WELCOME TO PERIOD 22
Homework Exercise #21 is due today.
CAUTION:
Keep your phones, tablets,
calculators, watches, etc. off of the tables.
PHYSICS 1103 – PERIOD 22
•How do electric motors work?
•What causes a changing magnetic field in
a motor?
•How do electric generators work?
•Put away your phones and electronics.
The large magnets will damage them.
Induced magnetism in electric motors
1) A changing current through a coil of wire induces a
changing magnetic field around the coil.
2) This changing magnetic field can be attracted to or
repelled by a second magnet field.
Electric motors
Motors convert electrical energy into kinetic energy using
the forces between magnetic fields to spin a rotor.
The rotor is a metal rod wrapped
in a coil of wire and mounted on a
pivot, which allows the rotor to spin.
Current in the wires induces a magnetic field around the
rotor.
The magnetic field of the rotor is attracted to and repelled
by nearby magnetic fields.
To keep the rotor spinning, its poles must reverse their
polarity. This requires a changing current in the coil of the
rotor.
Spinning a magnet with a magnet
N
S
Magnet on a pivot
N
S
Magnet in your hand
The magnet spins until its south end is aligned with the
north end of the magnet in your hand.
If you remove the magnet in your hand just as the south
pole of the spinning magnet approaches it, the spinning
magnet’s motion causes it to continue to spin.
 If you flip and replace the magnet in your hand at the
appropriate time, the spinning magnet again rotates to
align its south pole with the north pole of that magnet.
Spinning a magnet with an electromagnet
N
S
N
S
Magnet on a pivot
Electromagnet
The magnet spins until its south pole is aligned with the
north pole of the electromagnet.
If the current in the electromagnet is turned off just as the
permanent magnet approaches it, the permanent magnet’s
motion causes it to continue to spin.
If the current in the electromagnet is turned on at the
appropriate time, the permanent magnet again rotates to
align its south pole with the north pole of the electromagnet.
A simple motor
Permanent
magnet
Insulation is scraped
from both sides of
this wire.
End view of a wire with
insulation scraped from
one side of one end.
Insulation is scraped from
one side of this wire.
Paper clip supports
Why this motor spins
With each spin of the coil, the current turns on when
the bare wire touches the support and turns off when
the insulated wire touches.
When current flows through the coil, a magnetic field
is induced around the coil. Turning the current off
and on creates a changing magnetic field.
A magnet held near the coil provides the second
magnetic field needed to keep the coil spinning.
St. Louis motor
Permanent
magnets
Electrical contacts –
connect to batteries
Commutator
Rotor coils (electromagnets)
Rotor
Base
Shaft
St. Louis motor
Rotor
Rotor Coils
N
S
Spinning Electromagnet
N
S
Permanent Magnet
•The rotor of the St. Louis motor is an electromagnet on a
pivot. Permanent magnets attract and repel the rotor,
causing it to spin.
•To keep the rotor spinning, the magnetic poles of the
electromagnet reverse when the current reverses
direction.
What causes a changing current in a DC motor?
Commutators
Direct current motors can use
a commutator to create a
changing magnetic field.
A commutator is a conducting
ring with two slits. The ring
rotates as the rotor spins.
As the ring rotates, the current
in the rotor coils changes
direction.
When the current changes
direction, the magnetic field
around the rotor reverses poles.
The positive end of the battery is connected to half A of the
commutator ring. Current flows from left to right in the rotor coil.
1)
A
After the commutator turns ¼ of a turn to the right, the
brushes do not touch the commutator ring. No current flows.
B
S
Top view of
commutator
N
2)
A
Side view of
commutator and rotor.
B
After another ¼ turn to the right, the positive end of the battery
is connected to half B of the ring. Current flows from right to
left through the rotor coils. The poles of the rotor reverse.
3)
B
A
S
N
Motor with slit ring commutator
Motors and generators
Motors convert electrical energy into kinetic energy.
• A changing current flowing through a wire creates a
changing magnetic field around the wire.
• The changing magnetic field attracts and repels another
magnet.
• The interaction between these magnetic fields causes the
motor’s rotor to spin.
A generator is the opposite of an electric motor
Generators convert kinetic energy into electrical energy.
• A moving magnet creates a changing magnetic field.
• The changing magnetic field induces an electric current in
a nearby wire.
Electric generating plants
Generating plants convert kinetic energy into
electrical energy by rotating magnets near coils of
wire that are wrapped around iron cores.
When magnets spin near coils of wire, an electric
current is produced in the wire.
A turbine is used to spin the magnets.
Electric generating plant
Electric generating plants
To rotate the turbines that spin the magnets,
generating plants use…
1) water falling over a dam’s spillway,
2) wind energy from wind turbines, or
3) kinetic energy from steam. To heat water to steam,
generating plants burn fossil fuels or use the
thermal energy from nuclear fuel.
Hydroelectric generating plant
Wind turbine generator
(housing to cover the generator)
Coal burning generating plant
Nuclear generating plant
Fuels to generate electricity
What are the advantages and disadvantages of
• Fossil fuel generating plants?
• Nuclear generating plants?
BEFORE THE NEXT CLASS…
Read textbook chapter 23
Complete Homework Exercise 22
Bring a blank Activity Sheet 23 to class.