A Levels Physics

Trains supply coal to a power station. The table below gives quantities describing the operation of
the power station.
symbol
unit
power station output
P
number of trains per day
N
mass of coal on a train
M
kg
energy from 1 kg of coal
J
J
number of seconds in one day
S
Which expression gives the efficiency of the power station?
A
PS
PSN
в
NMJ
D
NM
NMJ
MJ
PS
PSJ


Choice A is correct because:

The efficiency of a power station is the ratio of the useful output power to the total input power.

The total output power of the power station is given by P.

The total input power is the energy supplied by the coal per day. This is given by the number of trains per day (N) multiplied by the mass of coal on each train (M) multiplied by the energy from 1 kg of coal (J) divided by the number of seconds in one day (S).

Therefore, the efficiency of the power station is given by the expression: .

Choice B is incorrect because:

Choice B suggests the efficiency is given by . This is incorrect as it does not represent the ratio of output power to input power.

Choice C is incorrect because:

Choice C suggests the efficiency is given by . This is incorrect as it does not consider the power output of the power station and the time factor in the denominator.

Choice D is incorrect because:

Choice D suggests the efficiency is given by S in the numerator. This is incorrect as it does not correctly represent the ratio of output power to input power per unit time. without the factor of 

Efficiency | 2012 - O/N - 9702/12 | Q22