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1.7.1Motion, Forces and Energy

Energy — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

5 min read 11 parts Lesson 58 of 91 1 quiz

📚 Cambridge IGCSE Physics 0625 Hub / Topical Revision
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Lesson summary

1.7.1 Energy — IGCSE PHYSICS notes: Identify energy stores, describe transfers and use conservation of energy, kinetic energy and gravitational potential energy.Energy = what makes things happen, measured in joules (J); 1 kJ = 1000 J.Energy is a scalar quantity.Stores of energy:K…

  • Mechanical working
  • Electrical working
  • Principle of conservation of energy
  • What the symbols mean
  • Change in gravitational potential energy (Extended)
  • Falling object without air resistance

Part 1 of 9Energy and energy stores

Energy
Energy what makes things happen, measured in joules (J); 1 kJ = 1000 J.
Energy is a scalar quantity.
Stores of energy:
  • Kinetic — moving objects.
  • Gravitational potential — objects raised above the ground.
  • Elastic (strain) — stretched or compressed objects.
  • Chemical — fuels, food and batteries.
  • Thermal (internal) — hot objects.
  • Nuclear — the nuclei of atoms.
  • Electrostatic and magnetic — charges and magnets in fields.

Part 2 of 9Energy transfers

Mechanical working
  • Mechanical working a force moves an object through a distance.
Electrical working
  • Electrical working an electric current carries energy.
Heating
  • Heating energy moves from hot to cold by conduction, convection or radiation.
Waves
  • Waves light, sound and other waves carry energy.
Light and sound are ways of transferring energy, not stores.
Heat is a transfer pathway; the store is thermal (internal) energy.

Part 3 of 9Conservation of energy

1Principle of conservation of energy
Principle of conservation of energy energy cannot be created or destroyed, only transferred from one store to another.
2
The total energy stays constant.
3
Wasted energy usually ends up in the thermal store of the surroundings, where it is dissipated and less useful.
4Energy in
Energy in useful energy out + wasted energy out.

Part 4 of 9Sankey diagrams

1
A Sankey diagram shows energy transfers as arrows.
2
The width of each arrow is proportional to the amount of energy.
3
The input arrow splits into useful output and wasted output.
4
The useful and wasted outputs add up to the input.

Part 5 of 9Kinetic energy

Kinetic energy
Kinetic energy the energy of a moving object (Extended).
Equation
  • Ek = ½mv2, with Ek in J, m in kg and v in m/s.
    What the symbols mean: Ek = kinetic energy (J) · m = mass (kg) · v = speed (m s−1)
Doubling the speed quadruples the kinetic energy.

Part 6 of 9Gravitational potential energy

Gravitational potential energy
Change in gravitational potential energy (Extended)
  • ΔEp = mgΔh.
    What the symbols mean: Ep = gravitational potential energy gained (J) · m = mass (kg) · g = 9.81 N kg−1 · h = change in height (m)
  • ΔEp is in J, m in kg, g in N/kg and Δh in m.
Only the vertical height change counts.
Falling object without air resistance
  • Falling object without air resistance loss of gravitational potential energy equals gain in kinetic energy, so mgΔh = ½mv2.

Part 7 of 9Efficiency

Efficiency (Extended)
(useful energy output ÷ total energy input) × 100%.
It also equals (useful power output ÷ total power input) × 100%.
Efficiency can never exceed 100%.

Part 8 of 9Method for a falling object

1
Identify the starting store (gravitational potential) and the final store (kinetic).
2
Calculate ΔEp = mgΔh.
3
Apply conservation of energy — Ek gained = ΔEp lost, minus any wasted energy.
4
Rearrange ½mv2 = Ek to find v.
5
State the answer with units and sensible significant figures.

Part 9 of 9More from the specification

More from the specification
Energy store
  • Energy store energy is held in stores such as thermal, kinetic, gravitational potential and chemical, and is transferred between them
System
  • System the object or group of objects being considered when energy changes are tracked
Kettle example
  • Kettle example electrical energy transfers to the thermal store of the water as the element heats it
Definition of energy transfer
  • Definition of energy transfer means moving energy from one store to another by heating, forces, electric current or waves, with total energy conserved
Energy store
  • Energy store energy is held in stores such as thermal, kinetic, gravitational potential and chemical, and is transferred between them
System
  • System the object or group of objects being considered when energy changes are tracked
Kettle example
  • Kettle example electrical energy transfers to the thermal store of the water as the element heats it
Definition of energy transfer
  • Definition of energy transfer means moving energy from one store to another by heating, forces, electric current or waves, with total energy conserved

Key terms

TermMeaning
EnergyQuantity measured in joules that makes things happen.
Energy storeWhere energy is held, such as kinetic or chemical.
Energy transferMovement of energy from one store to another.
Kinetic energyEnergy of a moving object, Ek = ½mv².
Gravitational potential energyEnergy stored by height in a gravitational field.
Conservation of energyTotal energy stays constant; it is only transferred.
Sankey diagramDiagram where arrow width shows energy amount.

Avoid these mistakes

1
Saying energy is used up or lost — it is transferred to other stores, mostly thermal.
2
Calling heat a store — heat is a transfer; the store is thermal energy.
3
Mixing up stores and pathways — work, heating, electrical working and waves are pathways.
4
Forgetting to square the speed in kinetic energy.

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