Download Welcome to 1161 Principles of Physics II

Chapter 21
Electric Current and DirectCurrent Circuits
Dr. Jie Zou
PHY 1161
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Outline
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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
Dr. Jie Zou
PHY 1161
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Electric Current
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Electric current, I: A flow of electric
charge from one place to another.
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Dr. Jie Zou
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
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Battery: A battery uses chemical
reactions to produce a difference in
electric potential between its two
ends, or two terminals.
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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.
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Dr. Jie Zou
Water flow as an analogy for electric
current
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
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Electromotive force, emf or : The
electric potential difference between the
terminals of a battery in an open circuit.
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Direction of the current flow:
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Dr. Jie Zou
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.
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
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Ohm’s law: V = IR
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R =  (L/A)
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Symbol for a resistor
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Dr. Jie Zou
V: potential difference or voltage (V); I:
Current (A); R: Resistance ()
: the resistivity of the material of the
wire; L: length of the wire; A: crosssectional 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)
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Energy and Power in Electric
Circuits
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Electrical power: P = IV
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In the case of a resistor, the electrical power
is dissipated in the form of heat.
The power dissipated in a resistor is:
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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.
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Dr. Jie Zou
SI units: watts (W)
(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
Dr. Jie Zou
Req = R1 + R2 + R3
1/Req = 1/R1 + 1/R2 + 1/R3
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Combination Circuits
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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 .
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Dr. Jie Zou
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.
Dr. Jie Zou
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Homework #4
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Chapter 21, P. 754-757, Problems:
#7, 19, 30, 52 (Physics, Walker, 4th
edition).
Dr. Jie Zou
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