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P1.7.4P1. Motion, Forces and Energy

Power — Combined Science 0653 - Physics Revision Notes & Topical Questions

5 min read 8 parts Lesson 13 of 15 1 quiz

📚 Combined Science 0653 - Physics Hub / Topical Revision
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Lesson summary

P1.7.4 Power — COMBINED SCIENCE notes: Define power as the rate of doing work or transferring energy and calculate it, including efficiency from power values.Power = the work done per unit time, or equivalently the energy transferred per unit time.Unit — the watt (W), where 1 W =…

  • Larger units
  • In terms of work
  • What the symbols mean
  • In terms of energy
  • Rearranged forms
  • Power rating

Part 1 of 6Power

Power
Power the work done per unit time, or equivalently the energy transferred per unit time.
Unit
  • Unit the watt (W), where 1 W = 1 J/s.
Larger units
  • 1 kW = 1000 W and 1 MW = 1 000 000 W.
Time
  • Time a more powerful machine transfers the same energy in a shorter time.
  • Two cars doing the same work reach the same speed, but the more powerful car gets there sooner.

Part 2 of 6Power equations

In terms of work
  • P = W ÷ t, with P in W, W in J and t in s.
    What the symbols mean: P = power (W = J s−1) · W or E = work done / energy transferred (J) · t = time (s) · F = force (N) · v = velocity (m s−1)
In terms of energy
  • P = ΔE ÷ t, which is the same relationship.
Rearranged forms
  • ΔE = P × t and t = ΔE ÷ P.
Units
  • Units use joules and seconds, and convert minutes and hours to seconds.

Part 3 of 6Power ratings

Source or deviceTypical power
Torch1 W
Light bulb100 W
Electric oven10 kW
Train1 MW
Large power station1 × 109 W
Sun1 × 1026 W
  • Power rating
    • the energy an appliance transfers every second.

Part 4 of 6Efficiency from power

Efficiency
(useful power output ÷ total power input) × 100%.
Wasted power
  • Wasted power input power − useful output power.
Lifting a load
  • Lifting a load the useful energy output is the gain in gravitational potential energy mgΔh, so useful power output = mgΔh ÷ t.
Power input
  • Power input total power input = useful power output ÷ efficiency.

Part 5 of 6Electrical power

Equation
  • P = I × V, with P in W, I in A and V in V.
    What the symbols mean: P = power (W) · I = current (A) · V = potential difference (V)
Link to energy
  • Link to energy the energy transferred by an appliance is E = P × t.

Part 6 of 6More from the specification

1
Thermodynamic energy accounting
  • Thermodynamic energy accounting tracking heat and work transfers across system boundaries provides an empirical means to verify the first law of thermodynamics.
2
Power
  • Power the rate of energy transfer, equal to work done divided by time and measured in watts
3
Energy stores
  • Energy stores kinetic, gravitational potential, elastic and thermal stores change when energy is transferred in a system
4
Kinetic energy
  • Kinetic energy half mass times speed squared, so a heavier or faster object stores more
5
Power calculation
  • Power calculation a motor lifting a load gives power as force times distance divided by time, and energy is conserved overall
6
Power
  • Power the rate of energy transfer, equal to work done divided by time and measured in watts
7
Energy stores
  • Energy stores kinetic, gravitational potential, elastic and thermal stores change when energy is transferred in a system
8
Kinetic energy
  • Kinetic energy half mass times speed squared, so a heavier or faster object stores more
9
Power calculation
  • Power calculation a motor lifting a load gives power as force times distance divided by time, and energy is conserved overall
10
Kinetic energy
  • Energy stored in a moving object.
11
Energy store
  • A place where energy is held, such as kinetic or thermal.
12
Thermal store
  • Energy store linked to temperature.
13
Gravitational field
  • A region in which a mass experiences a gravitational force.
14
Transfer pathway
  • The way energy moves between stores.
15
Heating
  • Energy transferred from a hotter to a colder object.

Key terms

TermMeaning
PowerThe rate of doing work or transferring energy.
WattThe unit of power, equal to 1 joule per second.
Kilowatt1000 watts.
Power ratingThe energy an appliance transfers per second.
EfficiencyUseful power output divided by total power input.

Avoid these mistakes

1
Mixing up work and power — power is work divided by time.
2
Leaving time in minutes or hours — convert to seconds before using P = W ÷ t.
3
Saying a more powerful machine does more work — it does the same work faster.
4
Calculating efficiency with input over output — use useful output over total input.

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