Electromagnetism

Question 1

When a current is supplied to the coil, a magnetic field forms and the metal rods move apart…why?

Answer 1

The rods are both magnetised they repel against each other.

Question 2

When the current in coil is replaced by alternating current what happens to the magnetic field? ( part of the rods question)

Answer 2

Magnetic field changes directions therefore the rods may not have time to be fully magnetised

Question 3

Explain why the coil starts to spin when the switch is closed?

Answer 3

The current-carrying coil creates a magnetic field around the coil and the magnetic field of the coil, as well as the magnet, interact hence, producing an upwards force on one side of the coil and a downwards force on the other side of the coil

Question 4

Describe the construction of an electromagnet

Answer 4

Electromagnets consist of a current carrying wire wrapped into a coil and on an iron core connected to a DC SUPPLY!!

Question 5

Why is the iron core made of a soft magnetic material

Answer 5

Iron demagnetises easily and the magnetic field changes

Question 6

Why is the voltage induced when the coil is turned?

Answer 6

The coil cuts through magnetic field lines

Question 7

Why is it harder to turn the coil when the lamp is connected

Answer 7

Energy is transferred by the lamp hence there is more work being done

Question 8

What is the purpose of the brushes and the commutator in the d.c. motor?

Answer 8

The brushes are always in contact with the current hence, they provide a connection and the commutator allows the current to be reversed every half turn to ensure force on the motor is always in the same direction so that the motor is turning

Question 9

How to find the shape of the magnetic field?

Answer 9

Sprinkle iron fillings on top of card, place magnet underneath and tap gently

Question 10

How do generators work

Answer 10

• Generators work by rotating a current-carrying coil in a magnetic field
• they have slip rings instead of commutators
• the slip rings are always connected to each end of the coil

Question 11

How do step-up transformers increase the voltage

Answer 11

There are more coils experiencing the change, so a larger voltage is induced.

Question 12

Describe the motor effect

Answer 12

The current-carrying wire has a magnetic field around it which interacts with the magnetic field of the magnet therefore exerting a force on the wire

Question 13

Explain how a current is induced in core of the transformer

Answer 13

• Magnetic field lines are cut by the core
• a voltage is induced
• conductor has free moving electrons that don’t experience a force

Question 14

Why are step-up and step-down transformers used in the National Grid

Answer 14

Step-up transformers decrease voltage and increase current hence, preventing cables from overheating and preventing energy lost as heat, therefore, making it safer as less voltage will be required by the homes and more efficient as the current is high

Question 15

Describe how a loudspeaker produces soundwaves

Answer 15

• The magnetic field around the current-carrying coil interacts with the magnetic field around the magnet to produce a force on the coil
• When alternating current switches direction, the magnetic field also switches direction frequently
• makes the cone of the speaker oscillate

Question 16

Why can a voltage be induced

Answer 16

The magnetic field around the coil is changing

Question 17

How can we increase the strength of a motor

Answer 17

• Increase the current
• Increase the strength of the magnet
• Increase the number of turns on the coil
• use an iron core

Question 18

How can we increase induced voltage

Answer 18

Increase speed of moving magnets
Increase number of turns in a coil
Increase strength of the magnet
Increase magnetic field strength

Question 19

Explain how there can be current from a wireless charging base

Answer 19

• There is an alternating field
• Alternating field is cut by coils
• A voltage is induced
• Therefore, there is a current

Question 20

What is left hand fleming rule

Answer 20

• thumb = force
• first finger = field direction
• second finger = current

Question 21

How to increase output current of the transformer?

Answer 21

• Increase number of turns on the PRIMARY COIL
• Use a larger current in the PRIMARY COIL

Question 22

How can we make the motor spin in the opposite direction

Answer 22

• Reverse the polarity of the magnetic field

Question 23

How can an ammeter be changed so that it could measure smaller currents

Answer 23

• Use a stronger magnet
• Use an iron core
• Increase number of turns on coil

Question 24

How to increase OUTPUT CURRENT of the transformer

Answer 24

• Decrease number of turns on secondary coil
• Increase number of turns on primary coil
• Increase the INPUT CURRENT

Question 25

what does it mean if iron demagnetises quickly?

Answer 25

• It’s magnetic field changes directions frequently

Question 26

When shown a uniform magnetic field on a diagram, how does the diagram show that the magnetic field is uniform?

Answer 26

• Lines are parallel
• Lines are evenly spaced

Question 27

Explain how does a transformer work

Answer 27

• A transformer steps up or down the voltage
• The alternating current in the coil produces a magnetic field
• The alternating current changes direction frequently
• Hence, causing the magnetic field in the iron core to change
• Both fields interact with the coil which induces a voltage in the secondary coil
• Note that transformers don’t work with a d.c. supply

Question 28

How to make coil spin in the opposite direction

Answer 28

• Reverse polarity of the supply
• Reverse the direction of the current

Question 29

How to use plotting compasses to show direction around the bar magnet

Answer 29

• Place the plotting compass around the magnet and mark its direction
• Repeat the process by placing the compass at different positions and mark the directions again
• Results show directions are linked together hence, the field line can be easily found