1. Current and Resistance
Chapter 22
Current and Resistance

2. 22 Current and Resistance
Slide 22-2

3. Slide 22-3

4. Slide 22-4

5. Slide 22-5

6. Properties of a Current
Slide 22-12

7. Definition of a Current
Slide 22-13

8. Example Problem
The discharge of the electric eel can transfer a charge of 2.0 mC in a time of 2.0 ms. What current, in A, does this correspond to?
Slide 22-14

9. Conservation of Current
Slide 22-15

10. Checking Understanding
Rank the bulbs in the following circuit according to their brightness, from brightest to dimmest.
A  B  C  D
A  B  C  D
A  D  B  C
B  C  A  D
Answer: C
Bulbs: A&D, B&C
Currents: C
Slide 22-16

11. Answer
Rank the bulbs in the following circuit according to their brightness, from brightest to dimmest.
A  B  C  D
A  B  C  D
A  D  B  C
B  C  A  D
Answer: C
Bulbs: A&D, B&C
Currents: C
Slide 22-17

12. Checking Understanding
The wires below carry currents as noted. Rate the currents IA, IB and IC:
Answer: C
Bulbs: A&D, B&C
Currents: C
Slide 22-18

13. Answer
The wires below carry currents as noted. Rate the currents IA, IB and IC:
Answer: C
Bulbs: A&D, B&C
Currents: C
Slide 22-19

14. Batteries
The potential difference between the terminals of a battery, often called the terminal voltage, is the battery’s emf.
Slide 22-20

15. Simple Circuits
The current is determined by the potential difference and the resistance of the wire:
Slide 22-21

16. Resistivity
The resistance of a wire depends on its dimensions and the resistivity of its material:
Slide 22-22

17. Checking Understanding
A battery is connected to a wire, and makes a current in the wire. Which of the following changes would increase the current?
Increasing the length of the wire; (2) keeping the wire the same length, but making it thicker; (3) using a battery with a higher-rated voltage; (4) making the wire into a coil, but keeping its dimensions the same; (5) increasing the temperature of the wire.
All of the above
1 and 5
1, 4, and 5
2 and 3
None of the above
Answer: D
i) B, C
ii) A, D
iii) E
Slide 22-23

18. Answer
A battery is connected to a wire, and makes a current in the wire. Which of the following changes would increase the current?
Increasing the length of the wire; (2) keeping the wire the same length, but making it thicker; (3) using a battery with a higher-rated voltage; (4) making the wire into a coil, but keeping its dimensions the same; (5) increasing the temperature of the wire.
All of the above
1 and 5
1, 4, and 5
2 and 3
None of the above
Answer: D
i) B, C
ii) A, D
iii) E
Slide 22-24

19. Ohm’s Law
Answer:
i) B, C
ii) A, D
iii) E
Slide 22-25

20. Checking Understanding: Measuring Light Intensity
A battery is connected to a photoresistor. When light shines on this resistor, it increases the number of charge carriers that are free to move in the resistor. Now, the room lights are turned off, so less light falls on the photoresistor. How does this affect the current in the circuit?
The current increases.
The current decreases.
The current is not affected.
Answer: B
Slide 22-26

21. Answer
A battery is connected to a photoresistor. When light shines on this resistor, it increases the number of charge carriers that are free to move in the resistor. Now, the room lights are turned off, so less light falls on the photoresistor. How does this affect the current in the circuit?
The current increases.
The current decreases.
The current is not affected.
Answer: B
Slide 22-27

22. Example Problem
The filament of a 100-W bulb carries a current of 0.83 A at the normal operating voltage of 120 V.
What is the resistance of the filament?
If the filament is made of tungsten wire of diameter mm, how long is the filament?
Slide 22-28

23. Conceptual Example Problem
If you use wire of the same diameter operating at the same temperature, should you increase or decrease the length of the wire from the value calculated in the previous example in order to make a 60 W light bulb? (Hint: The bulb is dimmer. What does this tell us about the current?)
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24. Power in Circuits
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25. Energy and Power in Resistors
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26. Checking Understanding
A resistor is connected to a 3.0 V battery; the power dissipated in the resistor is 1.0 W. The battery is traded for a 6.0 V battery. The power dissipated by the resistor is now
1.0 W
2.0 W
3.0 W
4.0 W
Answer: D
Slide 22-32

