Transmitting electricity

Question 1

What is electromagnetic induction?

Answer 1

Voltage is induced when a wire is moved in a magnetic field. If the piece of wire s part of a circuit, a current will flow. This is called electromagnetic induction.

Question 2

Why is voltage always induced?

Answer 2

A voltage is always induced when there is relative movement between a magnet and a coil of a wire. The induced voltage is larger when the magnet is moved more quickly.

Question 3

What happens in an electrical generator?

Answer 3

In an electrical generator, a coil is rotated in a magnetic field. As the coil rotates, it cuts the magnetic field lines to induce a voltage across the coil.

Question 4

What causes an alternating current to flow in the circuit?

Answer 4

The coil has slip rings, which are connected to a circuit via brushes. This causes an alternating current to flow in the circuit.

Question 5

How will the current increase?

Answer 5

The current will increase if the speed of motion increases, if a stronger magnet is used, or if there are more turns of wire in the coil.

Question 6

What is direct current?

Answer 6

Direct current otherwise known as d.c. always flows in the same direction. A cell provides direct current.

Question 7

What is an alternating current?

Answer 7

Alternating current otherwise known as a.c. changes direction at a frequency determined by the rotating coil.

Question 8

When is mains electricity generated?

Answer 8

Mains electricity is generated at a frequency of 50 Hz.

Question 9

What follows the variation of current with time?

Answer 9

The variation of current with time follows a sinusoidal curve.

Question 10

What is a transformer?

Answer 10

A transformer is a device that changes the size of an alternating voltage.

Question 11

How does a transformer work?

Answer 11

Two coils of wire are wrapped round a soft iron core. The alternating voltage supply is connected to the primary coil, and the output alternating voltage is induced across the secondary coil.

Question 12

What does a step-up transformer do?

Answer 12

A step-up transformer converts a low voltage input to a higher voltage output. The primary coil will have fewer turns than the secondary coil.

Question 13

What does a step-down transformer do?

Answer 13

A step-down transformer converts a high voltage input to a low voltage output. The primary coil will have more turns than the secondary coil.

Question 14

What is the National Grid?

Answer 14

The National Grid is the network of pylons and cables that transmit electrical energy from power stations to users across the UK.

Question 15

How are transformers used in the National Grid?

Answer 15

• step-up transformers increase the 23 kV power-stations output to higher voltages (275 kV or 400 kV). This reduces heat loss in the cables and improves efficiency of transmission.
• step-down transformers lower the voltage to 230 V, which is safer for homes.

Question 16

How can risks be minimised?

Answer 16

Risks can be minimised by burying cables underground, but this is expensive and makes it difficult to maintain cables. Suspending cables from pylons is a cheaper option.

Question 17

What does a.c. voltage in the primary coil of a transformer create?

Answer 17

The a.c. voltage in the primary coil of a transformer creates an ever-changing magnetic field around it.

Question 18

What does the magnetic soft-iron core do?

Answer 18

The magnetic soft-iron core channels the magnetic field through the secondary coil.

Question 19

What does the alternating magnetic field do?

Answer 19

The alternating magnetic field will continuously cut through the wires in the secondary coil and an a.c. voltage will be induced.

Question 20

Is the turns ratio equal to the voltage ratio?

Answer 20

Yes.
output voltage at secondary coil / input voltage at primary coil = number of turns on secondary coil / number of turns on primary coil