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1.7.4Motion, Forces and Energy

Power — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

5 min read 9 parts Lesson 76 of 91 1 quiz

📚 Cambridge IGCSE Physics 0625 Hub / Topical Revision
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Lesson summary

1.7.4 Power — IGCSE PHYSICS notes: Define power as work done or energy transferred per unit time and use P = W/t and P = ΔE/t.Power = the work done per unit time, or the energy transferred per unit time.Machines transfer energy continuously, and the rate of transfer is the power.…

  • Two cars doing the same work
  • Equations (Extended)
  • Rearrangements
  • Larger units
  • Electrical power can also be found from P
  • Energy from power

Part 1 of 7Definition

1Power
Power the work done per unit time, or the energy transferred per unit time.
2
Machines transfer energy continuously, and the rate of transfer is the power.
3
A more powerful machine transfers the same energy in less time, or more energy in the same time.
4Two cars doing the same work
  • Two cars doing the same work the one with more power reaches the same speed sooner.

Part 2 of 7Equations and units

1Equations (Extended)
  • P = W/t and P = ΔE/t.
  • P is power in watts (W), W or E is work or energy in joules (J), t is time in seconds (s).
2Rearrangements
  • W = Pt, E = Pt and t = W/P.
3Unit
  • Unit the watt (W), where 1 W = 1 J/s.
4Larger units
  • 1 kW = 1000 W and 1 MW = 1 000 000 W.
5
Times must be in seconds for joules and watts.
6Electrical power can also be found from P
IV.

Part 3 of 7Power ratings

AppliancePower rating
A torch1 W
An electric light bulb100 W
An electric oven10 000 W = 10 kW
A train1 000 000 W = 1 MW
Large power station1 × 10⁹ W
A star like the Sun1 × 10²⁶ W

Part 4 of 7Calculating power

1
Find the energy transferred or work done.
2
Find the time in seconds.
3
Calculate P = E ÷ t.
4
Give the answer in W or kW.
Lifting
  • Lifting power = ÷ time, since the work against gravity is mgh.
    • (weight × vertical height)
Energy from power
  • E = Pt, such as a 2000 W iron used for 300 s transferring 600 000 J.

Part 5 of 7Kilowatt-hours

A kilowatt-hour (kWh) is the energy used by a 1 kW device in 1 hour.
1 kWh
3 600 000 J.
Energy in kWh
Energy in kWh power in kW × time in hours.
Electricity bills use kilowatt-hours.

Part 6 of 7Measuring your power running upstairs

1
Measure your mass with bathroom scales and calculate weight W = mg.
2
Measure the height of one step and multiply by the number of steps for the vertical height.
3
Time yourself running up the stairs with a stopwatch.
4
Calculate work done = weight × vertical height.
5
Calculate power = work done ÷ time.

Part 7 of 7More from the specification

1
Sankey diagram efficiency calculation
  • Sankey diagram efficiency calculation comparing useful power output against total input energy provides a visual and mathematical means to evaluate machine efficiency.
2
Energy stores
  • Energy stores kinetic, gravitational potential, elastic, thermal and chemical stores are changed within a system by transfers.
3
Kinetic energy
  • Kinetic energy energy of a moving object, equal to half mass times speed squared.
4
Gravitational potential energy
  • Gravitational potential energy energy of an object raised in a field, equal to mass times g times height.
5
Power
  • Power the rate of energy transfer, equal to energy transferred divided by time, measured in watts.
6
Efficiency
  • Efficiency useful energy output divided by total input;
    • the rest is wasted, usually as heat to the surroundings.
7
Transfer pathway
  • The way energy moves between stores, such as heating or work done.
8
Energy stores
  • Energy stores kinetic, gravitational potential, elastic, thermal and chemical stores are changed within a system by transfers.
9
Kinetic energy
  • Kinetic energy energy of a moving object, equal to half mass times speed squared.
10
Gravitational potential energy
  • Gravitational potential energy energy of an object raised in a field, equal to mass times g times height.
11
Power
  • Power the rate of energy transfer, equal to energy transferred divided by time, measured in watts.
12
Efficiency
  • Efficiency useful energy output divided by total input;
    • the rest is wasted, usually as heat to the surroundings.
13
Surroundings
  • Everything outside the system.
14
Distance
  • Total length of path travelled, a scalar.
15
Kinetic energy
  • K = ½mv2, energy of motion.
16
Gravitational potential
  • Work done per unit mass in bringing a test mass from infinity to the point.
17
Energy store
  • Where energy is held, such as kinetic or chemical.
18
Wasted energy
  • Energy transferred to stores that are not useful, usually thermal in the surroundings.
19
Thermal store
  • Energy held in the moving particles of a material.
20
Gravitational field
  • Region where mass experiences gravitational force.
21
Efficiency
  • Useful output divided by total input.
22
Thermal
  • A convection current in the air.

Key terms

TermMeaning
PowerRate of energy transfer or work done.
Watt (W)One joule per second.
Kilowatt1000 W.
Kilowatt-hourEnergy used by 1 kW for 1 hour, 3 600 000 J.
Electrical powerP = IV.

Avoid these mistakes

1
Using minutes or hours in P = E/t — convert the time to seconds for watts.
2
Confusing power (rate of energy transfer) with energy.
3
Using the distance along a slope instead of the vertical height for lifting power.
4
Mixing kW with W — convert before calculating.

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