1. Unit 7, Chapter23
CPO Science
Foundations of Physics

2. Unit 7: Electricity and Magnetism
 23.1 Properties of Magnets
 23.2 Magnetic Properties of Materials
 23.3 The Magnetic Field of the Earth
Chapter23 Electricity and Magnetism

3. Chapter23 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.

4. Chapter23 Vocabulary Terms
 gauss
 right-hand rule
 coil
 solenoid
 magnetic field
 tesla
 Faraday’s law
 induction
 induced current
 magnetic flux
 commutator
 generator
 electromagnet
 polarity

5. 23.1 Electric Current and Magnetism
Key Question:
Can electric current create a magnet?
*Students read Section 23.1 AFTER Investigation 23.1

6. 23.1 Electric Current and Magnetism
 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.

8. 23.1 Electric Current and Magnetism
 Two wires carrying electric current exert force on each
other, just like two magnets.
 The forces can be attractive orrepulsive depending on
the direction of current in both wires.

10. 23.1 Electric Current and Magnetism
 The magnetic field around a single wire is too small to
be of much use.
 There are two techniques to make strong magnetic
fields fromcurrent 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 theircenter.

12. 23.1 Electric Current and Magnetism
 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.

14. 23.1 The true nature of magnetism
 The magnetic field of a coil is identical to the field of
a disk-shaped permanent magnet.

15. 23.1 Electric Current and Magnetism
 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.

16. 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
perpendicularto both the magnetic field and to the
direction of motion of the charge.

17. 23.1 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 meterpersecond.
 This relationship is how the unit of magnetic field is
defined.

18. 23.1 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.

19. 23.1 Magnetic field neara wire
 The field of a straight wire is proportional to the current in
the wire and inversely proportional to the radius from the
wire.
Magnetic field
(T)
Radius (m)
Current (amps)
B = 2x10-7
I
r

20. 23.1 Magnetic fields in a coil
 The magnetic field at the centerof a coil comes fromthe
whole circumference of the coil.
Magnetic
field
(T)
Radius
of coil (m)
Current
(amps)
No. of turns of
wire
B = 2π x10-7
NI
r

21. 23.1 Calculate magnetic field
 A current of 2 amps flows in
a coil made from400 turns
of very thin wire.
 The radius of the coil is 1
cm.
 Calculate the strength of
magnetic field (in tesla) at
the centerof the coil.

22. 23.2 Electromagnets and the Electric
Motor
Key Question:
How does a motor work?
*Students read Section 23.2 AFTER Investigation 23.2

23. 23.2 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
orsteel.
 Because iron is magnetic, it
concentrates and amplifies the
magnetic field created by the
current in the coil.

24. 23.2 Electromagnets and the Electric
Motor
 The right-hand rule:
 When yourfingers curl
in the direction of
current, yourthumb
points toward the
magnet’s north pole.

25. 23.2 The principle of the electric motor
 An electric motoruses electromagnets to convert
electrical energy into mechanical energy.
 The diskis called the rotorbecause it can rotate.
 The diskwill keep spinning as long as the external
magnet is reversed every time the next magnet in the
diskpasses by.
 One ormore stationary magnets reverse theirpoles to
push and pull on a rotating assembly of magnets.

26. 23.2 The principle of the electric motor

27. 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.

28. 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 commutatorthat switches the electromagnets
fromnorth to south at the right place to keep the
rotorspinning.

29. 23.2 Electric Motors
 If you take apart an electric motorthat runs on
batteries, the same three mechanisms are there; the
difference is in the arrangement of the
electromagnets and permanent magnets.

30. 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
motorand what it looks
like inside with one end of
the motorcase removed.

31. 23.2 Electric motors
 The permanent magnets
are on the outside, and
they stay fixed in place.
 The wires fromeach of the
three coils are attached to
three metal plates
(commutator) at the end of
the armature.
commutator

32. 23.2 Electric Motors
 As the rotorspins, the three plates come into contact
with the positive and negative brushes.
 Electric current flows through the brushes into the
coils.

33. 23.3 Induction and the Electric Generator
Key Question:
How does a generator
produce electricity?
*Students read Section 23.3 AFTER Investigation 23.3

34. 23.3 Induction and the Electric Generator
 If you move a magnet neara 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 magnetismand
conversely, changing magnetic fields also can
cause electric charge to move.

35. 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.

36. 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.

37. 23.3 Magnetic flux
 A moving magnet
induces current in
a coil only if the
magnetic field of
the magnet passes
through the coil.

38. 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.

39. 23.3 Faraday's Law

40. 23.3 Generators
 A generator is a device that uses induction to
convert mechanical energy into electrical energy.

41. 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.

42. 23.3 Transformers
 A relationship between voltages and turns fora transformer
results because the two coils have a different numberof
turns.

43. Application: Trains that Float by
Magnetic Levitation