Energy
1. What is Energy?
Energy is the capacity to DO WORK.
SI unit: Joule (J)
'Energy exists in MANY forms and can be transformed from one form to another. It is the CURRENCY of the universe — everything that happens requires energy.'
2. Forms of Energy
| Form | Description | Example |
|---|---|---|
| Kinetic Energy (KE) | Energy of MOTION | A moving car, flowing water |
| Potential Energy (PE) | Stored energy due to POSITION | Water in a dam, stretched bow |
| Heat energy | Energy of MOLECULAR motion | Fire, hot water |
| Chemical energy | Stored in CHEMICAL bonds | Food, batteries, fuel |
| Electrical energy | Flow of ELECTRIC charges | Lightning, household current |
| Light energy | Electromagnetic RADIATION | Sunlight, laser |
| Sound energy | Energy of VIBRATIONS | Music, speech |
| Nuclear energy | Energy from NUCLEAR reactions | Sun, nuclear reactors |
3. Kinetic Energy
The energy possessed by an object due to its MOTION.
Formula: KE = ½ mv²
Where m = mass of the object (kg), v = velocity (m/s).
Worked Example: A car of mass 1000 kg is moving at 20 m/s. Find its kinetic energy.
KE = ½ × 1000 × 20² = 500 × 400 = 200,000 J = 200 kJ
'Doubling the speed QUADRUPLES the kinetic energy (because v² is in the formula). A car at 80 km/h has FOUR times the KE of the same car at 40 km/h.'
4. Potential Energy
The energy possessed by an object due to its POSITION or CONFIGURATION.
Gravitational Potential Energy
Formula: PE = mgh
Where m = mass (kg), g = gravitational acceleration (9.8 m/s²), h = height (m).
Worked Example: A book of mass 2 kg is kept on a shelf at a height of 1.5 m. Find its potential energy. (Take g = 10 m/s²)
PE = 2 × 10 × 1.5 = 30 J
Elastic Potential Energy
Stored in STRETCHED or COMPRESSED objects (springs, rubber bands).
5. Law of Conservation of Energy
'Energy can NEITHER be created NOR destroyed. It can only be TRANSFORMED from one form to another.'
Total energy BEFORE = Total energy AFTER
Worked Example: A ball of mass 2 kg is dropped from a height of 10 m. Find its KE just before hitting the ground. (g = 10 m/s²)
PE at top = mgh = 2 × 10 × 10 = 200 J KE at top = 0 J Total energy at top = 200 J
By conservation, total energy just before hitting ground = 200 J. PE at bottom = 0 J (h = 0) So KE just before ground = 200 J
6. Power
Power is the RATE of doing WORK.
Formula: Power = Work done / Time taken
P = W/t
Unit: Watt (W). 1 W = 1 J/s.
1 kilowatt (kW) = 1000 W 1 megawatt (MW) = 10⁶ W 1 horsepower (HP) = 746 W
Worked Example: A motor does 5000 J of work in 25 seconds. Find its power.
P = 5000/25 = 200 W
7. Renewable vs Non-Renewable Energy
| Renewable Sources | Non-Renewable Sources |
|---|---|
| SUN (solar) | COAL |
| WIND | PETROLEUM (oil) |
| WATER (hydroelectric) | NATURAL GAS |
| TIDAL | NUCLEAR (uranium) |
| GEOTHERMAL | |
| BIOMASS |
Renewable: Can be REPLENISHED naturally in a short time. Environmentally FRIENDLY. Non-Renewable: LIMITED in supply. Will EVENTUALLY run out. Produce POLLUTION.
'India has a target of 500 GW of renewable energy capacity by 2030, with SOLAR power being the largest contributor.'
Common Mistakes and Fixes
| Mistake | Fix |
|---|---|
| 'KE = mv²' | KE = ½ mv² (half times mass times velocity squared). The ½ is ESSENTIAL |
| 'PE is always positive' | PE = mgh depends on the REFERENCE level. Below reference, PE is NEGATIVE |
| 'Power and energy are the same' | Power is RATE of using energy (energy per unit time). Energy is TOTAL capacity |
| 'Conservation means energy is 'used up'' | Energy is NEVER used up — it is TRANSFORMED into other forms (often less useful ones) |
ICSE Exam Focus (6–8 marks)
- 2-mark questions: Define energy, power, state SI units
- 3-mark questions: Numericals on KE = ½mv² or PE = mgh
- 4-mark questions: Conservation of energy numericals
- 5-mark questions: Power numericals (P = W/t)
- 6-mark questions: Classification of energy sources with reasoning
Self-Test
Q1. A body of mass 5 kg is moving with a velocity of 10 m/s. Find its KE. A1. KE = ½ × 5 × 10² = ½ × 5 × 100 = 250 J.
Q2. A stone of mass 2 kg is raised to a height of 5 m. Find its PE. (g = 10 m/s²) A2. PE = mgh = 2 × 10 × 5 = 100 J.
Q3. A machine does 600 J of work in 30 seconds. Find its power. A3. P = 600/30 = 20 W.
Q4. State the law of conservation of energy. A4. Energy can NEITHER be created NOR destroyed. It can only be transformed from one form to another. Total energy in an isolated system remains CONSTANT.
Q5. Give two examples of renewable and two examples of non-renewable sources of energy. A5. Renewable: Solar, wind, hydro, tidal. Non-renewable: Coal, petroleum, natural gas, nuclear fuel.
Q6. A ball of mass 500 g is thrown upward with a velocity of 20 m/s. Find its KE at the start. What is its PE at the highest point? (g = 10 m/s²) A6. KE = ½ × 0.5 × 400 = 100 J. At highest point, v = 0, so KE = 0. By conservation, PE = 100 J. Height = 100/(0.5×10) = 20 m.
