Energy and Power
Understand the concepts of Energy and Power with diagrams, flow charts, formulas, examples, and student-friendly explanations.
What is Energy?
Energy is the ability to do work. Every moving or working object needs energy.
ENERGY
↓
Ability to Do Work
SI Unit of Energy
The SI unit of energy is Joule (J).
Forms of Energy
Examples
- Battery stores chemical energy.
- Bulb produces light energy.
- Fan uses electrical energy.
- Speaker produces sound energy.
Mechanical Energy
Mechanical energy is the sum of kinetic energy and potential energy.
Kinetic Energy
Kinetic energy is the energy possessed by a moving object.
Where:
- m = mass
- v = velocity
Moving Car 🚗 --------> Moving objects have kinetic energy.
Example
Find kinetic energy of a 2 kg object moving at 4 m/s.
KE = 1 2 × 2 × 4²
KE = 16 Joule
Potential Energy
Potential energy is the energy stored in an object due to its position or height.
Where:
- m = mass
- g = acceleration due to gravity
- h = height
📦
|
| h
|
__________|________
Object at height stores potential energy.
Example
A 5 kg object is kept at 10 m height. Find PE.
PE = 5 × 9.8 × 10
PE = 490 Joule
Law of Conservation of Energy
Energy can neither be created nor destroyed. It can only change from one form to another.
Electrical Energy
↓
Bulb 💡
↓
Light + Heat Energy
What is Power?
Power is the rate of doing work or using energy.
SI Unit of Power
The SI unit of power is Watt (W).
More Work in Less Time
↓
More Power
Units of Power
| Unit | Value |
|---|---|
| 1 Watt | 1 Joule/second |
| 1 Kilowatt | 1000 Watt |
| 1 Horsepower | 746 Watt |
Electric Power
Electric power is the rate at which electrical energy is consumed.
Where:
- V = Voltage
- I = Current
Electricity ⚡ ---> Fan 🌀 Electrical energy converts to mechanical energy.
Work-Energy Theorem
The work done by a force on an object is equal to the change in kinetic energy of the object.
Force Applied ---> 🚗
Speed Increases
↓
Kinetic Energy Increases
Example
A force increases the kinetic energy of a body from 20 J to 80 J.
Work Done = 80 - 20
Work Done = 60 J
Relation Between Force and Potential Energy
Potential energy is associated with conservative forces such as gravitational force and spring force.
Here:
- F = Conservative force
- U = Potential energy
- x = Displacement
Ball at Height
⚽
|
| h
|
______|______
Gravitational potential energy stored.
Elastic Potential Energy
When a spring is stretched or compressed, energy is stored in it. This is called elastic potential energy.
Where:
- k = Spring constant
- x = Extension or compression
Wall =====/\/\/\/\==== 📦
Spring
Stretching spring stores energy.
Commercial Unit of Energy
Electrical energy used in homes is measured in kilowatt-hour (kWh).
Electric Meter
⚡
Measures energy in kWh
Example
A 1000 W heater works for 2 hours.
Energy Used = 1 kW × 2 h
Energy = 2 kWh
Efficiency of Machines
Efficiency tells how much useful energy is obtained from the total energy supplied.
Input Energy ⚡
↓
Machine
↓
Useful Output + Heat Loss
Difference Between Energy and Power
| Energy | Power |
|---|---|
| Ability to do work | Rate of doing work |
| Measured in Joule | Measured in Watt |
| Can be stored | Cannot be stored |
Solved Numericals
Example 1
A machine does 500 J work in 10 seconds. Find power.
P = W t
P = 500 / 10
P = 50 Watt
Example 2
Find kinetic energy of a 4 kg object moving at 3 m/s.
KE = 1 2 × 4 × 3²
KE = 18 Joule
Quick Revision Flow Chart
Practice Questions
- Define energy.
- What is kinetic energy?
- Write the formula of potential energy.
- Define power.
- What is the SI unit of power?
- State the law of conservation of energy.
- Find power if 200 J work is done in 20 seconds.
- Differentiate energy and power.
- What is mechanical energy?
- Write two examples of energy transformation.