Conservation & Dissipation of Energy
Understanding how energy is transferred, stored, and wasted in physical systems
Energy cannot be created or destroyedThe Law of Conservation of Energy
"Energy cannot be created or destroyed, only transferred from one store to another or dissipated to the surroundings."
Input Energy
Total energy going into a system
Useful Energy
Energy transferred to where it's wanted
Dissipated Energy
Energy wasted to surroundings
Key Principle
Total energy input = Useful energy output + Wasted energy output
No energy is ever lost - it just becomes less useful!
Energy Stores & Transfers
Kinetic Energy
Energy of moving objects
Thermal Energy
Energy of hot objects
Gravitational Potential
Energy due to height
Chemical Energy
Energy stored in chemical bonds
Elastic Potential
Energy in stretched/compressed objects
Nuclear Energy
Energy stored in atomic nuclei
Dissipation of Energy
When energy is transferred, some is always dissipated (wasted) to the surroundings, usually as thermal energy.
Energy Flow in a Light Bulb
Only 10% of electrical energy is converted to light - the rest is dissipated as heat!
Common Ways Energy is Dissipated:
- Friction - Kinetic energy → Thermal energy
- Sound - Energy transferred as noise
- Air Resistance - Objects heating the air
- Electrical Resistance - Wires heating up
Efficiency
Efficiency tells us how much of the input energy is transferred usefully.
As a Decimal
Useful Output ÷ Total Input
As a Percentage
(Useful ÷ Total) × 100%
Example: LED Bulb (90% Efficient)
Example: Filament Bulb (10% Efficient)
Efficiency of Common Devices:
Sankey Diagrams
Sankey diagrams show energy flow - the width of the arrows represents the amount of energy.
LED Light Bulb
Efficiency: 90%
Car Engine
Efficiency: 25%
The thicker the arrow, the more energy is transferred that way!
Interactive Efficiency Calculator
Calculate Efficiency
Efficiency
Wasted Energy: 25 J
Real-World Applications
Electric Vehicles
Electric cars are much more efficient than petrol cars.
Electric cars waste less energy as heat
Home Insulation
Reduces energy dissipation from houses.
Loft insulation, double glazing, cavity walls
LED Lighting
LED bulbs are 90% efficient vs 10% for filament bulbs.
Uses 90% less electricity for same light
Regenerative Braking
Captures kinetic energy that would otherwise be wasted as heat.
Used in electric and hybrid vehicles
Solved Examples
Example 1: Calculating Efficiency
Question: A motor transfers 500J of electrical energy. It produces 400J of kinetic energy. Calculate its efficiency.
Example 2: Finding Wasted Energy
Question: A television with 250W input power is 65% efficient. Calculate the power wasted.
Example 3: Sankey Diagram
Question: A kettle uses 2000J of electrical energy. 1500J heats the water, the rest is wasted. Draw a Sankey diagram and calculate efficiency.
Energy Efficiency of Common Devices
| Device | Input Energy | Useful Output | Wasted Output | Efficiency |
|---|---|---|---|---|
| LED Light Bulb | 100 J | 90 J (light) | 10 J (heat) | 90% |
| Electric Motor | 100 J | 85 J (movement) | 15 J (heat) | 85% |
| Solar Panel | 100 J | 20 J (electricity) | 80 J (heat) | 20% |
| Car Engine | 100 J | 25 J (movement) | 75 J (heat, sound) | 25% |
| Filament Bulb | 100 J | 10 J (light) | 90 J (heat) | 10% |
Reducing Energy Dissipation
Lubrication
Reduces friction between moving parts, reducing heat dissipation.
Example: Oil in car engines
Insulation
Traps air to reduce thermal energy transfer.
Example: Loft insulation, double glazing
Streamlining
Reduces air resistance, saving energy.
Example: Aerodynamic cars
Low Resistance
Using materials with lower electrical resistance.
Example: Superconductors, thicker wires