Topic 13 - Electromagnetic Induction

Question 1

What happens when an electrical conductor moves relative to a magnetic field?

Answer 1

• A potential difference is induced across the conductor.

Question 2

What happens to an electrical conductor when there is a change to the magnetic field that is placed in?

Answer 2

• The potential difference is also changed.

Question 3

How do you produce an electric current using a magnet and a conductor on a small-scale?

Answer 3

• Moving a coil of wire between magnetic poles.
• Moving a magnet within a coil of wire.
• Once a voltage is induced, if there is a complete circuit, a current will also be induced.

Question 4

How is electrical current produced on a large scale?

Answer 4

An electromagnet is rotated around a coil.

Question 5

What factors affect the size of an induced voltage?

Answer 5

• The number of turns on the coil of wire
• How strong the magnetic field is
• The speed of the movement

Question 6

What is the requirement for an induced potential difference to cause a current flow?

Answer 6

The conductor must form a closed loop or be part of a complete circuit.

Question 7

What can be said about the direction of the magnetic field produced by an induced current?

Answer 7

• The direction of the field is such that it opposes the change that induced the current.
• This change is either the movement of the conductor or a change to the field

Question 8

What are two ways that the generator affect is used to generate different types of current?

Answer 8

• In an alternator to produce alternating current
• In a dynamo to produce direct current

Question 9

How does a dynamo generate current?

Answer 9

• A coil of wire rotates inside a magnetic field.
• A commutator is used to ensure it continues rotating in the same direction.
• This keeps the current flowing in the same direction (d.c current).

Question 10

How is electromagnetic induction used in alternators to generate alternating current?

Answer 10

• A coil of wire rotates in a magnetic field.
• The end of this coil is connected to slip rings which will cause the current to change direction while rotating (a.c current).

Question 11

What electromagnetic effect does a microphone take advantage of and how?

Answer 11

• The generator effect
• It converts the pressure variations in sound waves into alternating current.

Question 12

How do loudspeakers make use of the motor effect?

Answer 12

• The motor effect is used to convert variations in the current of an electrical circuit into the pressure variations which produced audible sound

Question 13

Explain how a loud speaker works:

Answer 13

• A cone wrapped in wire is connected to an a.c power supply and is placed in a permanent magnetic field.
• When current flows through the wire, it creates a magnetic field which interacts with the permanent field.
• This produces a force which causes the cone to vibrate, producing sound.

Question 14

How is the pitch from a loudspeaker changed?

Answer 14

• The frequency of the a.c current is altered.
• This creates a different frequency of vibration in the cone

Question 15

How do microwaves convert sound into electrical signals?

Answer 15

• Sound produces pressure variations which causes the diaphragm to vibrate.
• A coil of wire is connected to the diaphragm, so as the dial moves, the coil also moves.
• Movement in a magnetic field = a p.d is induced
• As there is a complete circuit a current is also created

Question 16

Describe the makeup of a basic transformer:

Answer 16

A primary coil and a secondary coil of wire wrapped around an iron core.

Question 17

Why is iron used as the core for a transformer?

Answer 17

It is easily magnetised

Question 18

How does a transformer work?

Answer 18

• An alternating current flows through the primary coil
• This induces a changing magnetic field in the core
• This changing magnetic field induces an emf, which produces a current int the secondary coil

Question 19

Why must the current flowing through the primary coil of a transformer be alternating?

Answer 19

• For current to be induced in the secondary coil, the magnetic field in the core must be continuously changing.
• For the magnetic field to be changing, the current in the primary coil must be alternating.

Question 20

What can be said about the electrical power input and output of a 100% efficient transformer?

Answer 20

The electrical power input is equal to the electrical power output.

Question 21

Equation linking number of turns with p.d in transformers

Answer 21

primary p.d / secondary p.d = primary turns / secondary turns

Question 22

Power equation for transformers with 100% efficiency (uses voltage and current)

Answer 22

Vp x Ip = Vs x Is

Question 23

On a step-down transformer, does the secondary coil have more or fewer turns?

Answer 23

The secondary coil has fewer turns

Question 24

Advantage of a step down transformer in the national grid?

Answer 24

It means that voltage can be reduced to a value safe enough to be used in houses

Question 25

Why is it important to have step up transformers in the national grid?

Answer 25

- It can increase efficiency - For the same power, a higher voltage leads to a lower current (P = V x I) - The lower the current, the less energy that is lost. - Used for transmission across power lines.

Question 26

How does a step-up transformer work?

Answer 26

- The primary coil has fewer turns - An a.c current produced a magnetic field in the primary coil. - The iron core passes the magnetic field to the secondary coil - The magnetic field induces a higher voltage in the secondary coil - Power is conserved, so this produces a lower current

Question 27

Where are step-up and step-down transformers used in the national grid?

Answer 27

- Step-up transformers - used at power stations - Step-down transformers - used domestically.