Magnetism and Electromagnetism

Question 1

What are the forces produced when magnets are brought together?

Answer 1

When two like poles come together they repel each other.

Two unlike poles attract each other.

Question 2

What is the difference between permanent and induced magnets?

Answer 2

Permanent magnet - produces its own magnetic field - they always have a magnetic field
Induced magnet - becomes a magnet when placed in a magnetic field - when the permanent magnet is taken away these magnets loose their magnetism.

Question 3

Describe a magnet

Answer 3

A magnet has two poles, a north pole and south pole, this is where the force of the magnets are the strongest.

Question 4

What is the force between magnets called?

Answer 4

The attraction and repulsion between poles are called non contact forces.

Question 5

What is the force induced magnets always cause?

Answer 5

Attraction

Question 6

What is a magnetic field?

Answer 6

This is region around a magnet that where a force acts on another magnet or magnetic material. The closer to the magnet the stronger the field - closer to the poles.

Question 7

How do you plot magnetic field lines?

Answer 7

You can use a compass or iron fillings.
Compass - first place one near the north pole of the magnet than draw a cross at the north point of the compass move the compass so the south pole of the compass is on the cross. We continue until we have plotted a complete magnetic line join the dots. We show the direction by an arrow. We repeat this starting at different points around the magnet.

Question 8

What are the four types of magnetic material?

Answer 8

Iron
Steel
Cobalt
Nickel

Question 9

What direction does the magnetic field line go?

Answer 9

From North to South

Question 10

How can we tell the Earth has a magnetic field?

Answer 10

When a compass is away from a magnet the compass will always point in the North South direction. This is due to the Earths core.

Question 11

How does current flowing through a wire generate a magnetic field?

Answer 11

When a current flows through a conducting wire, a magnetic field is produced around the wire. We can prove this by a compass. The strength of the magnet depends on the amount of current flowing through the wire. The magnetic field is stronger closer to the wire. If the current direction is changed so is the magnetic field.

Question 12

Explain how a solenoid an increase the magnetic effect of a current

Answer 12

A solenoid increases the strength of the magnetic field by -
Increase the current
Increase the coils
Place a piece of iron inside the iron - iron core, this is called an electromagnet - we can turn electromagnets on or off and vary its strength.

Question 13

What is the magnetic field pattern for a straight wire carrying a current and for a solenoid?

Answer 13

When the current is on we get a strong uniformed magnetic field.

Question 14

Where are electromagnets used?

Answer 14

A relay - used in high voltage circuits.

A doorbell

Question 15

How do relays work?

Answer 15

It consists of two separate circuits a low voltage circuit with an electromagnet and a high voltage circuit with two metal contacts - one of the contacts is connected to a spring. There is an iron block attached to this contact. When the low voltage circuit is on the electromagnet produces a magnetic field this now attracts the iron block this cause the contacts to close. When the low voltage circuit is switched off the two contacts spring back and the circuit is broken.

Question 16

How do doorbells work?

Answer 16

The switch of the circuit is closed when the buzzer is pressed, this causes a current to flow a magnetic field is then produced by the electromagnetic the iron contact is now attracted to the magnetic field. When the contact moves the clapper now hits the bell this then breaks the circuit so there is now magnetic field and the contact springs back and the circuit is closed again, this is repeating it giving a ringing sound.

Question 17

What is the motor effect?

Answer 17

This is where a wire with a current is placed between two magnets this wire will move upwards due to a force called the motor effect. When a conductor such as a wire is carrying a current, it experiences a force in a magnetic field.

Question 18

How do you calculate the size of the force generated by the motor effect?

Answer 18

F= magnetic flux densitycurrentlength
F(N)=B(T)I(A)l(m)

This equation applies to a wire that is at right angles to the magnetic field.

Question 19

What is the left hand rule?

Answer 19

This is used to determine the direction of force

Question 20

What is the useful application of the motor?

Answer 20

Electric motors

Question 21

What happens when a loop of wire is placed in a magnetic field?

Answer 21

Each side of the loop which has an opposite direction of current experience force using flemmings left hand rule we can see the forces are different on either side one is moving upwards and the other downwards. This means the loop will rotate until 90 degrees.

