. What is the purpose of the foil and the plastic? How is this capacitor similar to the Leyden jar? 86

3 Name Section 6) Sources of Separated Electric Charge: Batteries and Capacitors : a) Charge a 1 farad (green) capacitor by connecting it to one 1.5 volt battery for about 30 seconds. Connect the charged capacitor to a toy car. Describe what happens. Do you think the car would run longer if it were connected to this capacitor or to a D cell battery? Which stores more energy: the capacitor or a battery? b) How much total charge did you store on the 1-farad capacitor when you charged it with a single 1.5 volt battery? c) Suppose this battery provided 0.18 amps of current while charging the capacitor. How many coulombs of charge would the battery provide in one hour? d) How many 1-farad capacitors would be needed to provide this amount of separated charge? (Hint: you found the amount of separated charge per capacitor in part b). e) Group Discussion Question: Since a capacitor stores much less charge than a battery, why are capacitors used? 7) Capacitor Discharge: Your instructor will demonstrate discharging a large capacitor by connecting it to a light bulb and by touching it with a metal-tipped rod. a) In which case did the capacitor discharge more quickly? b) In which case is more energy released? c) In which case is more power produced? d) Explain why more power is produced in this case. 87

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5 Name Section Period 14 Exercises: Electric Current and Capacitance Write answers to the questions below. Show your mathematical steps and the units of the quantities. This sheet with your answers should be turned in at the beginning of Period Identifying the elements of electric circuits: a) What is the difference between electric charge and electric current? b) What devices could produce a voltage boost (electrical energy increase) in a circuit? c) What happens to the voltage boost in an operating electric circuit? d) What conditions are necessary for an electric circuit to light a bulb? 2. Using ratios to solve problems: A 1.0 farad capacitor stores about 5 joules of electrical potential energy. How many such charged capacitors would it take to equal the 0.7 million joules of chemical energy stored in a bottle of gasoline the size of one of these capacitors? 3. Using units to solve problems: A rechargeable 1.25 volt AA battery is rated at 0.6 amp-hours. a) How many coulombs of charge is 0.6 amp-hours? (Hint: 1 hour = 3,600 sec) b) If this much charge is supplied by the battery at a voltage of 1.25 volts, how much energy is available from this battery? 89

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