Question bank › IGCSE Physics › Energy resources and energy transfers

Energy stores, conservation of energy, efficiency and Sankey diagrams

Answer on paper first, then reveal the mark scheme one mark at a time.

1Grade 4–53 marks

The Sankey diagram shows the energy transfers for a lamp each second.

Calculate the efficiency of the lamp. [3]

efficiency = useful energy output ÷ total energy input (× 100%) allow 'total energy output' (spec wording) or 'total energy supplied'(1)
$\dfrac{50}{200}$(1)
0.25 or 25% (no unit other than %); reject 0.25%(1)
Answer: 0.25 (25%)
Examiner tip: Efficiency has no unit and can never be greater than 1 (100%); check you have not divided input by output.

2Grade 6–76 marks

An electric motor is used to lift a box of mass 40 kg through a height of 6.0 m.

The motor has an input power of 250 W and takes 20 s to lift the box.

gravitational field strength $g$ = 10 N/kg

(a) Calculate the efficiency of the motor. [5]

(b) Describe what happens to the energy that is wasted. [1]

(a) GPE gained $= mgh = 40 \times 10 \times 6.0$(1)
(a) 2400 J(1)
(a) energy input = power × time $= 250 \times 20$ alternative method: useful power output $= 2400 \div 20$ scores this mark(1)
(a) 5000 J alternative method: 120 W(1)
(a) efficiency = 2400 ÷ 5000 = 0.48 or 48% alternative method: 120 ÷ 250 = 0.48; allow ecf; reject 48 with no % sign or any unit other than %(1)
(b) transferred to the surroundings as thermal energy / dissipated by heating (the motor and air) allow sound(1)
Answer: (a) 0.48 (48%) (b) dissipated to the surroundings as thermal energy
Examiner tip: Both energies must be in joules — dividing 2400 J by 250 W compares an energy with a power.

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