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
2. Explain the relationship between electric current and
3. Describe and construct a simple electromagnet.
4. Explain the concept of commutation as it relates to an
5. Explain how the concept of magnetic flux applies to
generating electric current using Faraday’s law of
6. Describe three ways to increase the current from an
4. Chapter23 Vocabulary Terms
5. 23.1 Electric Current and Magnetism
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
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
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
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
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
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
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
B = 2x10-7
20. 23.1 Magnetic fields in a coil
The magnetic field at the centerof a coil comes fromthe
whole circumference of the coil.
of coil (m)
No. of turns of
B = 2π x10-7
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
Calculate the strength of
magnetic field (in tesla) at
the centerof the coil.
22. 23.2 Electromagnets and the Electric
How does a motor work?
*Students read Section 23.2 AFTER Investigation 23.2
23. 23.2 Electromagnets and the Electric
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
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
The right-hand rule:
When yourfingers curl
in the direction of
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
One ormore stationary magnets reverse theirpoles to
push and pull on a rotating assembly of magnets.
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
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
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
This diagram shows a small
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
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
33. 23.3 Induction and the Electric Generator
How does a generator
*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
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
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
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
passing through the
coil (the flux)
40. 23.3 Generators
A generator is a device that uses induction to
convert mechanical energy into electrical energy.
41. 23.3 Transformers
because they efficiently
change voltage and
current, while providing
the same total power.
The transformer uses
induction, similar to a
42. 23.3 Transformers
A relationship between voltages and turns fora transformer
results because the two coils have a different numberof
An apparatus can be built that shows the magnetic field around a straight wire.
The compass needles all form a circle when the current is switched on in the wire.
The direction of the force can be deduced from the right-hand rule.
If you bend the fingers of your right hand as shown, your thumb, index, and middle
finger indicate the directions of the force, current and magnetic field.
When wires are bundled, the total magnetic field is the sum of the fields created by the current in each individual wire.
By wrapping the same wire around into a coil, current can be “reused” as many times as there are turns in the coil
Coils are used in electromagnets, speakers, electric motors, electric guitars, and almost every kind of electric appliance that has moving parts.
1) You are asked for the magnetic field in tesla.
2) You are given the current, radius, and number of turns.
3) Use the formula for the field of a coil: B = 2π x 10-7 NI ÷ R
B =(2π x 10-7)(400)(2A) ÷(.01m)= 0.05 T
To keep the disk spinning, the external magnet must be reversed as soon as magnet (B) passes by.
Once the magnet has been reversed, magnet (B) will now be repelled and magnet (C) will be attracted.
As a result of the push-pull, the disk continues to rotate counterclockwise.
As the motor turns, the plates rotate past the brushes, switching the electromagnets from north to south by reversing the positive and negative connections to the coils.
The turning electromagnets are attracted and repelled by the permanent magnets and the motor turn
Consider a coil of wire rotating between two magnets
When the coil is in position (A), the magnetic flux points from left to right.
As thecoil rotates (B), the number of field lines that go through the coil decreases.
As a result, the flux starts to decrease and current flows in a negative direction.
At position (C), the largest negative current flows because the rate of change in
flux is greatest.
The graph of flux versus time has the steepest slope at position (C), and that is why the current is largest.
At position (C), no magnetic field lines are passing through the coil at all and therefore the flux through it is zero.
As the coil continues to rotate (D), flux is still decreasing by getting more negative.
Current flows in the same direction, but decreases proportionally to the decreasing rate of change (the slope of flux versus time levels out).
At position (E), the flux through the coil reaches its most negative value.
The slope of the flux versus time graph is zero and the current is zero.
As the coil rotates through (F), the flux starts increasing and current flows in the opposite direction.
Because the magnet near the coil alternates from north to south as the disk spins,
the direction of the current reverses every time a magnet passes the coil.
This creates an alternating current.