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Chapter 21

Electric Current and Direct-Current Circuits

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Outline Electric current Batteries Electromotive force and the

direction of current flow Resistance and ohm’s law Energy and power in electric circuits Resistors in series and parallel, and

combination circuits

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Electric Current Electric current, I: A flow of

electric charge from one place to another.

I = Q/t SI unit: coulomb per second (C/s) =

ampere, or amp (A) When charge flows through a

closed path and returns to its starting point, we refer to the closed path as an electric circuit.

Example 21-1: The disk drive in a portable CD player is connected to a battery that supplies it with a current of 0.22 A. How many electrons pass through the drive in 4.5 s?

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Batteries Battery: A battery uses

chemical reactions to produce a difference in electric potential between its two ends, or two terminals.

Water flow as an analogy for electric current

When a battery is connected to a circuit, electrons move in a closed path from the negative terminal of the battery to the positive terminal.

The flashlight: a simple electrical circuit.

A mechanical analog to the flashlight circuit.

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Electromotive Force and the Direction of Current Flow

Electromotive force, emf or : The electric potential difference between the terminals of a battery in an open circuit.

SI unit: volts (V) – emf is not a force. For an ideal battery, the potential

difference between its terminals = its emf, even in a closed circuit.

Direction of the current flow: The direction in which a positive test

charge would move. The flow of electrons and the current flow

point in opposite directions. The average speed of electrons in a wire is

~ 10-4 m/s – rather slow, due to repeated collisions with atoms in the wire.

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Resistance and Ohm’s Law Ohm’s law: V = IR

V: potential difference or voltage (V); I: Current (A); R: Resistance ()

R = (L/A) : the resistivity of the material of

the wire; L: length of the wire; A: cross-sectional area of the wire.

Table 21-1: values of of different materials.

Example 21-2: A current of 1.82 A flows through a copper wire 1.75 m long and 1.10 mm in diameter. Find the potential difference between the ends of the wire . (For copper, = 1.68 x 10-8 m)

Symbol for a resistor

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Energy and Power in Electric Circuits

Electrical power: P = IV SI units: watts (W)

In the case of a resistor, the electrical power is dissipated in the form of heat.

The power dissipated in a resistor is: P = IV = I (IR) = I2 R, or equivalently P = IV = (V/R) V = V2/R.

Conceptual checkpoint 21-2: A battery that produces a potential difference V is connected to a 5-W light bulb. Later, the 5-W light bulb is replaced with a 10-W light bulb.

(a) In which case does the battery supply the greatest current?

(b) Which light bulb has the greater resistance?

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Resistors in Series and Parallel

A series circuit

A parallel circuit

Req = R1 + R2 + R3

1/Req = 1/R1 + 1/R2 + 1/R3

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Combination Circuits Example 21-7: In the

circuit shown, the emf of the battery is 12.0 V, and all the resistors have a resistance of 200.0 . Find the current supplied by

the battery to this circuit. Find the current in each

resistor.

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Another Example

Find the current I in this circuit and the current in each resistor.

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Homework #4

Chapter 21, P. 754-757, Problems: #7, 19, 30, 52 (Physics, Walker, 4th edition).