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## Electricity and Magnetism

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### Electricity and Magnetism Electricity and Magnetism Properties of Magnets Magnetic Properties of Materials The Magnetic Field of the Earth – PowerPoint PPT presentation

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Title: Electricity and Magnetism

1
Electricity and Magnetism
Electricity and Magnetism
• Properties of Magnets
• Magnetic Properties of Materials
• The Magnetic Field of the Earth

2
Chapter Objectives
1. Predict the direction of the force by using the
right-hand rule.
2. Explain the relationship between electric current
and magnetism.
3. Describe and construct a simple electromagnet.
4. Explain the concept of commutation as it relates
to an electric motor.
5. Explain how the concept of magnetic flux applies
to generating electric current using Faradays
law of induction.
6. Describe three ways to increase the current from
an electric generator.

3
Chapter Vocabulary Terms
• gauss
• right-hand rule
• coil
• solenoid
• magnetic field
• tesla
• induction
• induced current
• magnetic flux
• commutator
• generator
• electromagnet
• polarity

4
Electric Current and Magnetism
• Key Question
• Can electric current create a magnet?

5
23.1 Electric Current and Magnetism
• In 1819, Hans Christian Oersted, a Danish
physicist and chemist, placed a compass needle
near a wire
• When the switch was closed, the compass needle
moved just as if the wire were a magnet.

6
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7
Electric Current and Magnetism
• Two wires carrying electric current exert force
on each other, just like two magnets.
• The forces can be attractive or repulsive
depending on the direction of current in both
wires.

8
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9
Electric Current and Magnetism
• A single wire is too small to be of much use.
• There are two ways to make strong magnetic fields
from current in wires
• Bundling together, this allows the same current
to create many times the magnetic field of one
wire.
• Coiling the wires concentrate the magnetic field
in their center.

10
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12
Electric Current and Magnetism
• A wire coil with many turns called solenoid.
• A coil takes advantage of these two techniques
(bundling wires and making coils) for increasing
field strength.

13
The true nature of magnetism
• The magnetic field of a coil is identical to the
field of a disk-shaped permanent magnet.

14
Electric Current and Magnetism
• The orbiting electrons carry electric charge.
• Moving charge makes current the orbiting
electrons make currents
• These currents create the magnetic fields.
• Substances that have all their orbits going the
same way have permeant magnetic fields.

15
Magnetic force on a moving charge
• The magnetic force on a wire is really due to
force acting on moving charges in the wire.
• A charge moving in a magnetic field feels a force
perpendicular to both the magnetic field and to
the direction of motion of the charge.

16
Magnetic force on a moving charge
• A magnetic field that has a strength of 1 tesla
(1 T) creates a force of 1 newton (1 N) on a
charge of 1 coulomb (1 C) moving at 1 meter per
second.
• This relationship is how the unit of magnetic
field is defined.

17
Magnetic force on a moving charge
• A charge moving perpendicular to a magnetic field
moves in a circular orbit.
• A charge moving at an angle to a magnetic field
moves in a spiral.

18
23.1 Magnetic field near a wire
• The field of a straight wire is proportional to
the current in the wire and inversely
proportional to the radius from the wire.

Current (amps)
Magnetic field (T)
19
Magnetic fields in a coil
• The magnetic field at the center of a coil comes
from the whole circumference of the coil.

No. of turns of wire
Magnetic field (T)
Current (amps)
20
Calculate magnetic field
• A current of 2 amps flows in a coil made from 400
turns of very thin wire.
• The radius of the coil is 1 cm.
• Calculate the strength of magnetic field (in
tesla) at the center of the coil.

21
Electromagnets and the Electric Motor
• Key Question
• How does a motor work?

Students read Section 23.2 AFTER Investigation
23.2
22
Electromagnets and the Electric Motor
• Electromagnets are magnets that are created when
electric current flows in a coil of wire.
• A simple electromagnet is a coil of wire wrapped
around a rod of iron or steel.
• Because iron is magnetic, it concentrates and
amplifies the magnetic field created by the
current in the coil.

23
Electromagnets and the Electric Motor
• The right-hand rule
• When your fingers curl in the direction of
current, your thumb points toward the magnets
north pole.

24
The principle of the electric motor
• An electric motor uses electromagnets to convert
electrical energy into mechanical energy.
• The disk is called the rotor because it can
rotate.
• The disk will keep spinning as long as the
external magnet is reversed every time the next
magnet in the disk passes by.
• One or more stationary magnets reverse their
poles to push and pull on a rotating assembly of
magnets.

25
The principle of the electric motor
26
Commutation
• The process of reversing the current in the
electromagnet is called commutation and the
switch that makes it happen is called a
commutator.

27
Electric Motors
• Electric motors are very common.
• All types of electric motors have three key
components
• A rotating element (rotor) with magnets.
• A stationary magnet that surrounds the rotor.
• A commutator that switches the electromagnets
from north to south at the right place to keep
the rotor spinning.

28
Electric Motors
• If you take apart an electric motor that runs on
batteries, the same three mechanisms are there
the difference is in the arrangement of the
electromagnets and permanent magnets.

29
Electric motors
• The rotating part of the motor, including the
electromagnets, is called the armature.

30
23.2 Electric motors
• The permanent magnets are on the outside, and
they stay fixed in place.
• The wires from each of the three coils are
attached to three metal plates (commutator) at
the end of the armature.

commutator
31
23.2 Electric Motors
• As the rotor spins, the three plates come into
contact with the positive and negative brushes.
• Electric current flows through the brushes into
the coils.

32
23.3 Induction and the Electric Generator
• Key Question
• How does a generator produce electricity?

Students read Section 23.3 AFTER Investigation
23.3
33
23.3 Induction and the Electric Generator
• If you move a magnet near a coil of wire, a
current will be produced.
• This process is called electromagnetic induction,
because a moving magnet induces electric current
to flow.
• Moving electric charge creates magnetism and
conversely, changing magnetic fields also can
cause electric charge to move.

34
23.3 Induction
• Current is only produced if the magnet is moving
because a changing magnetic field is what creates
current.
• If the magnetic field does not change, such as
when the magnet is stationary, the current is
zero.

35
23.3 Induction
• If the magnetic field is increasing, the induced
current is in one direction.
• If the field is decreasing, the induced current
is in the opposite direction.

36
23.3 Magnetic flux
• A moving magnet induces current in a coil only if
the magnetic field of the magnet passes through
the coil.

37
• Faradays law says the current in a coil is
proportional to the rate at which the magnetic
field passing through the coil (the flux) changes.

38
39
23.3 Generators
• A generator is a device that uses induction to
convert mechanical energy into electrical energy.

40
23.3 Transformers
• Transformers are extremely useful because they
efficiently change voltage and current, while
providing the same total power.
• The transformer uses electromagnetic induction,
similar to a generator.

41
23.3 Transformers
• A relationship between voltages and turns for a
transformer results because the two coils have a
different number of turns.

42
Application Trains that Float by Magnetic
Levitation