Magnetism and Electromagnetism

Question 1

What does magnetically hard mean? (2 marks)

Answer 1

• A material which is difficult to magnetise and difficult to demagnetise
• It retains its magnetism
• Steel is an example of a magnetically hard material

Question 2

What does magnetically soft mean? (2 marks)

Answer 2

• A material which is easy to magnetise and easy to demagnetise
• It loses its magnetism
• Iron is an example of a magnetically soft material

Question 3

Why does the voltmeter reading change from zero when a magnet is moved near the loop of wire? (2 marks)

Answer 3

• The wire cuts magnetic field lines
• This induces a voltage

Question 4

A wire is moved up and down repeatedly in a magnetic field . Explain the effect this movement has on an ammeter reading (3 marks)

Answer 4

• The ammeter displays a current reading
• This is because a voltage is induced when the wire cuts the magnetic field lines
• The induced voltage gives a current
• The ammeter reading will go negative when the wire is moved in the opposite direction

Question 5

Explain the effect on the ammeter reading of moving the wire with the same amplitude but higher frequency? (4 marks)

Answer 5

• The ammeter reading will be higher
• This is because the wire is moving more quickly
• Meaning the magnetic field lines are being cut more per unit time
• This gives a larger induced voltage

Question 6

Explain why a coil of wire heats up when current is passed through it? (3 marks)

Answer 6

• The coil of wire has a resistance
• Electrons lose energy as they flow through the coil
• Due to collisions with ions in the coil
• This causes the coil to heat up

Question 7

How does a transformer work? (5 marks)

Answer 7

• It steps up or steps down the voltage
• Current in the Primary coil produces a magnetic field
• The current direction is changing as it is AC
• The current changing causes a magnetic field in the core
• The core strengthens the magnetic field
• The field lines interact with the Secondary coil
• This induces a voltage in the secondary coil
• Transformers wont work with a DC current

Question 8

Describe the structure of a step-down transformer (3 marks)

Answer 8

• The transformer has primary and secondary coils
• The transformer has a soft iron core
• The primary coil has more turns than the secondary coil

Question 9

What will be the impact on a transformer if it becomes warmer? (2 marks)

Answer 9

• Power will be lost
• Efficiency will be reduced
• Resistance increases with temperature

Question 10

Why is electrical energy transmitted using very high voltages? (4 marks)

Answer 10

• Power = Current x Voltage
• If a high voltage is used, then current can be lower
• This is more efficient
• Less energy is lost from the wires as heat

Question 11

Explain how the transmission of electrical power is made more efficient by using step up and step-down transformers (5 marks)

Answer 11

• A step-up transformer increases the voltage and therefore reduces current
• This therefore reduces the current
• Current is linked with heating
• Less heating of transmission cables
• Therefore, less energy is lost / wasted in transmission of electricity
• Step down transformer reduces the voltage and increases the current

Question 12

What is an alternating current?

Answer 12

• An alternating current changes direction between positive and negative frequently / continuously

Question 13

What is a direct current?

Answer 13

• Direct current is a steady, constantly flowing current in one direction in a circuit only
• A battery is an example of a Direct Current