Electromagnetism

Question 1

How can you plot the magnetic field around a bar magnet?

Answer 1

• Place the bar magnet on a piece of paper.
• Place the compasses around the bar magnet and mark the direction of the needle on the paper.
• Remove the compasses and join up the points to show the magnetic field lines, adding arrows to show the direction.

Question 2

The geographic south pole of the Earth is the magnetic ____ pole.

Answer 2

north

Question 3

What’s a solenoid?

Answer 3

A coil of wire that acts like a magnet when a current is passed through it.

Question 4

What’s an electromagnet?

Answer 4

A solenoid with an
iron core.

Question 5

A magnet can be p__ or i__.

Answer 5

permanent
induced

Question 6

What three things can you do to increase the magnetic field around a current-carrying wire?

Answer 6

• Increase the current.
• Move closer to the wire.
• Coil the wire into a solenoid.

Question 7

Give two advantages of an electromagnet over a solenoid.

Answer 7

• It can be turned on and off.
• The strength of the electromagnet can be varied by changing the current.

Question 8

Explain how a loudspeaker works. [4 marks]

Answer 8

• One iron core pole is surrounded by a coil and the other magnetic pole.
• This creates a magnetic field, and when a current flows through the wire, the magnetic fields interact which causes the wire to experience a force due to the motor effect.
• This causes the paper cone to vibrate forwards & backwards and so air particles vibrate at the same frequency as the AC. This creates variations in pressure of the air which form the sound waves.
• By changing the frequency of the AC, you can change the frequency of the sound wave produced (the frequency of the AC = the frequency of the sound wave produced).

Question 9

Explain how a microphone converts electrical signals into sound waves. [4 marks]

Answer 9

• The sound waves cause the diaphragm to vibrate.
• This causes the coil to move in the magnetic field, cutting magnetic field lines.
• This induces a potential difference due to the generator effect.
• The coil is connected to a circuit, so a current is induced.
• When the coil moves backwards, the current reverses, so an alternating current is induced.
• The frequency of the ac is the same as the frequency of the sound wave.

Question 10

Explain how an electric bell works. [4 marks?]

Answer 10

• When the switch is closed, a current flows through the circuit.
• The electromagnet turns on and becomes magnetised, creating an magnetic field which attracts the iron armature.
• This causes the contacts? to push together, which causes the hammer to hit the bell.
• However this breaks the circuit, so the electromagnet turns off - the armature is no longer attracted to the electromagnet, causing the arm to fall back to its original position.
• This completes the circuit, so the cycle repeats until the switch is opened.

Question 11

Define the motor effect.

Answer 11

A current-carrying wire/conductor in a magnetic field experiences a force (the magnet & conductor exert forces on each other as the magnetic fields interact).

Question 12

How can you increase the motor effect?

Answer 12

• Increase the strength of the magnetic field by using stronger magnets.
• Increase the current.

Question 13

How do electric motors work?

Answer 13

The two sides of the current-carrying coil will experience forces in opposite directions due to the motor effect & Fleming’s left hand rule.

Question 14

Why is a split-ring commutator used in a motor?

Answer 14

It reverses the direction of the current every half-turn so the coil continues to rotate in the same direction.

Question 15

What is the pupose of the carbon/graphite brushes in a motor?

Answer 15

The two halves of the coil swap from one carbon brush to the other.
The carbon brushes are electrical conductors and complete the circuit as the wire rotates - if they weren’t used, the wire would all get tangled up.

Question 16

Define the generator effect.

Answer 16

When a conductor moves relative to a magnetic field or a magnetic field changes around a conductor, magnetic field lines are cut so a potential difference is induced. If the conductor is connected to a complete circuit, a current is induced.

Question 17

Give three ways you can increase the size of the induced potential difference/current from the generator effect.

Answer 17

• Increase the speed of the conductor’s movement.
• Increase the strength of the magnetic field.
• Increase the number of turns on the coil.

Question 18

Give 2 ways to reverse the direction of the generator effect.

Answer 18

• Reverse the magnet’s polarity.
• Reverse the conductor’s direction of movement.

