Energy
What Is Energy?
Energy is the ability to do work or cause change. It exists in many different forms and can be transferred from one form to another, but it can never be created from nothing or destroyed — this is the law of conservation of energy.
Energy is measured in joules (J). Larger amounts are measured in kilojoules (kJ) or megajoules (MJ).
Forms of Energy
| Energy type | Description | Example |
|---|---|---|
| Kinetic (KE) | Energy of movement | Moving car, flowing water, wind |
| Gravitational potential (GPE) | Energy due to height above the ground | Ball at top of ramp, water in a reservoir |
| Elastic potential | Energy stored in stretched or compressed objects | Spring, elastic band, bow |
| Chemical | Energy stored in chemical bonds | Food, fuel, batteries |
| Thermal (heat) | Internal energy due to temperature | Hot water, steam |
| Electrical | Energy carried by moving charges | Electric current |
| Light (radiant) | Electromagnetic radiation energy | Sunlight, laser |
| Sound | Vibrations travelling through matter | Music, thunder |
| Nuclear | Energy stored in atomic nuclei | Nuclear fuel (uranium), the Sun |
The Law of Conservation of Energy
Energy cannot be created or destroyed. It can only be transferred from one store to another or transformed from one type to another.
Total energy input = Total energy output (including wasted energy)
This means whenever energy seems to "disappear," it has actually been converted to another form (usually heat, which spreads out into the surroundings and is hard to use again).
Energy Transfers
Energy transfers can be described using:
- Word equations: e.g., chemical energy → kinetic energy + thermal energy
- Sankey diagrams — arrows showing useful and wasted energy (width proportional to amount)
Common energy transfers:
| Situation | Energy transfer |
|---|---|
| Ball rolling down a hill | Gravitational potential → kinetic |
| Electric kettle | Electrical → thermal |
| Burning fuel | Chemical → thermal + light |
| Stretching a rubber band | Chemical (in muscles) → elastic potential |
| Loudspeaker | Electrical → sound + thermal |
| Solar panel | Light → electrical |
| Wind turbine | Kinetic → electrical |
| Mobile phone charging | Electrical → chemical |
| Car engine | Chemical → kinetic + thermal |
Calculating Kinetic Energy
KE = 0.5 x m x v²
Where m = mass (kg), v = speed (m/s), KE = kinetic energy (J)
Example: A car of mass 1000 kg travelling at 20 m/s: KE = 0.5 x 1000 x 20² = 0.5 x 1000 x 400 = 200,000 J (200 kJ)
Note: doubling speed quadruples kinetic energy (because v is squared). This is why speed is so dangerous in road accidents.
Calculating Gravitational Potential Energy
GPE = m x g x h
Where m = mass (kg), g = gravitational field strength (10 N/kg on Earth), h = height (m), GPE = gravitational potential energy (J)
Example: A 2 kg ball raised 5 m: GPE = 2 x 10 x 5 = 100 J
Efficiency
No machine is 100% efficient — some energy is always wasted (usually as heat or sound).
Efficiency = (useful energy output / total energy input) x 100%
Example: A light bulb converts 100J electrical energy into 10J light and 90J heat. Efficiency = (10 / 100) x 100% = 10%
The 90J of heat is the wasted energy — it spreads into the surroundings and cannot be recovered usefully.
Ways to improve efficiency:
- Lubrication — reduces friction in engines and machines
- Insulation — reduces heat loss in buildings and boilers
- LED lighting — more efficient than incandescent bulbs (10% → ~90% efficiency)
- Regenerative braking — electric cars recover kinetic energy as electrical energy when braking
Energy Resources
Non-Renewable Energy Sources
| Source | How it works | Advantages | Disadvantages |
|---|---|---|---|
| Coal | Burned to heat water → steam → turbine → generator | Reliable, cheap, abundant | Produces CO2, SO2; finite; polluting |
| Oil | Burned in engines or power stations | Energy-dense; versatile | CO2 emissions; finite; oil spills |
| Natural gas | Burned to heat water or directly | Cleaner than coal; flexible | CO2 emissions; finite |
| Nuclear | Fission of uranium heats water → turbine | No CO2; very energy-dense | Radioactive waste; expensive; risk of accidents |
Renewable Energy Sources
| Source | How it works | Advantages | Disadvantages |
|---|---|---|---|
| Solar | Photovoltaic panels convert light to electricity | No CO2; low maintenance | Intermittent; expensive to install |
| Wind | Wind spins turbines | No CO2; land can still be farmed | Intermittent; visual and noise impact |
| Hydroelectric | Falling water spins turbines | Reliable; can respond to demand | Floods valleys; impacts river ecosystems |
| Tidal | Tidal flow drives turbines | Predictable; no CO2 | Few suitable sites; expensive |
| Geothermal | Steam from underground heat drives turbines | Reliable; low emissions | Only viable in volcanic regions |
| Biomass | Burning biological material | Carbon-neutral (if replanted) | Produces CO2 when burned; land use |
Energy and the Environment
Burning fossil fuels releases carbon dioxide, contributing to the enhanced greenhouse effect and climate change.
Moving to renewable energy reduces CO2 emissions but requires:
- Storage solutions (batteries) for intermittent sources like solar and wind
- New infrastructure and investment
- Changes in behaviour and consumption
Key Terms
| Term | Definition |
|---|---|
| Energy | The ability to do work; measured in joules |
| Kinetic energy | Energy of a moving object; KE = 0.5 x m x v² |
| Gravitational potential energy | Energy due to height; GPE = m x g x h |
| Conservation of energy | Energy cannot be created or destroyed, only transferred |
| Efficiency | Ratio of useful energy output to total energy input, as a percentage |
| Renewable energy | Energy from sources that are naturally replenished |
| Non-renewable energy | Energy from finite sources (fossil fuels, nuclear) |
| Sankey diagram | Diagram showing energy transfers with arrow widths proportional to energy amounts |
Worked Example
Q: A 500g ball is dropped from a height of 10m. Calculate its kinetic energy just before it hits the ground. Assume all GPE converts to KE (g = 10 N/kg).
Step 1: Calculate the GPE at the top: GPE = m x g x h = 0.5 x 10 x 10 = 50 J
Step 2: By conservation of energy, all GPE converts to KE just before impact: KE = GPE = 50 J
Step 3 (bonus): Find the speed at impact using KE = 0.5 x m x v²: 50 = 0.5 x 0.5 x v² v² = 50 / 0.25 = 200 v = √200 ≈ 14.1 m/s
Common Mistakes
- Saying energy is "used up" — energy is never destroyed; it is always transferred to another form (usually heat spreading into surroundings).
- Confusing efficiency over 100% — no device can have efficiency greater than 100%; that would violate conservation of energy.
- Forgetting to square the velocity in the kinetic energy formula — KE = 0.5 x m x v² (v is squared).
- Calling nuclear energy "renewable" — nuclear fuel (uranium) is finite and not replenished, so it is non-renewable (though it produces very little CO2).
Tips and Tricks
- Efficiency mnemonic: useful OUT divided by total IN, times 100.
- In KE calculations, always check your units — mass in kg, speed in m/s, energy in J.
- Sankey diagrams: the total width going in = total width going out (conservation of energy); the narrow "wasted" arrow usually goes downward.
- When a question says "assume no energy is lost to friction/heat," all one form converts directly to another — use conservation of energy to find unknowns.