Forms of energy and the words we use
Energy is the ability to do work, measured in joules (J). Common forms:
- Kinetic: energy of moving things. Ek = ½mv².
- Gravitational potential: energy of height. Eg = mgh.
- Elastic: stored in a stretched spring or rubber band.
- Chemical: stored in food, fuel and batteries.
- Electrical: carried by moving charges.
- Thermal: from the motion of particles; we feel it as temperature.
- Light (radiant) and sound: carried by waves.
- Nuclear: stored in the centre of atoms.
An energy transformation is a change from one form to another. An energy transfer moves energy from one object to another. A system is the part we study (for example, a kettle and its water).
The law of conservation of energy
Energy cannot be created or destroyed; it only changes form or moves. For any device:
Input energy = useful output energy + wasted energy
Wasted energy usually leaves as heat (thermal energy) and sometimes as sound. It still exists but is spread out and hard to use.
A falling ball
At the top a 0.5 kg ball 10 m up has Eg = 0.5 × 9.8 × 10 = 49 J. Just before landing, if air resistance is tiny, Ek ≈ 49 J. On impact the 49 J becomes sound, heat and a little bounce.
Energy flow diagram
A Sankey diagram draws arrows whose widths match the energy. The input arrow splits into a useful arrow and a wasted arrow; the widths add up.
Efficiency and power
Efficiency tells how much input becomes useful output:
efficiency = useful output energy ÷ input energy × 100 % (or use power instead of energy).
No real device reaches 100 %, because some energy always turns into heat.
Power is how fast energy is changed: P = E ÷ t, in watts (W); 1 W = 1 J/s. A 60 W bulb uses 60 J every second.
| Power station | Typical efficiency |
|---|---|
| Hydroelectric | about 85–90 % |
| Combined-cycle gas | about 55–60 % |
| Wind turbine | about 35–45 % of wind energy |
| Coal | about 33–40 % |
| Nuclear | about 33 % |
| Solar panel (PV) | about 15–22 % |
Thermal stations (coal, gas, nuclear) lose most energy as heat in cooling towers.
Energy-transformation technologies and sources
- Simple devices: a hand-crank torch (kinetic → electrical → light), a kettle (electrical → thermal), a catapult (elastic → kinetic).
- Complex devices join many changes. A hybrid car: chemical (fuel) → thermal → kinetic in the engine; on braking, kinetic → electrical stored in the battery (chemical); then back to kinetic in the motor.
- Thermal power station: chemical or nuclear → thermal (boil water) → kinetic (steam turbine) → electrical (generator).
- Hydro: gravitational → kinetic → electrical. Wind: kinetic of air → kinetic of blades → electrical. Solar cell: light → electrical directly.
Renewable sources are refilled by nature (sun, wind, flowing water, tides, geothermal, biomass). Non-renewable sources take millions of years to form or are limited (coal, oil, natural gas, uranium).
Making a plan for better energy use
Good choices look at efficiency, cost, pollution and reliability together. A practical plan for a school or home:
- Measure: read the electricity meter for a week.
- Find big wasters: old bulbs, standby devices, poor insulation.
- Swap to efficient devices (LEDs, star-rated fans and fridges, heat pumps).
- Add renewable supply where possible (rooftop solar, solar water heaters).
- Check again and compare.
Try it at home
Rub your hands fast for 10 seconds. Kinetic energy became thermal energy. Now touch a working phone charger, a TV and an LED bulb (carefully). The warmest one wastes the most energy as heat. Then test your guess with the device picker in step 6.
Key formulas and definitions
- Efficiency = useful output ÷ total input × 100 %
- Input energy = useful output + wasted energy
- Power P = E ÷ t (1 W = 1 J/s)
- E_k = ½ m v² and E_g = m g h
- Energy E = P × t (1 kWh = 3.6 × 10⁶ J)
Worked examples
1. A motor uses 500 J of electrical energy and gives 400 J of kinetic energy. Find the wasted energy and the efficiency.
Wasted = 500 − 400 = 100 J. Efficiency = 400 ÷ 500 × 100 = 80 %.
2. An LED lamp is 40 % efficient. How much light energy does it give from 1200 J of electricity?
Useful = 40 % × 1200 = 0.40 × 1200 = 480 J of light. The other 720 J is heat.
3. A 2000 W kettle runs for 3 minutes. How much energy does it use?
t = 3 × 60 = 180 s. E = P × t = 2000 × 180 = 360 000 J = 360 kJ.
4. Water falls 50 m through a hydro turbine at 1000 kg every second. If the station is 90 % efficient, what electrical power does it give? (g = 9.8 m/s²)
Energy per second = mgh = 1000 × 9.8 × 50 = 490 000 J/s = 490 kW. Electrical power = 0.90 × 490 = 441 kW.
5. A coal station gives 600 MW of electricity at 35 % efficiency. What power does it take in from coal, and how much is wasted?
Input = 600 ÷ 0.35 ≈ 1714 MW. Wasted = 1714 − 600 ≈ 1114 MW, mostly heat.
6. A 0.2 kg ball is thrown up at 10 m/s. Ignoring air, how high does it rise?
½mv² = mgh, so h = v² ÷ (2g) = 100 ÷ 19.6 ≈ 5.1 m. The mass cancels.
Common mistakes
- Saying energy is "used up" or "lost". It changes into less useful forms, usually heat, but the total stays the same.
- Giving an efficiency above 100 %. Useful output can never be more than the input.
- Mixing up energy (J) and power (W). Power is energy per second.
- Forgetting to change minutes or hours into seconds before using P = E ÷ t.