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
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
Section 1 of 11Energy 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.
Section 2 of 11Energy 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.
Section 3 of 11Conservation 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.
Section 4 of 11Sankey 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.
Section 5 of 11Kinetic 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.
Section 6 of 11Gravitational 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.
Section 7 of 11Efficiency
Efficiency (Extended)
(useful energy output ÷ total energy input) × 100%.
It also equals (useful power output ÷ total power input) × 100%.
Efficiency can never exceed 100%.
Section 8 of 11Method 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.
Section 9 of 11More 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
Section 10 of 11Key terms
| Term | Meaning |
|---|---|
| Energy | Quantity measured in joules that makes things happen. |
| Energy store | Where energy is held, such as kinetic or chemical. |
| Energy transfer | Movement of energy from one store to another. |
| Kinetic energy | Energy of a moving object, Ek = ½mv². |
| Gravitational potential energy | Energy stored by height in a gravitational field. |
| Conservation of energy | Total energy stays constant; it is only transferred. |
| Sankey diagram | Diagram where arrow width shows energy amount. |
Section 11 of 11Exam technique
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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