Topic 13 - Electromagnetic Induction

Question 1

SP13a - Explain how a loop of wire can gain an induced current using a magnet.

Answer 1

• A magnet has a magnetic field around it.
• Once it moves throughout the wire, the field induces a P.d. in the wire which creates a magnetic field opposite to original change.

Question 2

SP13a - How does an alternator work?

Answer 2

• A coil of wire wrapped around an iron core is rotated inside a magnetic field
• Wires from the coil are connected to slip rings on the axle of the coil
• The slip rings make electrical contact through carbon brushes
• This produces an alternating current

Question 3

SP13a - How does a dynamo work?

Answer 3

Same concept as an alternator except:

• The wires are connected to a split ring commutator
• The commutator is connected to the carbon brushes
• Every half-turn the connections are swapped
• Thus, this produces a direct current

Question 4

SP13a - How do microphones work?

Answer 4

• (The opposite to loudspeakers)
• Sound waves cause vibrations in air pressure
• The pressure causes a diaphragm to move back and forth
• The diaphragm moves a coil of wire back and forth
• This causes the coil which is wrapped around a permanent magnet to gain an induced voltage and current
• The amplitude of the wave affects the volume
• The frequency affects the pitch
• Converting sound energy into electrical energy

Question 5

SP13a - How do loudspeakers work?

Answer 5

• (The opposite to microphones)
• An a.c. voltage passes througha coil
• This moves the coil along a permanent magnet
• The coil hits the diaphram causing it to vibrate
• This creates soundwaves
• The higher the current, the larger the frequency and amplitude
• And so the volume and pitch is higher
• Converting electrical energy into sound energy

Question 6

SP13b - What is the national grid?

Answer 6

The system that connects power stations to homes to provide electricity nationally

Question 7

SP13b - What are transformers, both step-up and step-down?

Answer 7

Transformers are (not robots in disguise) devices that alter the voltage and current of electricity.

• SU increase voltage; decrease current
• SD decrease voltage; increase current

Question 8

SP13b - Describe the process of transferring energy from power stations to houses and factories.

Answer 8

• Electricity is generated at the power station (11kV)
• Stepped up (400kV) for transmission lines
• Stepped down (33kV) Where some is used in large factories
• Some is stepped down further (11kV) for small factories
• Rest is stepped down instead (230V) to be used in houses shops etc.

Question 9

SP13b - Why is electricity stepped up before transmission?

Answer 9

• A higher voltage means a lower current.
• A higher current would mean a higher power loss (P = I²xR).
• So the lowest possible current is attained so that the least amount of energy is lost by heating.
• Low resistance wires are also used (thicker).

Question 10

SP13b - What is the structure of a transformer?

Answer 10

A primary coil (connected to an a.c supply) and a secondary coil with differing amounts of turns, wrapped around an iron core.

Question 11

SP13b - How do transformers work?

Answer 11

• Changing a.c current supplied
• This produces a changing magnetic field in primary coil
• The iron core is induced and carries this changing magnetic field
• This creates a changing magnetic field in the secondary coil
• This induces a changing a.c voltage in the wires connected to the secondary coil.

Question 12

SP13c - What is the voltage current transformer equation and what does it assume?

Answer 12

V(p) x I(p) = V(s) x I(s)

[assuming that the transformer is 100% efficient]

Question 13

SP13b - Which equation links voltage and turns across coils?

Answer 13

V(p)/V(s) = T(p)/T(s)

V = Voltage across coil

T = Turns in coil

Question 14

SP13c - What are the equations for power and energy?

Answer 14

• P = I x V
• E = P x t , so
• E = I x t x V