Electromagnetic Induction

Define magnetic flux

The measurement of total magnetic field that passes through a certain area

State Faraday’s Law

The magnitude of induced EMF in a circuit is directly proportional to the rate of change of magnetic flux

• Magnitude of induced EMF → deflection size

How can Faraday’s Law be observed?

Faster movement → greater deflection

• When the magnet is not moving in the coil, there is no change in the magnetic flux linking the coils → no induced EMF

• More turns in coil → more change in magnetic flux linkage → higher EMF induced

State Lenz’s Law

Direction of induced EMF is always such that its magnetic effect always opposes the motion or change producing it

What happens when the North pole of a magnet approaches a solenoid from the right?

• To oppose the change, the solenoid needs to have a north pole on the right to repel the approaching magnet

• The induced EMF is in a direction that will induce a North pole on the right of the solenoid

What happens when the North pole of a magnet leaves a solenoid from the right?

• To oppose the change, the solenoid needs to have a south pole on the right to attract the magnet

• The induced EMF is in a direction that will induce a south pole on the right of the solenoid

What is an AC generator?

An alternating current generator makes use of electromagnetic induction to convert mechanical energy into useful electrical energy *RHR

What happens in an AC generator when the coil is rotated clockwise?

• Force on AB is up, force on CD is down

• Induced current is anticlockwise

• Magnetic field lines cut across by AB & CD at max. rate as the rate of change of magnetic flux is max.

• Induced EMF is the greatest

What happens in an AC generator when the coil is vertical?

• AB & CD are moving parallel to the magnetic field so they do not cut magnetic field lines

• No rate of change of magnetic flux

• No induced EMF

What happens in an AC generator after the coil comes down from a vertical position?

• Force on AB is down, force on CD is up

• Induced current reverses

  • Reverses every half cycle, hence current generated is alternating

• Same thing as before the vertical position except for the reversal of directions

Graph representing induced EMF of an AC generator

How can the magnitude of induced EMF be increased?

• Increase the number of turns in the coil

• Increasing the frequency of rotation

• Winding the coil around a soft iron core to increase the magnetic flux linking the coil

• Using stronger permanent magnets

How does a fixed coil generator work?

1. A magnet connected to an axle and wheel rotates as the wheel rotates

2. Magnetic field lines linking the coil change as the magnet rotates, setting up and induced EMF in the coil

3. The induced EMF causes a current to flow through the output terminals to the load

What is a transformer?

A device that can change a high alternating voltage / current to a low alternating voltage / current or vice versa

What is the purpose of the laminated soft iron core in a transformer?

• Concentrates the magnetic field lines of forces so that magnetic field is stronger

• Soft iron is a soft magnetic material that allows the magnetic field to change direction easily in the core

• Lamination reduces heat loss due to eddy currents

What are eddy currents?

Undesirable currents induced (in the soft iron core) that result in thermal energy loss

What is are the differences between a step-up and step-down transformer

Step-up → more turns in secondary coil, increases voltage

Step-down → fewer turns in secondary coil, decreases voltage

Equations relating the primary and secondary coil in a transformer

Vs / Vp = Ns / Np = Is / Ip

p = primary

s = secondary

N = no. of turns in coil

How do we calculate input and output power?

VpIp = VsIs

input power = output power

What is the power transmission problem?

Resistance in the metal cables used to transport electrical energy from power stations to households and factories results in heat loss

How can the power transmission problem be solved?

• Stepping up the voltage using a transformer to increase voltage and reduce current to reduce power loss

• Reduce I

• Use thicker wires

  • Downsides are that they are heavy, require a stronger tower and expensive

Ploss = I²R