Electromagnetic Induction

Question 1

What happens when a conductor physically moves through a magnetic field in a direction that is not parallel or when a magnetic field changes in it?

Answer 1

• A voltage (potential difference) is induced in the conductor
• This is referred to as the generator effect

Question 2

What factors affect the size of the induced voltage in the generator effect?

Answer 2

• The speed at which the wire/magnet is moved
• The number of turns in the coil of wire
• The size/thickness of the coil (as there is a greater area cutting through field lines)
• The strength of the magnetic field

The direction of the induced voltage is determined by the orientation of the poles of the magnet or by moving it through the field lines in the opposite direction

Question 3

How is a current produced through the generator effect?

Answer 3

• A voltage is induced in the conductor
• This completes the circuit and since it is part of a completed circuit, current is induced (created)

Question 4

How can the generator effect be used to generate alternating current in an a.c. generator?

Answer 4

• A coil of wire is forced to spin in a uniform magnetic field (between two bar magnets)
• As the coil spins, voltage is induced in the wire but in alternating directions depending on the direction that the coil cuts the magnetic field lines
• Therefore the current which is induced is alternating as the direction of the voltage is alternating (due to the spinning motion)
• This can be observed on a meter, which the generator is connected to via slip rings and brushes

This can also be done by spinning a magnet around a conductor instead where the exact same thing will happen

Question 5

How can the generator effect be used to generate direct current in a dynamo?

Answer 5

• It works the same as the a.c. generator except it has a split ring communicator which switches the connection between the coil and the brushes every half turn (when it is perpendicular) in order to keep the induced voltage and therefore the current leaving the dynamo in the same direction
• While the induced voltage is never negative it varies from a maximum value to 0 each turn depending on how perpendicular the coil is to the magnetic field

Question 6

How does a transformer work to change the size of an alternating voltage?

Answer 6

• A transformer will consist of a primary coil, a secondary coil and an iron core and will utilise the generator effect
• An alternating current is supplied to the primary coil which creates a changing magnetic field and therefore induces voltage into the primary coil
• This induces a changing magnetic field inside the iron core (which magnetises easily) which will then induce an alternating voltage inside the secondary coil
• The size of the voltage induced in each coil will depend on the number of turns in that coil
• A transformer can therefore be used to change the size of an alternating voltage if the primary coil has a different number of turns to the secondary coil

Question 7

Why are step-up and step-down transformers important for the power grid and how does it work?

Answer 7

• Transformers change the voltage by having differering numbers of turns in the primary and secondary coils
• The secondary coil, which can have a higher or lower voltage induced in it compared to the primary coil depending on which type of transformer it is, will induce an alternating current in a wire leading away from the generator
• If the voltage is higher, the current will be lower and vice versa due to the rules regarding the conservation of energy
• High voltages are needed when electricity is transported to ensure a very low voltage and therefore a low loss of thermal energy (as current produces heat when flowing through a resistant conductor)
• However lower voltages are produced from the power stations and are needed for use in buildings - this is why transformers are necessary

Question 8

What is the relationship between input (primary) and output (secondary) voltages and the number of turns on the primary coil and number of turns on the secondary coil for a transformer?

Answer 8

Input (primary) voltage/output (secondary) voltage = primary turns/secondary turns

Question 9

What is the relationship between input power and output power in a transformer that is 100% efficient?

Answer 9

• Input power = output power (Pₚ = Pₛ)
• Vₚ × Iₚ = Vₛ × Iₛ

Transformers are never 100% efficient however