# Generation of Electricity 3

```GenerationFormal Homework 3: Generation and Transmission of Electricity
Q1.
A thermal power station burns Fossil Fuels and has an efficiency of 40%. A
combined heat and power station burns biomass and is more efficient; it uses heat
to produce hot water for homes as well as generating electrical energy. The energy
output for each power station is shown in the diagram below:
waste heat energy
40% electrical energy output
Thermal power station
20% waste heat energy
50% useful heat energy output
30% electrical energy output
Combined heat and power station
(a)
(i)
State one fossil fuel that might be burned in the thermal power station.
1
(ii) State one sources of biomass that might be burned in the combined heat
and power station.
2
(b)
(i)
Calculate the percentage of waste heat for the thermal power station. 3
(ii) Calculate the total percentage useful energy output of the combined heat
and power station.
2
(8)
Q2.
Electrical power is distributed by the National Grid system. Transformers are used
in this
grid system.
(a) State why transformers are used in this grid
1
(b) The voltages required at various stages of the grid system are shown:
Stage
Voltage (volts)
Super Grid
400 000
National Grid
132 000
Heavy industry
33 000
Light industry
11 000
Homes
230
A transformer transfers power from the National Grid to heavy industry. The
primary coil of the transformer has 6000 turns.
(i) State the voltage at the primary coil.
1
(ii) State the voltage output at the secondary coil
1
(iii) Calculate the number of turns in the secondary coil
3
(6)
Q3.
Model power transmission lines are set up to demonstrate how electricity is
distributed from a power station to consumers.
(a)
Why must a.c. be used in this circuit?
1
(b)
State the purpose of transformer X.
1
(c)
For safety reasons, in this model, the voltage in the transmission lines must not
exceed 25 V.
Show that the transmission voltage is within this limit.
(d)
3
The current in the transmission lines is 200 mA.
The transmission lines have a total resistance of 20 Ω.
Calculate the total power loss in these transmission lines.
3
(8)
Q4.
Electricity is often transported long distances from power stations, where it is
generated, to towns and cities, where it is needed. Typically, electricity is carried in
overhead power lines. However, in recent years, there has been increasing calls for
electricity cables to be buried under the ground.
(a) State two areas where electricity transmission cables might be buried
underground.
1
(b) Give two advantages of overhead power lines compared to underground cables.
2