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1.7.7Motion, Forces and Energy

Efficiency — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

5 min read 9 parts Lesson 81 of 91 1 quiz

📚 Cambridge IGCSE Physics 0625 Hub / Topical Revision
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Lesson summary

1.7.7 Efficiency — IGCSE PHYSICS notes: Calculate the efficiency of a device from energy or power values and explain how wasted energy can be reduced.Every device has an energy input and an output.Useful output — the energy we want, such as light from a lamp or movement from a mo…

  • Wasted output
  • Energy form
  • Power form
  • State clearly what the useful output is
  • Total wasted energy
  • Useful energy

Part 1 of 7Useful and wasted energy

Every device has an energy input and an output.
Useful output
  • Useful output the energy we want, such as light from a lamp or movement from a motor.
Wasted output
  • Wasted output the rest, usually thermal energy in the surroundings and sometimes sound.
The total output equals the total input because energy is conserved.
Efficiency measures how well a device transfers input energy into useful output.

Part 2 of 7The efficiency equation

Energy form
  • Energy form efficiency = × 100%.
    • (useful energy output ÷ total energy input)
Power form
  • Power form efficiency = × 100%.
    • (useful power output ÷ total power input)
Efficiency has no units and is a percentage or a decimal between 0 and 1.
It can never exceed 100%, because that would mean energy is created.
Friction, electrical resistance and heating make 100% impossible.
Wasted energy
Wasted energy input − useful output.

Part 3 of 7Typical efficiencies

DeviceTypical efficiency
Filament lampAbout 5%
LED lampAbout 30% to 50%
Petrol car engineAbout 25% to 35%
Electric motorAbout 80% to 95%
Power station (coal or gas)About 35% to 60%
  • A lamp that is 25% efficient transfers 25% of its input as light and wastes 75%.

Part 4 of 7Improving efficiency

1
Lubrication reduces friction in moving parts and the heating that goes with it.
2
Thermal insulation such as loft insulation, double glazing and pipe lagging reduces energy escaping.
3
Streamlined shapes reduce air resistance on vehicles.
4
Thicker wires reduce electrical resistance and the energy wasted as heat.
5
Better designs such as LEDs and efficient motors waste less.
6
Wasted energy spreads into the surroundings as a small temperature rise and is hard to recover.

Part 5 of 7Sankey diagrams and power stations

1
A Sankey diagram shows the input arrow splitting into a useful arrow and wasted arrows, with widths drawn to scale.
2
A thermal power station turns heat into steam, steam turns a turbine and the turbine turns a generator.
3
Energy is dissipated at each stage, and a typical thermal power station is about 30% efficient overall.
4
Combined heat and power schemes use waste heat to warm buildings and raise the overall efficiency.

Part 6 of 7Calculating efficiency

1
Identify the total input and the useful output in the same units and for the same time.
2
Divide useful by total and multiply by 100 for a percentage.
3
For wasted energy, subtract useful from input.
4
Check that wasted plus useful equals the input.
1State clearly what the useful output is
  • State clearly what the useful output is for a heater the thermal energy is useful, for a bulb it is waste.
2
If the useful output is a lift, useful energy is mgΔh and useful power is that energy divided by the time.

Part 7 of 7More from the specification

1
Mechanical energy transfers in joules
  • Mechanical energy transfers in joules calculating input and useful output energy quantities in joules provides a means to determine system efficiency.
2
Vehicle speed and kinetic energy dissipation
  • Vehicle speed and kinetic energy dissipation accelerating vehicles increase speed, requiring greater energy input while overcoming frictional drag forces within the system.
3
Joule
  • Unit of energy, 1 kg·m2/s2.
4
Distance
  • Total length of path travelled, a scalar.
5
Kinetic energy
  • K = ½mv2, energy of motion.
6
Gravitational potential
  • Work done per unit mass in bringing a test mass from infinity to the point.
7
Energy store
  • Where energy is held, such as kinetic or chemical.
8
Thermal store
  • Energy held in the moving particles of a material.
9
Gravitational field
  • Region where mass experiences gravitational force.
10
Transfer pathway
  • The way energy moves between stores, such as heating or work done.
11
Total wasted energy
  • Total wasted energy when energy is wasted in several ways on a Sankey diagram, add every wasted arrow together.

Key terms

TermMeaning
EfficiencyFraction or percentage of input energy transferred to useful output.
Useful energyThe energy output we want from a device.
Wasted energyEnergy transferred to stores that are not useful.
Sankey diagramDiagram where arrow widths show energy transfers.
InsulationMaterial that reduces thermal energy transfer.
LubricationUse of oil or grease to reduce friction.

Avoid these mistakes

1
Giving an efficiency over 100%.
2
Using the wasted energy as the useful energy in the equation.
3
Mixing power and energy in one calculation — use the same quantity for input and output.
4
Thinking friction can be removed completely — it can only be reduced.
5
Forgetting to multiply by 100 for a percentage.

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