Chapter 22, 23

Electric Current and Direct-Current Circuits

Units of Chapter 22 and 23

• Electric Current and circuit

• Resistance and Ohm’s Law

• Energy and Power in Electric Circuits

• Resistors in Series and Parallel

•Ammeters and Voltmeters

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What would happen?

• More electric potential difference - more charges move

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Video - electric current animation

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Electric current• The flow of charge particle from one

place to another. • The electric current is defined the rate of flow of electric charge.

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What do you need to light a bulb?

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Electric circuits - A closed path through which charge can flow, returning to its starting point.

• Conventional current – the flow of + charge

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Electric Current

The direction of current flow – from the positive terminal to the negative one.

Electric Current

A battery uses chemical reactions to produce a potential difference between its terminals. It causes current to flow through the flashlight bulb similar to the way the person lifting the water causes the water to flow through the

paddle wheel.

Electric Power

Power – the rate of energy

A generator transfers kinetic energy to electric energy each seconds.

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The electric power

• The Electric Power – the product of the current and the potential difference.

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Example

A 120 V motor operates at 13 A. Determine the power and the energy used in one hour of operation.

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Resistance and Ohm’s Law

Ohm's law states that the current through a conductor between two

points is directly proportional to the potential difference across the two poi

nts.

Resistance and Ohm’s Law

Resistance – the property determining how much current will flow.

Solving for the resistance, we find

The units of resistance, volts per ampere, are called ohms:

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Check Your Understanding

• Which of the following will cause the current through an electrical circuit to decrease? Choose all that apply.

a. decrease the voltage

b. decrease the resistance

c. increase the voltage

d. increase the resistance

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Check Your Understanding

• If the resistance of a circuit were tripled, then the current through the circuit would be ____.

a. one-third as muchb. three times as muchc. unchangedd. ... nonsense! There would be no way to make

such a prediction.

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Resistance and Ohm’s Law

Two wires of the same length and diameter will have different resistances if they are made of different materials. This property of a material is called the resistivity.

Resistance and Ohm’s Law

The difference between insulators, semiconductors, and conductors can be clearly seen in their resistivities:

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Use the Ohm's law equation to determine the missing values in the following circuits.

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Some circuit symbols used in schematic diagrams are shown below.

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Energy and Power in Electric Circuits

In materials for which Ohm’s law holds, the power can also be written:

This power mostly becomes heat inside the resistive material.

Energy and Power in Electric Circuits

When the electric company sends you a bill, your usage is quoted in kilowatt-hours (kWh). They are charging you for energy use, and kWh are a measure of energy.

Ammeters and VoltmetersAn ammeter is a device for measuring current, and a voltmeter measures voltages.

The current in the circuit must flow through the ammeter; therefore the ammeter should have as low a resistance as possible, for the least disturbance.

Ammeters and Voltmeters

A voltmeter measures the potential drop between two points in a circuit. It therefore is connected in parallel; in order to minimize the effect on the circuit, it should have as large a resistance as possible.

Summary

• Electric current is the flow of electric charge.

• Unit: ampere

• 1 A = 1 C/s

• A battery uses chemical reactions to maintain a potential difference between its terminals.

• The potential difference between battery terminals in ideal conditions is the emf.

• Work done by battery moving charge around circuit:

Summary

• Direction of current is the direction positive charges would move.

• Ohm’s law:

• Relation of resistance to resistivity:

• Resistivity generally increases with temperature.

• The resistance of a superconductor drops suddenly to zero at the critical temperature, TC.

Summary

• Ammeter: measures current. Is connected in series. Resistance should be as small as possible.

• Voltmeter: measures voltage. Is connected in parallel. Resistance should be as large as possible.