Download Chapter 25 Induction-3-2

Electromagnets
Which is stronger? Without Core or With Core?
What is a galvanometer?
A galvanometer is an electromagnet that interacts with
a permanent magnet. The stronger the electric current
passing through the electromagnet, the more is
interacts with the permanent magnet.
Galvanometers are
used as gauges in cars
and many other
applications.
The greater the current passing through the wires, the stronger
the galvanometer interacts with the permanent magnet.
Galvanometer Question
1. A galvanometer whose resistance is 2 kW is used
to make an ammeter that deflects full-scale for 20
mA.
a. What is the potential difference across the
galvanometer (2.0 kW resistance) when a current of
60uA passes through it?
b. What is the equivalent resistance of parallel resistors
having the potential difference calculated in (a) for a
circuit with a total current of 20 mA?
c. What resistor should be placed parallel with the
galvanometer to make the resistance calculated in
part (b)?
Simple Motor
What are electric motors?
An electric motor is a device which changes electrical
energy into mechanical energy.
How does an electric motor work?
Go to the next slide 
Simple as that!!
Which would be
a more strong
magnet?
A. A
B. B
C. They would be the
same
B
D. Not enough
information to decide
A
Which would be
a more strong
magnet?
A. A
B. B
C. They would be the
same
D. Not enough
information to decide
A
B
Which compass shows the correct
direction of the magnet field at point
A?
A.
B.
C.
D.
A
Which compass shows the correct
direction of the magnet field at point
A?
A.
B.
C.
D.
A
Chapter 25
Electromagnetic Induction
We have seen how electricity can produce a magnetic
field, but a magnetic field can also produce electricity!
How?
What is electromagnetic induction?
Moving a loop of wire through a magnetic field produces
an electric current. This is electromagnetic induction.
A generator is used to convert
mechanical energy into electrical energy by
electromagnetic induction.
Carefully study the next diagrams:
Generators vs. Motors
Generator
Electric Motor
Mechanical to Electrical
Electrical to Mechanical
Steam engine, Water wheel,
Wind turbine, hand crank,
gas powered
Fans, appliances, power
tools,
Np > Ns
Electromotive Force
 Voltage caused by the magnetic field
 Equal to the magnitude of the magnetic field, times the
length of the wire times the component of the velocity
of the wire in the field perpendicular to the field.
 EMF = BLv (sin q)
Electromagnetic Induction
 Generate Current through a circuit by moving it
perpendicular through a magnetic field.
 Fourth Right Hand Rule – Fingers in the direction of
Magnetic Field, Thumb in the direction of motion =
Palm in the direction of CONVENTIONAL current
 EMF = Palm
 Motion = Thumb
 Magnetic Field = Fingers
Practice Problem
 A straight wire, 0.20 m long, moves at a constant speed
of 7.0 m/s perpendicular to a magnetic field of strength
8.0 x 10-2 T.
a. What EMF is induced in the wire?
b. What direction is the EMF induced?
c. The wire is part of a circuit that has a resistance of 0.50
W. What is the current through the wire?
Review from 3/1/11
1. What are some key terms in how an electric motor
works?
2. How does an Electric Motor work?
3. How does a Generator work?
4. What is Faraday’s Law of Induction?
5. What are the 4 things that increase the strength of an
electromagnet?
6. Why does an iron core increase the strength of an
electromagnet?
Review from 3/1/11
1. What are some key terms in how an electric motor works?
 Electromagnet, permanent magnet, current, like poles,
opposite poles, solenoid, rotation
2. How does an Electric Motor work?
 1st: Converts electrical energy to mechanical energy.
 2nd: Current flows which turns the solenoid into a
electromagnet. The changing attraction and repulsion of the
electromagnet and permanent magnet cause the motor to
rotate.
Review from 3/1/11
1. One more time…..How does an Electric Motor work?
 2nd: Current flows, turns the solenoid into an electromagnet,
the like poles of the permanent magnet and electromagnet
repel, then the opposite poles of the electromagnet and
permanent magnet attract, this causes the electromagnet to
rotate, if the current stayed going in the same direction, the
solenoid would stop rotating, but the current reverses
direction, so the poles of the solenoid reverse which repels
the solenoid from the permanent magnet which continues the
rotation.
Review from 3/1/11
3. How does a Generator work?
 1st: Mechanical Energy converts to electrical energy
 2nd: A hand crank rotates a solenoid in a magnetic field.
Therefore, the wires are experiencing a changing
magnetic field. This change in magnetic field, produces
an electromotive force on the electrons.
4. What is Faraday’s Law of Induction?
 The EMF generated is proportional to the rate of change
of the magnetic field.
Review from 3/1/11
5. What are the 4 things that increase the strength of an
electromagnet?
1. Addition of Iron Core
2. Increase diameter of Coils
3. Increase number of Coils
4. Increase voltage
6. Why does an iron core increase the strength of an
electromagnet?
 It’s a temporary magnet so the domains line up with the
electromagnet. Now there are two magnets, which at
least doubles the magnetism.
Direct current versus alternating current –
AC vs DC : What’s the difference?
•Direct current is electrical current which comes from a battery
which supplies a constant flow of electricity in one direction.
•Alternating current is electrical current which comes from a
generator. As the electromagnet is rotated in the permanent
magnet the direction of the current alternates once for every
revolution.
•You can see that the DC source is a battery – current flows in
one direction. The AC source is the generator and the current
alternates once for each revolution.
•In the US, electric utilities use a 60-Hz frequency, meaning that
the current alternates direction (forward to backward and back to
forward) 60 times in one second.
Effective Current & Voltage
 Effective Current
 Effective Voltage
Effective Current & Voltage
Effective Current & Voltage
Practice Problem
1. A generator develops a maximum voltage of 170 V.
a. What is the effective voltage?
b. A 60-W light bulb is placed across the generator with an Imax
of 0.70 A. What is the effective current through the bulb?
c. What is the resistance of the light bulb when it is working?
2. The RMS voltage of an AC household outlet is 117 V. What
is the maximum voltage across a lamp connected to the
outlet? If the RMS current through the lamp is 5.5 A. What
is the current in the lamp?
Lenz’s Law
 If you have one current that generates a magnetic field.
That magnetic field will produce a current in the
opposite direction of the first current.
 If you have a magnetic field that generates a current,
the current will generate a magnetic field in the
opposite direction of the first magnet.
 MIT Video
Transformers
 The primary voltage (on the left)
induces a magnetic field in the
core, which creates the secondary
voltage (on the right).
 What makes transformers so useful
is that if you change the number
of turns from one side to the other,
you change the voltage in the wire
on the right!
 Transformers can change a high
voltage to a lower one, or a low
voltage to a higher one.
Step Up Transformer
 A step-up transformer converts a low voltage to a higher
one.
 If you increase the number of turns on the right, the
voltage coming off the transformer will increase in
proportion.
 The right side has 4 times more turns so the voltage on
the right has increased 4 times.
 So the voltage has been stepped up by a factor of 4.
Step Down Transformer
 Step-down transformer reduces voltage.
 If you decrease the number of turns on the right, the
voltage coming off the transformer will decrease in
proportion.
 The right side has 1/5 the number of turns, so the voltage is
only 1/5 as large.
 So the voltage has been stepped down by 5.
Transformer Math
 The ratio of the number of turns is the same as the ratio of
the voltages
Vp N p

