Core - Topic 5 - Generating Electricity

Question 1

Explain why step-down transformers are used in the transmission of electricity in the National Grid. (2)

Answer 1

An explanation linking two from MP1 (so that they) decrease the (high) voltages (1) MP2 high voltages used for efficiency/energy saving (1) MP3 (step-down transformers) used {near / for} {homes / factories/appliances} (1) MP4 (so that it is) safer (1)

Question 2

A transformer has 2400 turns on the primary coil and 100 turns on the secondary coil. Calculate the secondary voltage if the primary voltage is 12 V. (3)

Answer 2

Transposition (1) Vs = Vp x ns/np Substitution (1) (Vs =) 12 x 100 2400 Evaluation (1) 0.5 (V)

Question 3

When the lamp is first switched on, it heats up. It then reaches a constant temperature. Explain why the temperature of the lamp remains constant. (2)

Answer 3

explanation linking:  energy supplied and radiated (1)  (at) equal (rate) (1)

Question 4

A wind power system costs £6000 to install. It saves £250 each year. Calculate the payback time. (2)

Answer 4

 substitution (1) 6000 250  evaluation (1) 24 (years)

Question 5

Explain why step-up transformers are used in the transmission of electricity in the National Grid. (2)

Answer 5

An explanation linking any two of:  increase voltage (1)  decrease current (1)  reduce {loss / waste} of {energy / heat} (1)

Question 6

The diagram shows a magnet moving into a coil of wire. The coil of wire is attached to a sensitive ammeter. The moving magnet and the coil of wire are producing an electric current. The size and direction of the current can be changed in a number of ways. Describe changes that can be made to produce different currents and the effect of each change. (6)

Answer 6

A description to include some of the following points  speed of movement  stronger / more powerful (ORA) magnet  more turns / coils (ORA)  iron core  reversing movement  turning the magnet round  effect of any / each change  more conducting / less resistant wire  allow stronger current  allow ammeter reading / recording / voltage for current  allow moving coil Correct ideas but using inaccurate scientific terminology  larger / bigger magnet  more / longer movement Ignore  irrelevant information  speeds up current or more electricity

Question 7

State one example of a non-useful energy transfer in the motor.

(1)

Answer 7

(transfer of energy to) thermal

(1)

Question 8

The electric motors which drive the wheels are painted black.

Suggest why the motors are painted black.

(1)

Answer 8

Any one from

black is a good thermal radiator

(1)

(helps to) prevent motors

overheating (1)

Question 9

There are several large-scale energy resources which are suitable alternatives to

fossil fuels in some situations.

Two of these alternatives are hydro-electric power and solar power.

Compare hydro-electric power with solar power as energy resources for the

large-scale generation of electricity.

(6)

Answer 9

A discussion including some of the following points

• Both HEP and Solar power are renewable
• Both HEP and Solar power would save fossil fuels
• HEP only possible in some locations
• HEP requires reservoirs and damming of rivers
• This can damage environment /takes a lot of land out of

use

• Energy from solar power installation is currently much

less than energy from fossil fuel powered station

• Solar power only suitable in certain locations
• Solar power reliability dependent on constant sunshine
• Neither of them cause atmospheric pollution

Question 10

When the heater is switched on, it quickly warms up and then stays at a constant

temperature.

Explain why the heater stays at a constant temperature.

(2)

Answer 10

An explanation linking

• dissipating heat (1)
• at same (rate)/as quickly

as energy is being

supplied (1)

Question 11

The magnet is made to spin at a steady speed.

The ammeter gives a reading of 1.5 A.

The voltmeter gives a reading of 6 V.

(i) Calculate the output power of the generator.

(2)

Answer 11

Substitution (1)

1.5 x 6

Evaluation (1)

9 (W)

Ignore any unit given by

candidate.

Question 12

State two changes to the design of the generator that would give a larger

output power for the same speed of rotation.

(2)

Answer 12

• More turns on the coil (1)

• More powerful/stronger magnet(s) (1)

Question 13

This generator supplies an alternating current (AC) to the lamp.

Other types of generators supply a direct current (DC).

Describe the difference between charge movement in a direct current and in

an alternating current.

(2)

Answer 13

A description including

• in one direction only for DC

(1)

• reversing direction for AC (1)

Question 14

The first public power station was built in the centre of New York.

It used generators to supply direct current at 110 V. The cables had to go

underground and they could only supply nearby shops and offices.

The electricity was mainly used for electric light.

The development of alternating current generators led to major changes in the

way electricity is transmitted and used.

Compare the modern National Grid system with the early system in New York.

(6)

Answer 14

A comparison including some of the following ideas

• Transformers can be used or {voltages/currents} can be

{changed/transformed}

• AC (can transmit) at lower current/high(er) voltage
• National Grid is (usually) over ground (DC cables (were)

underground)

• Less energy lost in transmission
• National Grid system can supply to customers further away
• Possible to create a grid linking power stations
• More flexibility in voltage for consumer
• Consumer can draw large(r) current
• More flexibility in power drawn
• Great(er) range of devices can be powered

Ignore methods of electricity production