Energy
1. What Is Energy?
ENERGY is the capacity to DO WORK.
SI unit: joule (J). Also measured in calorie (cal), kilowatt-hour (kWh).
1 J = 1 N·m (one newton-metre).
Key Facts About Energy
- Energy CANNOT be created or destroyed (Law of Conservation of Energy).
- Energy can be TRANSFORMED from one form to another.
- Energy exists in MANY forms.
2. Different Forms of Energy
| Form of Energy | Description | Examples |
|---|---|---|
| Mechanical | Energy of motion or position | Moving car, stretched spring |
| Heat (Thermal) | Energy of moving particles | Fire, sunlight, body heat |
| Light | Energy that travels as waves | Sunlight, bulb, laser |
| Sound | Energy of vibrations | Music, speech, thunder |
| Electrical | Energy of moving charges | Batteries, power lines |
| Chemical | Energy stored in chemical bonds | Food, petrol, batteries |
| Nuclear | Energy from nucleus of atoms | Nuclear power plants, Sun |
| Magnetic | Energy of magnetic fields | Magnets, electromagnets |
3. Kinetic Energy and Potential Energy
Kinetic Energy (KE)
Energy possessed by a moving object due to its MOTION.
Formula: KE = ½ mv² m = mass of object, v = velocity
Factors affecting KE:
- KE increases with MASS (doubling mass doubles KE).
- KE increases with SQUARE of SPEED (doubling speed QUADRUPPLES KE).
Examples: A moving car, a flowing river, wind, a running athlete.
Potential Energy (PE)
Energy STORED in an object due to its POSITION or STATE.
Gravitational Potential Energy: PE = mgh (m = mass, g = gravity, h = height).
Examples:
- A book on a shelf (gravitational PE).
- A stretched rubber band (elastic PE).
- Water stored in a dam (gravitational PE).
Worked Example (ICSE 2024, 3 marks)
'A ball of mass 2 kg is lifted to a height of 5 m. Find its potential energy. (g = 10 m/s²)'
Solution: PE = mgh = 2 × 10 × 5 = 100 J.
4. Law of Conservation of Energy
Energy can neither be CREATED nor DESTROYED. It can only be TRANSFORMED from one form to another.
Total energy in an isolated system ALWAYS remains constant.
Simple Demonstration
A swinging pendulum:
- At the highest point: Maximum PE, Zero KE.
- At the lowest point: Maximum KE, Zero PE.
- At any point: PE + KE = Constant (ignoring air resistance).
Energy Transformation Examples
| Device | Input Energy | Output Energy |
|---|---|---|
| Electric bulb | Electrical | Light + Heat |
| Fan | Electrical | Mechanical (Kinetic) |
| Car engine | Chemical (petrol) | Mechanical + Heat |
| Solar panel | Light (sun) | Electrical |
| Microphone | Sound | Electrical |
5. Sources of Energy
Renewable Energy Sources
Energy sources that can be REPLENISHED naturally and are essentially INEXHAUSTIBLE.
| Source | How It Works |
|---|---|
| Solar energy | Sunlight is converted to electricity (solar panels) or heat |
| Wind energy | Wind turns turbines to generate electricity |
| Hydro energy | Flowing water turns turbines |
| Tidal energy | Ocean tides generate power |
| Geothermal energy | Heat from within the Earth |
| Biomass | Organic matter burned or converted to fuel |
Non-Renewable Energy Sources
Energy sources that are FINITE and will eventually be EXHAUSTED.
| Source | Examples |
|---|---|
| Fossil fuels | Coal, petroleum, natural gas |
| Nuclear fuels | Uranium, plutonium |
Comparison
| Feature | Renewable | Non-Renewable |
|---|---|---|
| Availability | Abundant | Limited |
| Environmental impact | Low (clean) | High (pollution) |
| Cost over time | Decreasing | Increasing |
| Examples | Sun, wind, water | Coal, oil, gas |
6. ICSE Exam Focus
| Topic | Marks | Frequency |
|---|---|---|
| Forms of energy | 2 marks | High |
| KE and PE (concept and calculation) | 3 marks | Very High |
| Conservation of energy | 2 marks | Medium |
| Energy transformations | 2-3 marks | High |
| Renewable vs non-renewable | 2 marks | Very High |
Common Mistakes
- Confusing KE and PE (KE is motion, PE is position/state).
- Forgetting the ½ in KE = ½mv².
- Saying energy is 'used up' (it is TRANSFORMED, not destroyed).
- Not specifying the TYPE of potential energy (gravitational, elastic, chemical).
Self-Test (5 Questions)
Q1. The SI unit of energy is: (1 mark)
- A) Newton
- B) Joule
- C) Watt
- D) Pascal
Q2. 'A 10 kg object is at a height of 8 m. Find its PE (g = 10 m/s²).' (2 marks)
Q3. Which energy transformation occurs in a television? (2 marks)
Q4. Name ONE renewable and ONE non-renewable energy source. (2 marks)
Q5. 'What is the KE of a 1 kg ball moving at 4 m/s?' (2 marks)
Answers
A1. B) Joule. A2. PE = 10 × 10 × 8 = 800 J. A3. Electrical → Light + Sound + Heat. A4. Renewable: Solar energy / Wind / Hydro. Non-renewable: Coal / Petroleum / Natural gas. A5. KE = ½ × 1 × 4² = ½ × 1 × 16 = 8 J.
