P7 : Magnetism and Electromagnetism

What type of force is magnetism?

A non-contact force

What are the 2 ends of a bar magnet called + what are their individual names?

• Poles

  • North pole

  • South pole

What do two like poles do?

They repel each other

What do two unlike poles do?

They attract each other

What are the 2 types of magnets?

• Permanent magnets

• Induced magnets

What is the difference between a permanent magnet and an induced magnet?

A permanent magnet produces its own magnetic field whereas an induced magnet is a material that becomes a magnet when it is placed in a magnetic field

What does an induced magnetism cause?

A force of attraction

What happens when an induced magnet is removed from a magnetic field?

The induced magnet loses most/all of its magnetism quickly and stop producing a magnetic field

What are the 4 magnetic materials?

• Iron

• Steel

• Cobalt

• Nickel

What does the strength of the magnetic material depend on?

The distance from the magnet

Where is the strength of the magnetic field the strongest?

At the poles of the magnet

Where do the field lines in a bar magnet go from and to?

The field line go from the north pole to the south pole

What is the relationship between the strength of the magnet and the placement of the field lines?

The closer together the field lines are, the stronger the magnet is

Draw the field lines

What is the compass needle attracted to?

The south pole of any magnet

How do we know that the earth has a magnetic field?

Because the compass needle always points in the direction of the earths magnetic field which is north → south which shows the earth’s core must be magnetic

When is a magnetic field created around a wire?

When there is a current flowing

What shape are the field lines of a wire?

Concentric circles

What does the direction of the magnetic field of a wire depend on + what rule can be used to determine this?

• The direction the current is flowing

• The right-hand thumb rule

Explain the right-hand thumb rule

In a wire what does the strength of the magnetic field depend on? (2)

• The current through the wire

• The distance from the wire

How do you increase the strength of the magnetic field of a wire? (3)

• The larger the current through the wire

• The closer the distance of the wire

• Wrapping a wire in a solenoid

What is a solenoid?

A wire wrapped in a coil

What is the magnetic field inside a solenoid like?

Strong and uniform

Why does a solenoid increase the strength of the magnetic field?

Because the field lines around each loop of wire interact with each other and increase the overall magnetic field strength

How do you increase the strength of the magnetic field of an electromagnet? (3)

• Increase the current that flows through the solenoid

• Increase the number of coils of the solenoid

• Add an iron core in the centre of the solenoid

What happens to the iron core of a solenoid when a current is flowing?

It becomes an induced magnet

What is an electromagnet?

A solenoid with an iron core

What happens if you turn off the current of a wire?

The magnetic field disappears

What are the uses of electromagnets? (2)

• Used in cranes to attract and pick up magnetic materials

• Used in other circuits to act as switches

Explain the motor effect

When a conductor carrying a current is placed in a magnetic field the magnet producing the field and the conductor exert a force on each other

What rule can be used to find the direction of the force?

Fleming’s left-hand rule

Describe + explain Fleming’s left-hand rule

To experience the full force what angle should the wire be at to the magnetic field?

90°

What 3 things effects the force acting on a wire in a magnetic field?

• Magnetic flux density (strength of the magnetic wire)

• Size of current

• Length of wire

When the current is at 90° to the magnetic field it is in, what is the equation to find the force?

If the conductor is parallel to the magnetic field it is in, what will happen?

It will not experience a force

How does an electric motor work?

A coil of wire carrying a current in a magnetic field tends to rotate

What does a split-ring communicator do + what do this lead to? (3)

• It swaps the contacts every half turn to keep the motor rotating in the same direction

• Which leads to the direction of the current swapping every half turn

• Which means the coil will continue to rotate in the same direction

What are the 2 ways the direction of the motor can be reversed?

