Magnetism - Paper 1 - Section 6

What is a magnetic field?

What is a magnetic field?

A region where magnetic materials (e.g. iron) experience a force.

What is magnetic flux density (strength of a magnetic field) measured in?

Teslas (T)

What are examples of magnetic materials?

Alloys of iron, nickel and cobalt. E.g. steel

Define magnetically hard

Materials that stay magnetic (eg. steel) They produce a magnet field which cannot be turned off.

Define magnetically soft

Materials that only become magnetic when placed in a magnetic field. (e.g. soft)

What happens when you bring a magnet near a piece of iron?

The iron-atom magnets line up with the applied magnetic field. The north poles of the iron atoms all point in the same direction. This cause the piece of iron to become magnetic, and it is attracted to the original magnet.

How do you plot the magnetic field lines of a bar magnet?

• Lines of force run from the North pole to South pole of the magnet

• The lines of force cannot overlap

• The lines of force get further apart, the further they are away from the magnet, due to the weakening of the magnetic field at this point.

What happens when a current flows through a wire (or any conductor)?

A magnetic field is created around the wire.

• The direction of the magnetic field depends on the direction of the current.

• The larger the electric current, the stronger the magnetic field.

How can you increase the strength of a magnetic field around a solenoid?

By adding a magnetically “soft” iron core through the middle of the coil. E.g. iron

What is the Motor Effect?

The interaction of two magnetic fields.

• When a current-carrying wire is placed between the poles of two magnets, the magnetic field around the wire interacts with the magnetic field between the magnets.

• This interaction causes the wire and magnet to exert a non-contact force on each other -- causing the wire to move.

• To get the maximum full force effect, the wire should be a 90 degrees to the direction of the magnetic field flux. If the wire is parallel, the magnet fields won’t interact and there will be no force experienced.

What should you use to tell you which way the force acts?

Fleming’s Left Hand rule.

• First finger = (magnetic) Field

• SeCond finger = current

• thuMb - Motion of the force

Explain how a loudspeaker works.

• A.C. electrical signals from an amplifier are def to a coil of wire in the speaker, which is wrapped around the base of a cone.

• The coil is surrounded by a permanent magnet, so the A.C. signals cause a force on the coil and make it move back and forth.

• These movements make the cone vibrate -- which produces sound waves.

Explain how a Simple D.C. Electric Motor works.

Due to the Motor Effect, the two arms of the spindle are forced one up, one down, causing a rotating effect. they are attached to a coil which then rotates.

• The split-ring communicator swaps the contacts every half turn, swapping the +/- polarity, which swaps the direction of the resulting force -- keeping the rotation in the same direction.

• Brush contacts - enable rotational movement to continue.

• The direction of the motor can be reversed by either swapping the polarity of the D.C. supply, or swapping the magnetic poles over.

• The speed can be increased by adding more turns to the coil, increasing the current, increasing the strength of the magnetic field, or by adding a soft iron core.

What is Electromagnetic Induction?

The creation of a voltage (and maybe a current) in a wire which is experiencing a change in magnetic field.

Explain the Dynamo Effect

Uses electromagnetic induction to generate electricity using energy from KE stores. Magnetic field + movement = induces a current.

Describe the two different situations where you get EM induction.

1. When an electrical conductor (a coil of wire) is moved through a magnetic field.

2. The magnetic field through an electrical conductor changes (gets bigger, or smaller, or reverses).

How can you test the Dynamo effect with an ammeter?

Connect an ammeter to a wire and move the wire through a magnetic field (between two magnets).

The ammeter will show the magnitude and direction of the induced current.

To increase the voltage in electromagnetic induction, you can…

• Increase the strength of the magnet

• Increase the number of turns on the coil

• Increase the speed of movement

Explain how A.C. Generators work.

• Generators rotate a coil in a magnetic field.

• As the coil spins, a current in induced in the coil. This current changes every half turn.

• In stead of a split-ring communicator, there are slip rings and brushes so the contacts don’t swap every half turn.

• This means they produce A.C voltage. Faster revolutions (rotations) produce more peaks + a higher overall voltage too.