Watch first
VIDEO
Cognito Efficiency and Sankey diagrams
Key facts & equations
Efficiency Efficiency = useful energy output / total energy input (× 100 for %). Or: efficiency = useful power output / total power input. Efficiency is always between 0 and 1 (or 0% to 100%). Can never exceed 1. Energy is always conserved — "wasted" energy is usually thermal energy (heat). Higher efficiency = less energy wasted = lower running costs. Sankey diagrams Sankey diagrams show energy flow. Width of arrow ∝ amount of energy. Input energy arrow enters from the left. Useful energy arrow goes straight ahead. Wasted energy arrows branch downward. Total width of output arrows = input arrow width (energy conserved). E.g. light bulb: 100 J in → 10 J light (useful) + 90 J heat (wasted). Efficiency = 10%. Power Power = energy transferred / time = work done / time. P = E/t = W/t.Units: watts (W). 1 W = 1 J/s. kW = 1000 W. Power = force × velocity. P = Fv. Used when object moves at constant speed. Electrical power: P = IV = I²R = V²/R. A 60 W bulb uses 60 J every second. Improving efficiency Reduce friction: lubrication, ball bearings, streamlined shapes. Reduce heat loss: insulation (in buildings, boilers, pipes). Recover waste energy: e.g. regenerative braking (EVs convert KE to electrical energy). LED lighting: much more efficient than filament bulbs. Trade-off: increasing efficiency often requires investment (insulation, new equipment). Exam questions — 4 questions · 10 marks · AQA 8463 style
⚡ Show all working — the AI marks against the AQA mark scheme. Method marks awarded for correct working even if the final answer is wrong.
3 marks Efficiency calculation AQA 8463 P1 style 🔢 Calculator
A motor uses 500 J of electrical energy. It does 350 J of useful mechanical work. (a) Calculate the efficiency. (b) How much energy is wasted? (2 marks)
Hint: (a) efficiency = useful/total. (b) Wasted = input - useful output.
Mark my answer +30 XP
1 mark Sankey diagram AQA 8463 style
In a Sankey diagram, what does the width of each arrow represent?
A The speed of energy transfer
B The type of energy
C The amount of energy
D The efficiency of the device
3 marks Power calculation AQA 8463 P1 style 🔢 Calculator
A car engine exerts a driving force of 2000 N at a constant speed of 25 m/s. (a) Calculate the power output. (b) If the car is 35% efficient, what is the power input from the fuel? (3 marks)
Hint: (a) P = Fv. (b) Rearrange efficiency formula: P_input = P_useful / efficiency.
Mark my answer +30 XP
1 mark Energy conservation in Sankey AQA 8463 style
A hairdryer uses 2000 W total. It produces 1800 W of useful heating and 200 W of sound. Which statement is correct?
A Energy has been created by the hairdryer
B Total output power = total input power — energy is conserved
C The hairdryer is 90% efficient and creates no sound
D The hairdryer violates the law of conservation of energy
Module complete! 🎉 Score loading...