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• Current

• Conservation of current

• Batteries

• Resistance and resistivity

• Simple circuits

Chapter 22Current and Resistance

Topics:

Sample question:

How can the measurement of an electric current passed through a person’s body allow a determination of the percentage body fat?

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Reading Quiz 1. The charge carriers in metals are

A. electrons B. positrons C. protons D. a mix of protons and electrons

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1. The charge carriers in metals are A. electrons

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Answer

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Reading Quiz 2. A battery is connected to a resistor. Increasing the resistance

of the resistor will A. increase the current in the circuit. B. decrease the current in the circuit. C. not affect the current in the circuit.

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2. A battery is connected to a resistor. Increasing the resistance of the resistor will

B. decrease the current in the circuit.

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Answer

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Reading Quiz 3. A battery is connected to a resistor. As charge flows, the

chemical energy of the battery is dissipated as A. current B. voltage C. charge D. thermal energy

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3. A battery is connected to a resistor. As charge flows, the chemical energy of the battery is dissipated as

D. thermal energy

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Answer

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Batteries

The potential difference between the terminals of a battery, often called the terminal voltage, is the

battery’s emf.

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∆Vbat = = Wchem

q____

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Definition of a Current

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1. What happens as conductor gets longer

2. What happens as conductor gets wider

3. What happens as conductor gets hotter

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Student Current Demo

Potential, Charge, and Field Circuit ModelCharge and Current (need to load before class)

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Conservation of Current

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Rank the bulbs in the following circuit according to their brightness, from brightest to dimmest.

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Simple Circuits

The current is determined by the potential difference and the resistance of the wire:

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I =∆Vchem

R_____

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Resistivity

The resistance of a wire depends on its dimensions and the resistivity of its material:

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Checking Understanding

A battery is connected to a wire, and makes a current in the wire.

i. Which of the following changes would increase the current?

ii. Which would decrease the current?

iii. Which would cause no change?

A. Increasing the length of the wire

B. Keeping the wire the same length, but making it thicker

C. Using a battery with a higher rated voltage

D. Making the wire into a coil, but keeping its dimensions the same

E. Increasing the temperature of the wire

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Power in Circuits

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A resistor is connected to a 3.0 V battery; the power dissipated in the resistor is 1.0 W. The battery is now traded for a 6.0 V battery. The power dissipated by the resistor is nowA. 1.0 WB. 2.0 WC. 3.0 WD. 4.0 W

Checking Understanding

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A resistor is connected to a 3.0 V battery; the power dissipated in the resistor is 1.0 W. The battery is now traded for a 6.0 V battery. The power dissipated by the resistor is now

D. 4.0 W

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Answer

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1. 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?

2. For the electric blanket of the above 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?

Electric Blankets

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Additional Clicker Questions

1. 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 powerA 10 V 1 WB 8 V 1 WC 12 V 2 WD 6 V 2 WE 3 V 3 W

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1. 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 powerA 10 V 1 W

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Answer

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Kirchhoff’s Laws

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