Physics chpt23

CPO Science
Foundations of Physics

Unit 7, Chapter 23

Unit 7: Electricity and Magnetism
Chapter 23 Electricity and Magnetism
 23.1 Properties of Magnets
 23.2 Magnetic Properties of Materials
 23.3 The Magnetic Field of the Earth

Chapter 23 Objectives
1. Predict the direction of the force on a moving charge or
current carrying wire in a magnetic field 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 Faraday’s law of
induction.
6. Describe three ways to increase the current from an
electric generator.

Chapter 23 Vocabulary Terms
 gauss
 right-hand rule
 coil
 solenoid
 magnetic field
 tesla
 Faraday’s law

 induction
 induced current
 magnetic flux
 commutator
 generator
 electromagnet
 polarity

23.1 Electric Current and Magnetism
Key Question:
Can electric current create a magnet?

*Students read Section 23.1 AFTER Investigation 23.1

 In 1819, Hans Christian
Oersted, a Danish physicist
and chemist, and a professor,
placed a compass needle near
a wire through which he could
make electric current flow.
 When the switch was closed,
the compass needle moved
just as if the wire were a
magnet.

 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.

 The magnetic field around a single wire is too small to
be of much use.
 There are two techniques to make strong magnetic
fields from current flowing in wires:
1. Many wires are bundled together, allowing the same
current to create many times the magnetic field of a
single wire.
2. Bundled wires are made into coils which concentrate
the magnetic field in their center.

 The most common form of
electromagnetic device is a
coil with many turns called a
solenoid.
 A coil takes advantage of
these two techniques
(bundling wires and making
bundled wires into coils) for
increasing field strength.

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

 The electrons moving around
the nucleus carry electric
charge.
 Moving charge makes electric
current so the electrons
around the nucleus create
currents within an atom.
 These currents create the
magnetic fields that determine
the magnetic properties of
atoms.

23.1 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.

 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.

 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.

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)

B = 2x10-7 I
r

Radius (m)

23.1 Magnetic fields in a coil
 The magnetic field at the center of a coil comes from the
whole circumference of the coil.
Magnetic
field
(T)

B = 2π x10-7 NI
r

No. of turns of
wire
Current
(amps)
Radius
of coil (m)

23.1 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.

23.2 Electromagnets and the Electric
Motor

Key Question:

How does a motor work?


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.

Motor
 The right-hand rule:
W
hen your fingers curl
in the direction of
current, your thumb
points toward the
magnet’s north pole.

23.2 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.

23.2 The principle of the electric motor

23.2 Commutation
 The process of reversing the current in the electromagnet
is called commutation and the switch that makes it
happen is called a commutator.

23.2 Electric Motors


Electric motors are very common.



All types of electric motors have three key
components:
1. A rotating element (rotor) with magnets.
2. A stationary magnet that surrounds the rotor.
3. A commutator that switches the electromagnets
from north to south at the right place to keep the
rotor spinning.



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.

23.2 Electric motors
 The rotating part of the
motor, including the
electromagnets, is called
the armature.
 This diagram shows a small
battery-powered electric
motor and what it looks
like inside with one end of
the motor case removed.

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



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.

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


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.

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.

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.

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

23.3 Faraday's Law
 Faraday’s law says the
current in a coil is
proportional to the
rate at which the
magnetic field
passing through the
coil (the flux)
changes.

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

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.

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

Application: Trains that Float by
Magnetic Levitation

Physics chpt23

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