Question 19

What is the difference between an alternator and a dynamo?

Answer 19

An alternator produces ac (the dirction of the current changes every half revolution) whereas a dynamo produced dc.

Question 20

Explain how an alternator works.

Answer 20

• Coil of wire rotates in a magnetic field.
• The ends of the coil are connected to slip rings and carbon brushes, ensuring there’s a continuous connection allowing the induced current to flow through the coil in a circuit (due to the generator effect).
• There is no split ring commutator, so the direction of the current changes every half revolution - this is ac.

Question 21

Explain how a dynamo works.

Answer 21

• Coil of wire rotates in a magnetic field.
• The ends of the coil are connected to a split ring commutator, which makes the ends of the coil swap contacts (from one carbon brush to the other) every half revolution.
• This means the induced current (due to the generator effect) reverses every half revolution so it is always flowing in the same direction - this is dc.

Question 22

Give three ways you can increase the maximum potential different induced by an alternator.

Answer 22

• Increase the rate of rotations.
• Increase the strength of the magnetic field.
• Increase the number of turns on the coil.

Question 23

When the coil in an alternator is at 0°/180° to the magnetic field, what is the induced potential difference?

Answer 23

0V as the coil moves parallel to the direction of the magnetic field - no magnetic field lines are cut.

Question 24

When the coil in an alternator is at 90° to the magnetic field, what is the induced potential difference?

Answer 24

Maximum potential difference as the coil moves perpendicular to the direction of the magnetic field - the most magnetic field lines are cut.

Question 25

When the coil in an alternator is at 270° to the magnetic field, what is the induced potential difference?

Answer 25

Maximum potential difference but in the negative direction as the coil moves perpendicular to the direction of the magnetic field - the most magnetic field lines are cut.

Question 26

When the coil in a dynamo is at 0°/180° to the magnetic field, what is the induced potential difference?

Answer 26

0V as the coil moves parallel to the direction of the magnetic field - no magnetic field lines are cut.

Question 27

When the coil in a dynamo is at 90° to the magnetic field, what is the induced potential difference?

Answer 27

Maximum potential difference as the coil moves perpendicular to the direction of the magnetic field - the most magnetic field lines are cut.

Question 28

When the coil in a dyamo is at 270° to the magnetic field, what is the induced potential difference?

Answer 28

Maximum potential difference still in the positive direction as the coil moves perpendicular to the direction of the magnetic field - the most magnetic field lines are cut.
The split ring commutator ensures the induced potential difference remains in the same direction.

Question 29

Give three ways you can increase the maximum potential different induced by a dynamo.

Answer 29

• Increase the rate of rotations.
• Increase the strength of the magnetic field.
• Increase the number of turns on the coil.

Question 30

How does increasing the rate of rotations of an alternator/dynamo affect a graph of the induced potential difference against time?

Answer 30

It would increase the maximum potential difference and decrease the distance between peaks.

Question 31

What does a transformer consist of?

Answer 31

Two coils (primary and secondary) wrapped around an iron core.

Question 32

Why is an iron core used in a transformer?

Answer 32

Iron is magnetically soft, meaning it can be easily magnetised and demagnetised.

Question 33

How does the number of turns on the secondary coil compare to that of the primary coil in a step down transformer?
How does this affect the potential difference?

Answer 33

There are more turns on the primary coil - you are ‘stepping down’.
The potential difference decreases - the potential difference across the secondary coil is lower.

Question 34

The more turns on a coil in a transformer, the ____ the potential difference.

Answer 34

greater

Question 35

How do transformers work?

Answer 35

• A primary potential difference supplies an alternating current, which flows through a coil of wire wrapped around one side of the iron core.
• The ac induces an alternating magnetic field (Fleming’s right hand rule) in the iron core.
• This induces an alternating potential difference in the secondary coil of wire on the other side of the iron core (due to the generator effect).
• This induces an alternating current in the secondary coil.

Question 36

Bob connects a battery to a transformer. He wants to know if it will work. Explain your answer.

Answer 36

It won’t work as transformers need alternative current for magnetic field lines to be cut to induce a changing potential difference by the generator effect.