Conservation of Energy
The Principle of Conservation of Energy
Energy cannot be created or destroyed; it can only be transferred from one store to another.
This is one of the most fundamental laws of physics and applies to every process — no exceptions.
Energy Stores
Edexcel 1PH0 uses the energy stores and pathways model:
| Store | Formula | Key variables |
|---|---|---|
| Kinetic energy (KE) | KE = ½mv² | m in kg, v in m/s |
| Gravitational potential energy (GPE) | GPE = mgh | m in kg, g in N/kg, h in m |
| Elastic potential energy (EPE) | EPE = ½ke² | k = spring constant (N/m), e = extension (m) |
| Thermal (internal) | — | Temperature × mass × specific heat |
| Chemical | — | (food, fuel, batteries) |
| Nuclear | — | (radioactive decay, fission) |
Energy Transfers
Energy transfers between stores via pathways:
- Mechanical (forces doing work)
- Electrical (current in a circuit)
- Heating (thermal conduction, convection, radiation)
- Radiation (light, sound, EM waves)
Work done = force × distance (in the direction of the force): W = Fd (unit: joule, J = N m)
When a resultant force does work on an object, it changes its kinetic energy: W = ΔKE.
Efficiency
Efficiency = useful output energy ÷ total input energy × 100%
Or using power: efficiency = useful output power ÷ total input power.
A machine with 80% efficiency wastes 20% as heat/sound. No real machine is 100% efficient due to friction and heating effects.
Sankey diagrams show energy flow: arrow width represents the amount of energy. The useful output arrow and waste arrow(s) branch from the main input arrow.
Example: A light bulb uses 100 J of electrical energy → 10 J light (useful) + 90 J heat (wasted). Efficiency = 10/100 = 10%.
Core Practical 3 — Investigating specific heat capacity
Equipment: electric heater, metal block (aluminium), joulemeter (or ammeter + voltmeter + stopwatch), thermometer, insulating jacket, digital balance.
Method:
- Measure mass of block (m).
- Place heater and thermometer in the holes of the block.
- Record initial temperature. Switch on heater. Record temperature every minute for 10 minutes.
- Record energy supplied: E = V × I × t from joulemeter.
- Calculate c: E = mcΔT → c = E ÷ (m × ΔT).
Sources of error: heat lost to surroundings (use insulating jacket); thermal contact between thermometer and block. Repeat and average for reliability.
GPE–KE Transfers
A falling object converts GPE to KE (ignoring air resistance): mgh = ½mv² → v = √(2gh)
Example: ball dropped from 5 m: v = √(2 × 9.8 × 5) = √98 ≈ 9.9 m/s at ground.
In reality, some GPE converts to thermal energy due to air resistance.
AI-generated · claude-opus-4-7 · v3-edexcel-physics