Question 22

How do electric motors work?

Answer 22

Using a loop of wire the current in an electric field experience different forces, the loop will rotate until 90 degrees then the current is switched every 90 degrees by a split ring commutator. Meaning the loop will continue to loop and not stay at 90 degrees.

Question 23

How does a moving coil loudspeaker work?

Answer 23

We find these on stereos - There is a cone with a coil of wire wrapped around it supplying AC current and a permanent magnet inside the coil of wire. As the current passes through the coil, it generates a magnetic field. The magnetic field interacts with the permanent magnet this either repels or attracts this produces a resultant force this causes the cone to move when the current is switched the force is also switched this movement generates sound waves. We can also change the frequency of the AC supply this allows us to replicate high pitched or low pitched sound. If we increase the size of the current we increase the amplitude this is the volume of sound.

Question 24

What is the generator effect?

Answer 24

If we have a complete circuit with induced potential difference we get induced current, the direction of the current switches when the direction of movement switches. When the movement stops so does the current.

Question 25

What is induced potential?

Answer 25

This is where a wire cuts through the magnetic field when the wire stops moving the potential difference.

Question 26

What are the factors that affect the size and direction of the induced potential?

Answer 26

The induced potential difference and current is larger when:
If we use a stronger magnetic field
If we move the wire more rapidly
If we shape the wire into a coil the more coils the greater the induced potential difference

Question 27

How can we switch the direction of induced current?

Answer 27

Move the magnet

Switch the poles of the magnet

Question 28

How does an alternator generate alternating current?

Answer 28

We use the generator effect and use commutators to switch the charge of the current when the wire is vertical it sweeps directly through the magnetic field lines. When the wire is vertical the potential difference is zero because the coil is moving parallel to the field so it is not cutting through the magnetic lines. When the wire keeps moving around we get potential difference when the wire cuts through the field lines but the current is constantly switched making an AC current.

Question 29

How does a dynamo generate a direct current?

Answer 29

The key feature is the split ring commutator this has two sides separated by a gap. When the coil is vertical the split ring does not switch the current this is DC current.

Question 30

How does a moving-coil microphone work?

Answer 30

The set up is similar to a speaker however we have a coil of wire attached to a thin sheet of plastic this is called a diaphragm the end of the coil sits over a permanent magnet. The sound waves hit the diaphragm, they cause it to vibrate. The coil of wire moves in and out through the magnetic field this induces a potential difference, this switches direction. The frequency of the sound waves is the same as the induced. The changing pattern of potential difference is now passed through an amplifier and then into a moving coil loudspeaker.

Question 31

How are transformers used to change the potential difference of an electricity supply?

Answer 31

The primary coil produces a changing magnetic field due to its AC current. This is then carried by an iron core, the magnetic field passes through the secondary coil, this induces a potential difference.

Question 32

What is a step up transformer?

Answer 32

This is where the primary coil has less coils than the secondary coil has more coils. The potential difference induced in the secondary coil will be greater than the potential difference in the primary coil.

Question 33

What is a step down transformer?

Answer 33

This is where the primary coil has more coils than the secondary coil the potential difference is stepped down. The amount of difference in coils relates to how the potential difference changes.

Question 34

What is necessary for transformers to function?

Answer 34

There must be an AC current supply. That is because we need a changing magnetic field.
An iron core to increase the strength of the magnetic field
A direct current produces a constant magnetic field and therefore it does not work in transformers.

Question 35

How are transformers used in power transmission?

Answer 35

They are used in the national grid
Step up - if we are transmitting a large amount of power we need either a large current or voltage. Power is wasted as heat in transmission cables due to the amount of current, step up transformers increase the potential difference and lower the current sop that the power can be transferred with less wasted energy.
Step down - this is where the potential difference is lowered to a safe level for the homes from 400,000V to 230V.

Question 36

How do you calculate the potential difference in transformers?

Answer 36

Vp/Vs=Vp/Ns

Power = Power of
of primary coil secondary
coil

VpIp=VsIs (only if the transformer is 100% efficient)