Vs N s
 An ideal transformer, which we will always assume,
dissipates no power (waste heat). The power of the primary
circuit is equal to
the power of the secondary current
P
P
p
s
V
I
V
I
p
p
s
s
 We use this assumption about power to find the current in
the secondary circuit
Is N p

Ip Ns
Final Thoughts on Transformers
Step-Up Transformer
Step-Down Transformer
Vp < Vs
Vp > Vs
Ip > I s
Ip < I s
Np < Ns
Np > Ns
Final Thoughts on Transformers
Step-Up Transformer
Step-Down Transformer
Consists of two inductively coupled
coils wound on a laminated iron core.
Consists of two inductively coupled
coils wound on a laminated iron core.
More turns in the secondary coil than
the primary coil
Less turns in the secondary coil than
the primary coil
Higher output voltage than input voltage Lower output voltage than input voltage
Lower output current than input current
Higher output current than input current
Used at Power stations to increase
voltage and reduce current for longdistance transmission
Used at substations and in towns to
reduce transmission line voltage for
domestic and industrial use.
Used in television sets to increase
voltage to operate the picture tube
Used in computers, radios, and CD
players to reduce household electricity
to very low voltages for electronic
components.
Transformer Practice
Problem
1. A transformer is designed to change 120.0 V into 1.0 x 104
V, and there are 5.00 x 103 turns in the primary coil. How
many turns are in the secondary, assuming 100%
efficiency?
2. A transformer has 420.0 turns in the primary and 120.0 in
the secondary. Assuming 100% efficiency.
a. What kind of transformer is this?
b. By what factor does number of turns change the voltage?
c. By what factor does it change the current?