Level: JHS, Basic 9 | Subject: Integrated Science | Strand: Forces and Energy | Topic: Energy Conservation and Efficiency
Curriculum reference: B9.4.1.1 Demonstrate understanding of energy transformation, conservation laws, and efficiency. Indicator B9.4.1.1.1: Apply the principle of conservation of energy to practical devices. All the exemplars are merged into one set of notes.
1. Conservation of Energy
Energy cannot be created or destroyed. It changes from one form to another, and the total stays the same. In every device, input energy = useful output energy + wasted energy.
2. Efficiency
Efficiency tells how much of the energy put into a device becomes useful output.
Efficiency = (useful energy output / total energy input) x 100%
It can also be written with work or power: efficiency = (useful work out / work in) x 100%, or (useful power out / power in) x 100%. No machine is 100% efficient, because some energy is always wasted as heat, sound or friction.
3. Energy Changes in Common Devices
| Device | Useful output | Wasted energy |
|---|---|---|
| Filament bulb | Light | Mostly heat |
| LED bulb | Light | A little heat (more efficient) |
| Electric fan | Moving air (kinetic) | Heat and sound |
| Electric kettle or iron | Heat | Heat lost to the surroundings |
| Car engine | Kinetic energy | Heat, sound and friction |
| Generator | Electrical | Heat and sound |
4. Worked Examples
Example 1 (electrical). A bulb takes in 100 J of electrical energy and gives out 20 J of light. Efficiency = 20 / 100 x 100 = 20%. Wasted energy = 80 J as heat.
Example 2 (mechanical). A machine does 600 J of useful work when 800 J of energy is supplied. Efficiency = 600 / 800 x 100 = 75%.
Example 3 (lifting a load). A pulley system lifts a 40 N load through 3 m using 200 J of effort. Useful work = 40 x 3 = 120 J. Efficiency = 120 / 200 x 100 = 60%.
Example 4 (finding useful output). A motor of efficiency 80% takes in 500 J. Useful output = 80 / 100 x 500 = 400 J.
Example 5 (electrical energy and cost). Energy in kilowatt hours (kWh) = power in kW x time in hours. A 2 kW heater used for 3 hours uses 2 x 3 = 6 kWh.
5. Improving Efficiency
- Oil moving parts to reduce friction.
- Use LED bulbs instead of filament bulbs.
- Insulate pipes, ovens and buildings to reduce heat loss.
- Service machines and vehicles regularly.
- Use efficient stoves and good pots with lids.
6. Energy Conservation for National Development
Energy conservation means using less energy to do the same job, so that supplies last longer and costs fall. In Ghana, the main steps are:
- Switching off lights and equipment that are not in use.
- Using energy-saving appliances and LED lighting.
- Reducing power losses on transmission lines and fixing illegal connections.
- Using renewable energy: solar, hydro, wind and biomass.
- Promoting efficient cookstoves and cleaner fuels such as LPG, which also protect forests.
- Public education in schools, homes and industry.
The benefits are lower bills, less pollution, fewer power cuts and more money for development.
Remember: Efficiency is always less than 100%. The rest of the energy is wasted, not lost.
Ready to practise?
Education Always (education.inspiringalways.com). Original notes written to follow the NaCCA Basic 9 Integrated Science curriculum.