Electromagnetic Induction Flashcards
State Faraday’s law of electromagnetic induction
The magnitude of induced emf in a conductor is directly proportional to
- the rate at which magnetic field lines are cut by the conductor
- the rate of change of magnetic flux linked with the conductor
Lenz’s law of electromagnetic induction states that
The direction of the induced current is always in a direction to oppose the change producing it
How to increase the induced emf
- move/rotate coil faster
- use a stronger magnet
- increase the number of turns of the coil
- insert a soft iron core into the coil
State the function of slip rings
To allow firm contact between the coil and the two carbon brushes; at the same time maintaining freedom of rotation (without twisting wires of the coil)
State function of carbon brushes
To maintain good electrical contact between the external circuit and rotating slip rings to transfer alternating emf generated to the circuit
Describe mutual induction
A changing current in a coil generates a varying magnetic field which can induce an emf in another coil nearby
What is a transformer?
A transformer is a device that changes a high alternating voltage (at low current) to a low alternating voltage (at high current) and vice versa
What are transformers used for?
- electrical power transmission from power stations to households and factories
- regulating voltages for proper operation of electrical appliances
Why is electrical power transmitted at high voltage?
When electrical power is transmitted at high voltage, the current in the transmission cables would be small. This would minimise the power loss (through heat to surroundings) in the cables
Furthermore, smaller currents would allow relatively thinner cables to be used for power transmission to save cost
State two advantages of using alternating voltages
A.C. Voltages can easily be steeped up or down by transformers cheaply and efficiently with very little loss of power (dc voltages can be changed in magnitude but it is difficult and expensive)
A.C. Voltages produced by AC generators are cheaper to produce than direct voltages which are converted from chemicals
How does effect of current in primary coil induce emf in secondary coil?
The varying current sets up a varying magnetic field in the primary coil which is linked to the secondary coil (through iron core)
A varying magnetic field is hence induced in the secondary coil and a varying current is induced to oppose the change producing it (by lenz’s law), therefore by faraday’s law the varying rate of change in magnetic flux linked to the secondary coil drives a varying induced emf
How is an emf induced in the coil of an AC generator as it rotates?
When the coil is rotating, it cuts the magnetic field lines between the magnetic poles.
The rate of cutting of magnetic field lines decreases from a maximum (when coil is horizontal) to a minimum (when coil is vertical) and then increases in the opposite direction. By faraday’s law, a varying emf is hence induced in the coil
For AC generator, what is the effect when speed of rotation of the coil is doubled?
Both the frequency of the alternating current and rate of cutting of magnetic field lines will be doubled. Hence, maximum output voltage is also doubled
(For graph, amplitude doubles and frequency doubles)
What is the effect when number of turns of the coil in an AC generator is doubled but speed of rotation remains the same?
The maximum output voltage is doubled
Amplitude of graph is doubled but frequency remains the same
