Physics - Electromagnetic Induction

Question 1

How can you induce a voltage across a conductor?

Answer 1

Relative motion between the conductor and magnetic field or if the field changes

Question 2

Describe induction*

Answer 2

Occurs due to the ‘cutting’ of field lines or changing flux through a loop of wire

Question 3

List the factors that affect the size of the induced voltage

Answer 3

Rate of change, strength of field and number of turns on conductor if coiled

Question 4

Distinguish between the motor effect and electromagnetic induction

Answer 4

The motor effect – force on a current carrying wire in a magnetic field

Electromagnetic induction – potential difference across a wire due to the motion in a magnetic field

Question 5

How is the induced field affected by the cross-sectional area of the coil and its alignment relative to the field?

Answer 5

Surface area perpendicular to field for maximum potential. Larger the surface the greater the potential

Question 6

Describe the basic principle of an AC generator

Answer 6

Magnet rotating within a coil or coil rotating within a magnetic field induces an EMF due to the changing magnetic field lines through the coil

Question 7

Describe the function of the slip-rings and brushes

Answer 7

Slip rings and brushes allows a current to flow through the contacts as the coil rotates

Question 8

What factors affect the size of the generated voltage?

Answer 8

Rate of rotation, strength of field, number of turns on coil

Question 9

Where is the AC generator used?

Answer 9

Mains electricity generation in power stations

Question 10

How are DC generators different to AC generators?

Answer 10

AC generators always have the terminal touching the same slip ring

DC generators use a commutator so the current only flows in one direction.

Question 11

Draw a diagram of a transformer

Answer 11

(See diagram) Must include: the type of power supply, primary coil, secondary coil, iron core, primary circuit, secondary circuit, load

Question 12

What are the stages that produce a current in the secondary coil?

Answer 12

• Power supply causes a current to flow in the wire.
• The current causes a magnetic field to be generated in the primary coil
• The iron core directs the magnetic field to the secondary coil
• The current is alternating, so produces an alternating magnetic field in the secondary coil
• The changing magnetic field in the secondary coil induces a current in the secondary coil.
• This current flows round the secondary circuit and powers the device.

Question 13

Why can we not say the electrical energy from the power supply is transferred directly to the load in the secondary circuit?

Answer 13

There are no electrons passing from the primary to the secondary coil. At no point are they connected

Question 14

Describe how to change the voltage in the secondary coil relative to the primary coil

Answer 14

Have different numbers of coils in the primary and secondary coils

Question 15

What is the equation that allows you to calculate how the voltage changes depending on the number of turns in the coils of the primary and secondary coils.

Answer 15

V_p/V_s =N_p/N_s

Primary voltage divided by secondary voltage = number of primary coils divided by number of secondary coils

Question 16

Describe what a step-up and step-down transformer does

Answer 16

Step-up: Increases the voltage in the secondary relative to the primary

Step-down: decreases the voltage in the secondary relative to the primary

Question 17

Assuming 100% energy efficiency, write down an equation that links the current and voltage of the primary and secondary coils

Answer 17

P_p=P_s
I_p x V_p=I_s x V_s

Primary power = secondary power
Primary current x primary voltage = Secondary current x secondary voltage

Basically the power is the same

Question 18

What can cause inefficiency in a transformer and how can these be reduced?

Answer 18

Resistance of the wire in the coil - Better conductors or superconductors

Eddy currents can form in the iron core - This can be reduced by laminating sheets of iron to reduce the current

Leakage of field lines, not all the field lines produced in the primary cut the secondary coil - Reduce any gaps in the coil.

Question 19

Define flux linkage

Answer 19

It is the amount of magnetic flux going through a coil or loop of wire.

It is therefore the magnetic field strength times the area of the coil/loop

Question 20

Write down the equation describing Ohm’s Law

Answer 20

V=IR

Question 21

Write down the equation for power

Answer 21

P = IV = V^2/R = I^2 R

Power = current x voltage = current squared x resistance

Question 22

Explain how the power lost when passing electricity through a wire depends on the current and the resistance of the wire

Answer 22

The power lost in a wire is the resistance of the wire and this is increased as the current (flow of electrons) increases. The power lost is given by the equation P=I^2 R

Question 23

Use the equation about power loss to explain whether we want high voltage or low voltage going through power cables

Answer 23

High voltage, as it leads to low current (therefore less resistance and less energy lost)

Question 24

What type of transformer would we use when taking electrical power form a power station and transferring it through power cables?

Answer 24

Step-up so the voltage is increased and the resistance decreases. This means that when it is travelling through the power lines less energy is lost

Question 25

Why can we not use the same voltage in the power lines in our homes?

Answer 25

It is too dangerous, because they are more likely to discharge through normally insulating materials (e.g. jump through the air)

Question 26

What type of transformer would we use when taking electricity from power lines and transferring it to homes?

Answer 26

Step-down so the voltage is reduced to a safe level for the home.

Question 27

Describe how a DC rectifier works

Answer 27

Uses diodes to only allow current through in one direction