Download Electromagnetism

Electromagnetism 1
SPH4U – Grade 12 Physics
Unit 1
Magnetism Review

Every magnet has a north and south pole
N
S
S
 Like Poles Repel
N
N
S
N
S
 Opposite Poles Attract
Magnetism Review

The magnetic field around a magnet is strongest
at the poles.

The north pole and the south pole of the same
bar magnet are in general, equally strong.

If you cut a magnet in two pieces, each piece
will have a north and south pole. You can keep
cutting to make smaller and smaller magnets –
but each one will be weaker in strength.
Magnetism Review

A magnetic field is the space around a magnet
where a magnetic force can be felt. It is very
similar in theory to an electric field. The symbol
for magnetic field is 
B

Lines of magnetic force show how the magnetic
force acts around the magnet. The force is
strongest at the poles (where the lines are close
together) and weaker the further out you go.
Magnetism Review

When drawing Magnetic Field lines,
remember:
 Lines
go from North to South outside the
magnet
 Lines go from South to North inside the
magnet
 Lines never cross
Magnetism Review
Magnetism Review

The direction of a
line of force is
defined as the
direction in which
the north pole of a
compass points
when placed along
that line. Therefore,
lines of magnetic
force point to the
South pole.
Magnetism Review

A horseshoe magnet
Predicting Magnetic Forces

Parallel fields from two different magnets show
us that there is a repulsion.
Predicting Magnetic Forces

Opposite fields from two different magnets
show us that there is an attraction.
The Earth
Electricity & Magnetism

Electricity and Magnetism are very closely
related, since both are based on the properties
of electrically charged particles.

The Electromagnetic force is one of the four
fundamental forces in nature (including the
strong nuclear, the weak nuclear, and gravity).
Principal of Electromagnetism

Moving electric charges will produce a magnetic
field. (this was discovered by Oersted).

This means that when an
electric current moves through
a wire, a magnetic field
is produced.
Magnetic Fields and Current


The magnetic field exists in circular rings
around a straight conductor.
The direction of the magnetic field depends on
the direction of the current.
Magnetic Fields and Current

The Right-hand Rule:
when holding a straight
conductor with your right
hand and thumb pointing
in the direction of
conventional current,
your curled fingers will
point in the direction of
the magnetic field lines.
Example 1

Draw the magnetic field lines around each
wire.
Current going into the page
Current going out of the page
Example 1

Draw the magnetic field lines around each
wire.
Magnetic Field around a Loop

If you make a circular loop from a straight wire
and run an electric current through the wire, the
magnetic field will circle around each segment
of the loop.
Magnetic Field around a Loop


You can still use the right hand rule to find the
direction of the magnetic field for a single loop.
The field will be stronger inside the loop than on
the outside.
Magnetic Field around a Solenoid

A solenoid is a conducting wire that is wound
up into many loops forming a coil.
Magnetic Field around a Solenoid

If you run an electric current through a solenoid,
the magnetic field is the sum of all the magnetic
fields of each loop.

The field will be strongest inside the loop
because the field lines are closer together.
Magnetic Field around a Solenoid

The more tightly you wind the coil, the stronger
the magnetic field will be.
Magnetic Field around a Solenoid

When we run a current through a solenoid like
this, the magnetic field that is created looks just
like a bar magnet. So it is as if a temporary bar
magnet is created. One end is North and one is
South.
Magnetic Field around a Solenoid

To determine the direction of the magnetic field
around a solenoid, use the right-hand rule for a
solenoid:
Right-hand rule for a
solenoid: If you curl your
fingers in the direction of the
conventional current, your
thumb will point in the
direction of the magnetic
field lines in the core. This
means your thumb points
towards “North”.
The Motor Principal



Since a current carrying wire has a magnetic
field around it, an external magnetic field
around the wire can cause the wire to move.
This is because of the attraction or repulsion of
the two magnetic fields (the one around the wire
and the external one).
http://www.youtube.com/watch?v=tUCtCYty-ns
The Motor Principal

This property is called the Motor Principal:
A current-carrying conductor that cuts across
external magnetic field lines experiences a
force that is perpendicular to both the
magnetic field and the direction of the
electric current.
The Motor Principal
The force is caused
because the magnetic
field lines on the magnet,
and on the wire are going
in the same direction.
Therefore there is a
repulsion.
The Motor Principal

Right-hand Rule for the
Motor Principal:
If the fingers of your open
right hand point in the
direction of the external
magnetic field, and your
thumb points in the
direction of the
conventional current,
then your palm faces in
the direction of the force
on the conductor.
Example 2

Determine the direction of the force on the
wire.
X
Example 2

Determine the direction of the force on the
wire.
X
Solution: Wire will go
into the page by the
right hand rule.
Electromagnetic Induction


Because of the relationship between electricity
and magnetism, when a magnetic field changes
near a conductor, an electric current can be
produced.
This is called Electromagnetic Induction. The
Law of Electromagnetic induction states:
An electric current is induced in a conductor
whenever the magnetic field in the region of the
conductor changes with time.
Electromagnetic Induction

This is how electricity is produced in a
power-plant:
 When
water (or steam) pushes the turbine, it
will rotate a magnet.
 When the magnet is rotated, the magnetic
field changes.
 When the magnetic field changes, a current is
produced.
Video
Overview of the electricity:

http://www.youtube.com/watch?v=XiHVe8
U5PhU
Homework

Read Sections 8.1, 8.2, 8.3,
 Make
additional notes to supplement the
lesson notes. (For now, you do not need to
know the formulas for these sections)

Complete the following questions:
 Pg.
385 #2, 3, 4,
 Pg. 391 # 2