27. Answer
A resistor is connected to a 3.0 V battery; the power dissipated in the resistor is 1.0 W. The battery is traded for a 6.0 V battery. The power dissipated by the resistor is now
1.0 W
2.0 W
3.0 W
4.0 W
Answer: D
Slide 22-33

28. Example Problem
An electric blanket has a wire that runs through the interior. A current causes energy to be dissipated in the wire, warming the blanket. A new, low-voltage electric blanket is rated to be used at 18 V. It dissipates a power of 82 W. What is the resistance of the wire that runs through the blanket?
Slide 22-34

29. Conceptual Example Problem: Electric Blankets
For the electric blanket of the previous example, as the temperature of the wire increases, what happens to the resistance of the wire? How does this affect the current in the wire? The dissipated power?
Slide 22-35

30. Example Problem
Many websites describe how to add wires to your clothing to keep you warm while riding your motorcycle. The wires are added to the clothing; a current from the 12-V battery of the motorcycle passes through the wires, warming them. One recipe for a vest calls for 10 m of 0.25-mm-diameter copper wire. How much power will this vest provide to warm the wearer?
Slide 22-36

31. Example Problem
Standard electric outlets in the United States run at 120 V; in England, outlets are 230 V. An electric kettle has a coiled wire inside that dissipates power when it carries a current, warming the water in the kettle. A kettle designed for use in England carries 13 A when connected to a 230 V outlet.
What is the resistance of the wire?
What power is dissipated when the kettle is running?
The kettle can hold 1.7 L of water. Assume that all power goes to heating the water. How long will it take for the kettle to heat the water from 20ºC to 100ºC?
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32. Summary
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33. Additional Questions
A set of lightbulbs have different rated voltage and power, as in the table below. Which one has the highest resistance?
Bulb Rated voltage Rated power
A 10 V 1 W
B 8 V 1 W
C 12 V 2 W
D 6 V 2 W
E 3 V 3 W
Answer: A
Slide 22-39

34. Answer
A set of lightbulbs have different rated voltage and power, as in the table below. Which one has the highest resistance?
Bulb Rated voltage Rated power
A 10 V 1 W
B 8 V 1 W
C 12 V 2 W
D 6 V 2 W
E 3 V 3 W
Answer: A
Slide 22-40

35. Additional Questions
2. A set of lightbulbs have different rated voltage and power, as in the table below. Which one has lowest resistance?
Bulb Rated voltage Rated power
A 10 V 1 W
B 8 V 1 W
C 12 V 2 W
D 6 V 2 W
E 3 V 3 W
Answer: E
Slide 22-41

36. Answer
2. A set of lightbulbs have different rated voltage and power, as in the table below. Which one has lowest resistance?
Bulb Rated voltage Rated power
A 10 V 1 W
B 8 V 1 W
C 12 V 2 W
D 6 V 2 W
E 3 V 3 W
Answer: E
Slide 22-42

37. Additional Questions
3. A battery makes a circuit with a lightbulb as shown. Two compasses are near the wires before and after the bulb in the circuit. Which compass experiences a larger deflection?
Compass 1 experiences a larger deflection.
Compass 2 experiences a larger deflection.
C. Both compasses experience the same deflection.
Answer: C
Slide 22-43

38. Answer
3. A battery makes a circuit with a lightbulb as shown. Two compasses are near the wires before and after the bulb in the circuit. Which compass experiences a larger deflection?
Compass 1 experiences a larger deflection.
Compass 2 experiences a larger deflection.
C. Both compasses experience the same deflection.
Answer: C
Slide 22-44

39. Additional Questions
4. In Trial 1, a battery is connected to a single lightbulb and the brightness noted. Now, in Trial 2, a second, identical, lightbulb is added. How does the brightness of these two bulbs compare to the brightness of the single bulb in Trial 1?
The brightness is greater.
The brightness is the same.
The brightness is less.
Answer: C
Slide 22-45

40. Answer
4. In Trial 1, a battery is connected to a single lightbulb and the brightness noted. Now, in Trial 2, a second, identical, lightbulb is added. How does the brightness of these two bulbs compare to the brightness of the single bulb in Trial 1?
The brightness is greater.
The brightness is the same.
The brightness is less.
Answer: C
Slide 22-46

41. Additional Example Problem
How much time does it take for 1.0 C to flow through each of the following circuit elements?
A 60 W reading light connected to 120 V.
A 60 W automobile headlamp connected to 12 V.
Slide 22-47