SP13 - Electromagnetic Induction

Question 1

How can potential difference be induced?

Answer 1

A changing magnetic field or a moving wire in a magnetic field can induce a potential difference.

Question 2

What does the size of an induced potential difference rely on?

Answer 2

• number of turns of wire
• strength of magnetic field
• how fast the magnetic field changes / the wire moves

Question 3

How do generators work?

Answer 3

A generator consists of a coil of wire that is rotated within a magnetic field. As the coil turns, a voltage is induced in the wire. The ends of the coil are connected to slip rings. Electrical contact with an external circuit is made with carbon brushes. These generators produce alternating current, so are often called alternators.

Question 4

How do dynamos work?

Answer 4

Same as alternators, except that a commentator switches over the connections every half-turn of the coil so produces direct current.

Question 5

How do microphones work?

Answer 5

They convert the pressure variations in sound waves into variations in current in electrical circuits. The sound waves cause variations in air pressure, which cause the diaphragm to vibrate. This moves a coil of wire backwards and forwards around a permanent magnet, which induces a current and makes the microphone work.

Question 6

How do loudspeakers work?

Answer 6

Loudspeakers convert variations in current to sound waves. The varying current flows through a coil of wire in a magnetic field, producing a force on the coil, which moves backwards and forwards as the current varies. The coil is connected to a diaphragm, and the movements of the diaphragm produce sound waves.

Question 7

What is the national grid?

Answer 7

A series of wires and cables which send electricity from power stations to homes, schools and factories.

Question 8

Why are transformers necessary for the national grid?

Answer 8

When electricity flows through a wire, the wire gets warm. The wasted energy is significant for transmission lines in the national grid. Transformers reduce the amount of energy wasted in the wires by increasing the voltage, which decreases the current, while keeping power the same. Transformers also reduce the voltage and increase the current before the electricity is sent into buildings.

Question 9

What do step-up transformers do?

Answer 9

They increase the voltage and decrease the current.

Question 10

What do step-down transformers do?

Answer 10

They make the voltage lower and the current higher.

Question 11

What are the values of potential difference at different stages of the national grid?

Answer 11

Power station - 25kV
Transmission lines - 400kV
Factories - 33kV or 11kV
Homes, shops, offices etc. - 230V

Question 12

How do transformers work?

Answer 12

Alternating current is supplied to the primary coil. This creates a continuously changing magnetic field, which is carried to the secondary coil through the iron core. The changing magnetic field induces a changing potential difference in the secondary coil. The potential difference is greater in the secondary coil of it has more turns than the primary coil.

Question 13

What is the transformer equation?

Answer 13

Potential difference across primary coil / potential difference across secondary coil = number of turns in primary coil / number of turns in secondary coil

Vp Np
— = —
Vs Ns

Question 14

What is the transformer equation about power?

Answer 14

Potential difference across primary coil x current in primary coil = potential difference across secondary coil x current in secondary coil

Vp x Ip = Vs x Is