O-LevelPhysicsEnergy & Work5054/21 · Oct/Nov 2014 · Q915 Marks

Fig. 9.1 shows the dam and reservoir of a hydroelectric power station. [Figure 9.1] A hydroelectric power station uses a renewable energy source.

(a)State two other renewable energy sources.[2]
(b)(i)1.When the power station operates at full capacity, the electrical power output is 6.8 × 10⁸W. Calculate the electrical energy output of the power station if it operates at full capacity for one year.[1]
(b)(i)2.Suggest why, in practice, the power station does not operate at full capacity throughout the year.[1]
(b)(ii)1.The water surface in the reservoir that supplies the hydroelectric power station is at a vertical height of 170m above the turbines. In one hour, 1.6 × 10¹⁰kg of water flows from the reservoir through the turbines. The gravitational field strength g is 10N/kg. Calculate the gravitational potential energy that is converted into other forms of energy in one hour.[2]
(b)(ii)2.Calculate the efficiency of the power station when operating at full capacity.[2]
(b)(ii)3.Suggest two reasons why the efficiency of the power station is less than 1.0 (100%).[2]
(c)(i)Before the electrical energy is transmitted along power lines, the voltage is increased to a very large value. State and explain why a very high voltage is used to transmit electrical energy over long distances.[3]
(c)(ii)State the name of the device that is used to increase the voltage.[1]
(c)(iii)Explain why the generators in the power station must be a.c. (alternating current) generators.[1]

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Mark scheme

Marking points from the official mark scheme (Oct/Nov 2014). The mark or mark type is shown in brackets.

[2] (a) any two from:
biomass/wood; geothermal power; solar power; tidal power; wave power; wind power
[1] (b) (i) 1. 2.1(4) × 10¹⁷ J (allow 2.1(5) × 10¹⁷ J if candidate uses 365.24/5)
[1] (b) (i) 2. any one from: not enough water (to maintain maximum flow); rainfall varies (during the year); periods of low demand
[2] (b) (ii) 1. (GPE = ) mgh or 1.6 × 10¹⁰ × 10 × 170
2.7(2) × 10¹³ J
[2] (b) (ii) 2. 2.7(2) × 10¹³/3600 or 6.8 × 10⁸ × 3600 or 6.8 × 10⁸/7.5(55) × 10⁸ or 2.4(48) × 10¹³/2.7(2) × 10¹³
0.90 or 90%
[2] (b) (ii) 3. any two from: friction (of water) with pipe/turbine/; viscosity of water; friction at bearings; resistance/heat in the wires; KE of water leaving turbine
[3] (c) (i) less energy lost/wasted or more efficient
(for a given power) a high voltage results in a small(er) current
less heat generated in wires or I²R or less resistive losses
(not if changed resistance mentioned)
[1] (c) (ii) transformer
[1] (c) (iii) transformers only work with an a.c. supply

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