Electromagnetism 1

SPH4U – Grade 12 Physics

Unit 1

Magnetism Review

Every magnet has a north and south pole

Like Poles Repel Opposite Poles Attract

S NN S N S N S

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

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.

B

Magnetism Review

When drawing Magnetic Field lines, remember:Lines go from North to South outside the

magnetLines go from South to North inside the

magnetLines 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 magnet 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

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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