Topic 3B.2 - Electromagnetic Induction

Question 1

Define magnetic flux

Answer 1

The product of magnetic field strength B and the area perpendicular to the magnetic field

Question 2

State the equation for magnetic flux

Answer 2

O = B A cos0

A = area of the loop
B = strength of external magnetic field 
0 = angle between B direction and direction normal to the loop

Question 3

Describe electro-magnetic induction and how it is created

Answer 3

Electromagnetic Induction occurs when a changing magnetic flux induces a potential difference in a wire.
It occurs when a wire is moved through a magnetic field or when a magnetic field in moved around the wire.

Question 4

What is electro motive force?

Answer 4

Electro Motive force (EMF) is the produced potential difference and is measured in Volts.

EMF is NOT a force

Question 5

State Faraday’s Law with associated equation

Answer 5

If a wire is moved to cut across lines of a B field then a current is induced in the wire.

EMF = N (change in) EM induction / time

EMF = N B A cos 0 / t

Question 6

What EMF proportional to?

Answer 6

1. the number of coils/turns in the wire
2. the speed of the magnet or coil
3. the strength of the magnet

Question 7

State Lenz’s Law

Answer 7

The direction of the induced current is such that it opposes the change producing it.

Question 8

How is the direction of EMF found? (which rule)

Answer 8

Fleming’s Right Hand rule (the gansta rule)

Question 9

How is Lenz’s Law derived from the Law of Conservation of Energy? (probably a 6 to 8 mark question)
(6 or 7 dots points/ steps)

Answer 9

As the direction of the current opposes the motion of the magnet, work must be done to move the magnet.

This external work that is added to the system is converted to electrical energy given off as heat by the wire (or current).

On the other hand, if the polarity of the ends of the solenoid does not oppose the motion of the magnet, the magnet will speed up without any external work required.

As such, no energy has been added to the system so no electrical current/kinetic energy can be produced as there is no energy to convert.

Since, Lenz’s Law dictates that a force is produced, external work must be done.

TF, the LoCE shows that the direction of the current must oppose the direction of motion of the magnet in order for a current to be produced.

TF, Lenz’s Law is the product of the LoCE.

Question 10

Define an eddy current.

What is the direction of an eddy current

Answer 10

An eddy current is a localised electric current induced in a conductor by a changing B field.
The direction of the eddy current opposes the change in the magnetic flux (Lenz’s law).

Question 11

State one application of eddy currents

Answer 11

Magnetic Brakes

Question 12

Name the main components of a generator and the concept they are based on

Answer 12

1. magnetic field
2. A moving wire
3. slip rings
4. carbon brushes

As the wire moves it cuts the magnetic field lines and as such an electric current is induced in the wire. The slip rings and carbon brushes ensure that the wire stays in contact with the external circuit as it rotates.

Question 13

State the use of transformers

Answer 13

Transformers are used to increase or decrease the voltage to make it at an appropriate level for use at different stages.

Question 14

Name the key components of a transformer (3)

Answer 14

1. Soft Iron core
2. Primary coils
3. Secondary coils

Question 15

What equation is used calculating transformer voltage

Answer 15

output Volts / input volts = turns output / turns input

V2 / V1 = n2 / n1

Question 16

Why is a high voltage used when transmitting voltage over long distances?

Answer 16

A higher voltage decreases the current and decreases the heat.
TF, thinner, lighter and cheaper cables can be used to transmit the electricity.