Chapter 23 Electric Current
(cont.)
• Direct current and
alternating current
• Electric power
• Electric circuits
04/07/04
Dr. Jie Zou
PHY 3050G
Department of Physics
1
Direct Current and Alternating
Current
• Electric current may be dc or ac.
• By dc, we mean direct current, which
refers tot he flowing of charges in one
direction.
Pulsating dc
Time
Time graphs of dc.
04/07/04
– Even if the current occurs in unsteady pulses, so
long as electrons move in one direction only, it is
dc.
– A battery produces direct current (dc) in a circuit
because the terminals of the battery always have
the same sign.
Dr. Jie Zou
PHY 3050G
Department of Physics
2
Alternating Current
• By ac, we mean alternating current:
electrons in the circuit are moved first in one
direction and then in the opposite direction,
alternating to and fro about relatively fixed
positions.
Time graphs of ac.
04/07/04
– This is accomplished by alternating the polarity
of voltage at the generator or other voltage
source.
– 60-hertz current: nearly all commercial ac circuits
in North America involve voltages and currents
that alternate back and forth at a frequency of 60
cycles per second (60 -hertz).
Dr. Jie Zou
PHY 3050G
Department of Physics
3
Converting ac to dc
• Household current is ac. The current in a
battery-operated device such as a pocket
calculator is dc.
• You can operate these devices on ac instead of
batteries with an ac-dc converter.
• The ac-dc converter uses a diode.
• Diode: a tiny electronic device that acts as a
one-way valve to allow electron flow in only
one direction. A diode is represented by the
symbol
. Current flows in the direction
of the arrow but not in the reverse direction.
a. When input to a diode is ac,
b. Output is pulsating dc.
04/07/04
Dr. Jie Zou
PHY 3050G
Department of Physics
4
Electric Power
• The rate at which electric energy is
converted into another form such as
mechanical energy, heat, or light is called
electric power.
• Electric power is equal to the product of
current and voltage. Power = current ×
voltage
The power and voltage
on the light bulb read
“100 W 120 V”. How
many amperes of
current will flow
through the bulb?
04/07/04
– Units of current: amperes
– Units of voltage: volts
– Units of electric power: watts
• For example, if a lamp rated at 120 watts
operates on a 120-volt line, it will draw a
current of 1 ampere. How about a 60-watt
lamp on a 120-volt line?
Dr. Jie Zou
PHY 3050G
Department of Physics
5
Electric Circuits
• Circuit: any path along which electrons can flow is a
circuit.
• For a continuous flow of electrons, there must be a
complete circuit with no gaps.
– A gap is usually provided by an electric switch that can be opened
or closed to either cut off or allow energy flow.
• Devices are commonly connected in a circuit in one of two
ways:
– Series or
– Parallel.
04/07/04
Dr. Jie Zou
PHY 3050G
Department of Physics
6
Series Circuits
•
Series circuit: when connected in series, the devices in
the circuit form a single pathway for electron flow
between the terminals of the battery or a generator.
•
Important characteristics of a series circuit
– Electric current has but a single pathway through the
circuit. Thus the current passing through each electrical
device is the same.
– The voltage, or potential difference, across each device is
proportional to its resistance.
– The total voltage across the circuit is the sum of all
voltage across individual devices.
– Ohm’s law applies to each individual devices.
•
Main disadvantage of a series circuit: If one device (e.g.
a bulb) fails, current in the whole circuit ceases.
04/07/04
Dr. Jie Zou
PHY 3050G
Department of Physics
7
Parallel Circuits
•
Parallel circuits: when connected in parallel, devices
in the circuit form branches, each of which is a
separate path for the flow of electrons. A break in
any one path does not interrupt the flow of charge in
the other paths. Each device operates independently
of the other devices.
•
Major characteristics of parallel circuits:
– Each device connects the same two points A and B of
the circuit and the voltage is the same across each
device.
– The total current in the circuit divides among the
parallel branches. The amount of current in each
branch is inversely proportional tot he resistance of the
branch.
– The total current in the circuit equals the sum of the
currents in its parallel branches.
– Ohm’s law applies to each individual devices.
04/07/04
Dr. Jie Zou
PHY 3050G
Department of Physics
8
Some Examples
•
Alternating current is normally produced by a
– A) battery. B) generator. C) both of these. D) neither of these.
•
On some early automobiles both headlights went out when one bulb
burned out. The headlights must have been connected in
– A) parallel. B) series.
•
The electric power of a lamp that carries 2A at 120 V is
– A) 1/6 watts. B) 2 watts. C) 60 watts. D) 20 watts. E) 240 watts.
•
The current through two identical light bulbs connected in series is
0.25 A. The voltage across both bulbs is 110 V. The resistance of a
single light bulb is
– A) 22 ohms. B) 44 ohms. C) 220 ohms. D) 440 ohms. E) none of
these.
04/07/04
Dr. Jie Zou
PHY 3050G
Department of Physics
9
Homework 11 (04/07/04)
• Chapter 23, P. 456, Exercises: 24, 43, 44,
45.
04/07/04
Dr. Jie Zou
PHY 3050G
Department of Physics
10