Transcript Document

Electromagnetic Induction
Faraday’s Law
Lenz’s Law
CHAPTER 25
Electromagnetic Induction
Induced Current in a Circuit
Motional electromotive force (emf)
e  vlB
Magnetic Flux
Lenz’s Law
Checking Understanding
A magnetic field goes through a loop of wire, as below. If the
magnitude of the magnetic field is increasing, what can we say about
the current in the loop?
A.
B.
C.
The loop has a clockwise current.
The loop has a counterclockwise current.
The loop has no current.
Answer
A magnetic field goes through a loop of wire, as below. If the
magnitude of the magnetic field is increasing, what can we say about
the current in the loop?
A.
B.
C.
The loop has a clockwise current.
The loop has a counterclockwise current.
The loop has no current.
Checking Understanding
A magnetic field goes through a loop of wire, as below. If the
magnitude of the magnetic field is decreasing, what can we say about
the current in the loop?
A.
B.
C.
The loop has a clockwise current.
The loop has a counterclockwise current.
The loop has no current.
Answer
A magnetic field goes through a loop of wire, as below. If the
magnitude of the magnetic field is decreasing, what can we say about
the current in the loop?
A.
B.
C.
The loop has a clockwise current.
The loop has a counterclockwise current.
The loop has no current.
Checking Understanding
A magnetic field goes through a loop of wire, as below. If the
magnitude of the magnetic field is constant, what can we say about
the current in the loop?
A.
B.
C.
The loop has a clockwise current.
The loop has a counterclockwise current.
The loop has no current.
Answer
A magnetic field goes through a loop of wire, as below. If the
magnitude of the magnetic field is constant, what can we say about
the current in the loop?
A.
B.
C.
The loop has a clockwise current.
The loop has a counterclockwise current.
The loop has no current.
Checking Understanding
A battery, a loop of wire, and a switch make a circuit below. A second
loop of wire sits directly below the first. Just before the switch is
closed, what can we say about the current in the lower loop?
A.
B.
C.
The loop has a clockwise current.
The loop has a counterclockwise current.
The loop has no current.
Answer
A battery, a loop of wire, and a switch make a circuit below. A second
loop of wire sits directly below the first. Just before the switch is
closed, what can we say about the current in the lower loop?
A.
B.
C.
The loop has a clockwise current.
The loop has a counterclockwise current.
The loop has no current.
Checking Understanding
A battery, a loop of wire, and a switch make a circuit below. A second
loop of wire sits directly below the first. Immediately after the switch is
closed, what can we say about the current in the lower loop?
A.
B.
C.
The loop has a clockwise current.
The loop has a counterclockwise current.
The loop has no current.
Answer
A battery, a loop of wire, and a switch make a circuit below. A second
loop of wire sits directly below the first. Immediately after the switch is
closed, what can we say about the current in the lower loop?
A.
B.
C.
The loop has a clockwise current.
The loop has a counterclockwise current.
The loop has no current.
Induced Fields
A changing magnetic field
induces an electric field.
A changing electric field induces a
magnetic field too.
Checking Understanding
A plane electromagnetic wave has electric and magnetic fields at all
points in the plane as noted below. With the fields oriented as shown,
the wave is moving
A.
B.
C.
D.
E.
into the plane of the paper.
out of the plane of the paper.
to the left.
to the right.
toward the top of the paper.
Answer
A plane electromagnetic wave has electric and magnetic fields at all
points in the plane as noted below. With the fields oriented as shown,
the wave is moving
A.
B.
C.
D.
E.
into the plane of the paper.
out of the plane of the paper.
to the left.
to the right.
toward the top of the paper.
Summary
Additional Questions
A bar magnet sits inside a coil of wire that is connected to a meter.
The bar magnet is at rest in the coil. What can we say about the
current in the meter?
A.
B.
C.
The current goes from right to left.
The current goes from left to right.
There is no current in the meter.
Answer
A bar magnet sits inside a coil of wire that is connected to a meter.
The bar magnet is at rest in the coil. What can we say about the
current in the meter?
A.
B.
C.
The current goes from right to left.
The current goes from left to right.
There is no current in the meter.
Additional Questions
A bar magnet sits inside a coil of wire that is connected to a meter.
The bar magnet is pulled out of the coil. What can we say about the
current in the meter?
A.
B.
C.
The current goes from right to left.
The current goes from left to right.
There is no current in the meter.
Answer
A bar magnet sits inside a coil of wire that is connected to a meter.
The bar magnet is pulled out of the coil. What can we say about the
current in the meter?
A.
B.
C.
The current goes from right to left.
The current goes from left to right.
There is no current in the meter.
Additional Questions
A bar magnet sits inside a coil of wire that is connected to a meter.
The bar magnet is completely out of the coil and at rest. What can we
say about the current in the meter?
A.
B.
C.
The current goes from right to left.
The current goes from left to right.
There is no current in the meter.
Answer
A bar magnet sits inside a coil of wire that is connected to a meter.
The bar magnet is completely out of the coil and at rest. What can we
say about the current in the meter?
A.
B.
C.
The current goes from right to left.
The current goes from left to right.
There is no current in the meter.
Additional Questions
A bar magnet sits inside a coil of wire that is connected to a meter.
The bar magnet is reinserted into the coil. What can we say about the
current in the meter?
A. The current goes from right to left.
B. The current goes from left to right.
C. There is no current in the meter.
Answer
A bar magnet sits inside a coil of wire that is connected to a meter.
The bar magnet is reinserted into the coil. What can we say about the
current in the meter?
A. The current goes from right to left.
B. The current goes from left to right.
C. There is no current in the meter.
Transformers
Checking Understanding
If the primary coil of wire on a transformer is kept the same and the
number of turns of wire on the secondary is increased, how will this
affect the voltage observed at the secondary?
A.
B.
C.
The voltage will increase.
The voltage will stay the same.
The voltage will decrease.
Answer
If the primary coil of wire on a transformer is kept the same and the
number of turns of wire on the secondary is increased, how will this
affect the voltage observed at the secondary?
A.
B.
C.
The voltage will increase.
The voltage will stay the same.
The voltage will decrease.
Two-Phase Power
to Your Home
Root-Mean-Square Current and Voltage
If we define
IR
Irms 
2
and
VR
Vrms 
2
then we can write
 IR 
2
PR  
R

(
I
)
rms R

 2
2
and
The expressions for AC power are identical to those used for DC
currents if rms currents and voltages are used.
Example Problem
The following devices are plugged into outlets on the same 120 V
circuit in a house. This circuit is protected with a 15-A circuit breaker.
Device
Power
Computer
250 W
Heater
900 W
Lamp
100 W
Stereo
120 W
Is there too much current in the circuit—that is, does the circuit
breaker blow?
Example Problem
The following devices are plugged into outlets on the same 120 V
circuit in a house. This circuit is protected with a 15-A circuit breaker.
Device
Power
Computer
250 W
Heater
900 W
Lamp
100 W
Stereo
120 W
Is there too much current in the circuit—that is, does the circuit
breaker blow? Note that the V, I, and P values are all root-meansquare (rms), so DC formulas apply.
I = P/V; I computer = 250/120 = 2.08 amps: I heater = 900/120 = 7.50
amps:
I lamp = 100/120 = 0.83 amp: I stereo = 120/120 = 1.00 amp:
Total current used is 11.41 amps. Circuit does not blow.
Physiological Effects and Electrical Safety
Electrical Safety
V
V
480 V
I


 48 mA
Req Rboots 10 MV