Lesson 6: The Generator Effect

Question 1

When is a potential difference induced across the ends of a conductor?

2 situations!

Answer 1

1.) If an electric conductor moves relative to a magnetic field.
2.) If there is a change in the magnetic field around the conductor.

Question 2

What is the generator effect?

Answer 2

• If an electric conductor moves relative to a magnetic field/ if there is a change in magnetic field around a conductor - a potential difference will be induced across the ends of the conductor.
• If the conductor is part of a complete circuit, a current is induced in the conductor.

Question 3

What are the factors that would increase the induced potential difference/ current in a wire (when it moves through a magnetic field?)

Answer 3

1.) Using a stronger magnetic field (ie. by using a stronger magnet.)
2.) Move the wire more rapidly through the magnetic field.
3.) Increase the number of turns on a coil of wire.

Question 4

What 3 factors affect the direction of induced potential difference/ current in a wire?

Answer 4

1.) The direction that the wire is moving in (through the magnetic field.)
2.) The direction of magnetic field changes (ie. move a magnet into coil of wire in opposite direction.)
3.) Switch the poles of a magnet (direction of magnetic field.)

Question 5

What does an induced current in a wire create around the wire?

Answer 5

• Induced current in wire creates magnetic field around wire.

Question 6

How does the magnetic field around a coil of wire that carries an induced current behave when a magnet passes through the coil of wire?

Answer 6

• The magnetic field opposes the original change.
• Ie. If the magnet is put into the coil of wire at the north end, that end of the coil of wire becomes a north end so the poles repel - making it harder to push the magnet in.
• If the magnet moves out of the coil from the north end, the end of the coil of wire becomes a south end so the poles attract - making it harder to pull the magnet out.

Question 7

What is the function of a micro-phone?

Answer 7

• A micro-phone uses the generator effect to convert the pressure variations (in sound waves) into variations in current in electrical circuits.

Question 8

Explain how a moving- coil microphone works.

Answer 8

1.) Sound waves hit the diaphragm - causing the diaphragm to vibrate.
2.) The diaphragm causes the coil to vibrate.
3.) The coil of wire moves back and forth THROUGH the (permanent) magnetic field of the permanent magnet.
4.) A potential difference is induced across the ends of the coil of wire.
(Electric signals- variations in potential difference/ current- travel to a loud-speaker where sound is amplified.)

Question 9

What is the PD of the coil of wire in a moving- coil microphone like?

Answer 9

• The PD is alternating (switches direction as the coil moves back and forth through magnetic field.)

Question 10

When is the potential difference of the coil of wire (in an alternator) = 0?

Answer 10

• When the coil of wire is parallel to the direction of the magnetic field. ie. no magnetic field lines are being cut.

Question 11

MARK-SCHEME EXPLANATION.

How does an alternator produce an alternating current?

Answer 11

• Every half turn of the coil, the PD reverses direction (because each side of the coil changes direction it is going in.)
• So, every half turn, the current changes direction.

Question 12

How would we increase the alternating current produced by an alternator?

2 main ways

Answer 12

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

Question 13

If we increase the speed at which the coil of wire in an alternator rotates, how will this change the size and the frequency of the AC current produced?

Answer 13

• Increases size of current produced.
• Increases frequency of current produced.

Question 14

When is the potential difference of the coil of wire the greatest (in an alternator/dynamo?)

Answer 14

• When the coil of wire is perpendicular to the direction of the magnetic field.
• Ie. the most magnetic field lines are being cut.

Question 15

What type of current is produced by a dynamo?

Answer 15

• Direct current.

Question 16

What device has a dynamo got to ensure that direct current is produced?

Answer 16

• Split ring commutator.

Question 17

On the dynamo graph of potential difference of wire against time, how many peaks are there in each full rotation of a coil? Why?

Answer 17

• There are 2 peaks in each full rotation.
• Each side of coil passes through magnetic field twice in one rotation.

Question 18

TYPICAL EXAM Q

Explain why an alternating current is induced in the coil (alternator.)

Answer 18

1.) The coil cuts magnetic field lines.
2.) Potential difference induced across the ends of the coil.
3.) Complete circuit (so current is induced in the coil.)
4.) Every half-turn, potential difference reverses direction.
5.) So, every half turn, current changes direction

Question 19

Suggest the purpose of the slip rings (in an alternator.)

Refer to June 2020 paper.

Answer 19

• Provides a connection between the coil and the brushes.

Question 20

Explain how a dynamo works to produce direct current.

Answer 20

1.) The coil cuts through magnetic field lines.
2.) Potential difference induced across the ends of the coil.
3.) Complete circuit (so current is induced in the coil.)
4.) Every half turn, potential difference remains in the same direction (the half - each side of coil- of the coil connected to each brush always moves in the same direction.)
5.) So, a direct current is produced.