• By swapping the polarity of the dc supply (reversing the current)

• By swapping the magnetic poles over (reversing the field)

Explain how the force on a conductor in a magnetic field causes the rotation of the coil in an electric motor (4)

1. In a conductor the force on the left hand side acts upwards and the force on the right hand side acts downwards

2. This will cause the loop to rotate in the clockwise direction

3. However the loop will stop rotating at 90°

4. But by using a split ring commutator, the motor will be able to keep on rotating in the same direction

How do loudspeakers and headphones work? (short version)

By using the motor effect to convert variations in current in electrical circuits to the pressure variations in sound waves

What effect is used in loudspeakers and headphones?

Motor effect

Explain how loudspeakers and headphones work (4)

• An alternating current (ac) is sent through a coil of wire attached to the base of a paper cone

• The coil surrounds one pole of a permanent and is surrounded by the other pole, so the current causes a force on the coil which causes the cone to move

• When the current reverses, the force acts in the opposite direction, which, causes the cone to move in the opposite direction too

• So variations in the current make the cone vibrate which makes the air around the cone vibrate and creates the variations in pressure that causes a sound wave

How can you alter the sound wave produced + why?

• By controlling the frequency of the ac

• This is because the frequency of the sound wave is the same as the frequency of the ac

Explain the generator effect in terms of induced potential difference (5)

• If we move a wire up through the magnetic field, then a potential difference is induced across the ends of the wire

• If the wire stops then the potential difference is lost

• If we move the wire back down through the magnetic field then we get the potential difference again

• However the potential difference has now reversed direction

• This process is called induced potential difference and the generator effect

Explain the generator effect in terms of induced current (4)

• If we move a circuit up through the magnetic field, then a current is induced across the circuit

• If the circuit stops then the current is lost

• If we move the circuit back down through the magnetic field then we get current again

• This process is called induced current and the generator effect

What does an induced current do?

Generates a magnetic field which opposes the original change, either the movement of the conductor or the change in magnetic field

What are the 3 ways you can change the size of induced potential?

• Increasing the speed of the movement

• Increasing the strength of the magnetic field

• Shaping the wire into a coil

What is the generator effect used in + what does it generate? (2)

• Used in an alternator to generate ac

• Used in a dynamo to generate dc

What do alternators generate?

Ac

What do alternators have that dynamos don’t have and vice versa?

• Alternators have split rings and brushes

• Dynamos have split-ring commutators

What is the difference between generators and dynamos in terms of the currents direction?

• Current in generators change direction every half turn

• Current in generators flow in the same direction every half turn

What do dynamos generate?

Dc

What does ac produce in terms of magnetic fields + what does dc produce in terms of magnetic fields?

• Ac produces a changing magnetic field

• Dc produces a constant magnetic field

How do microphones work? (short version)

By using the motor effect to convert variations in current in electrical circuits to the pressure variations in sound waves

What effect is used in microphones?

The generator effect

Explain how microphones work (4)

1. Sound waves hit a flexible diaphragm that is attached to a coil of wire wrapped around a magnet

2. This causes the coil of wire to move in the magnetic field which generates a current

3. The movement of the coil and the generated current depends on the properties of the sound wave

4. Louder sounds make the diaphragm mover further

What type of current is used for transformers?

Ac

What is the job of transformers?

To change the size of the potential difference of an alternating current

What does a transformer consist of?

A primary coil and a secondary coil wound on an iron core

Why is iron used in a transformer?

Because it is easily magnetised

What happens when a potential difference is applied across the primary coil? (3)

• The iron core magnetises and demagnetises quickly

• This changing magnetic field induces an alternating potential difference in the secondary coil

• If the secondary coil is part of a complete circuit, a current will be induced

What does a step up transformer do + which coil has more turns?

• Increases potential difference

• Secondary coil has more turns

What does a step down transformer do + which coil has more turns?

• Decreases potential difference

• Primary coil has more turns

What does the ratio of the potential differences across the primary and secondary coils of a transformer depend on + what is the equation for this?

It depends on the ratio of the number of turns on each coil

What is the equation to calculate power or current in coils?