# Energy – Topper Level Notes (Physics, Grade 10 NEB)

## 1. What is Energy?
- **Definition:** Energy is the capacity of a body to do work.
- **SI Unit:** Joule (J)

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## 2. Types of Energy

**A. Kinetic Energy (KE)**
- Energy due to motion.
- **Formula:** KE = ½ mv²  
  (m = mass in kg, v = velocity in m/s)

**B. Potential Energy (PE)**
- Energy due to position or state.
- **Gravitational PE:** PE = mgh  
  (g = 9.8 m/s², h = height in m)
- **Elastic PE:** PE = ½ kx²  
  (k = spring constant, x = extension/compression)

**C. Other Important Types**
- **Chemical, Electrical, Thermal, Nuclear, Light, and Sound Energy**

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## 3. Law of Conservation of Energy 
- **Statement:** Energy can neither be created nor destroyed, only changed from one form to another.
- **Mathematical Form:**  
  Initial Total Energy = Final Total Energy  
  KE₁ + PE₁ = KE₂ + PE₂

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## 4. Sources of Energy
**A. Renewable:** Solar, Wind, Hydro, Biomass, Geothermal  
**B. Non-renewable:** Coal, Oil, Natural Gas, Nuclear

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## 5. Major Energy Transformations

| Device/Process        | Original Form   | Final Form        |
|----------------------|-----------------|-------------------|
| Torch                | Chemical        | Light             |
| Hydroelectric Plant  | Potential       | Electrical        |
| Car Engine           | Chemical        | Kinetic/Heat      |
| Electric Bulb        | Electrical      | Light/Heat        |

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## 6. Work-Energy Principle  
- **Work-Energy Theorem:**  
  The net work done on an object equals the change in its kinetic energy.  
  W = KE₂ – KE₁

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## 7. Efficiency
- **Formula:** Efficiency (%) = (Useful output energy / Total input energy) x 100

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## 8. Energy Crisis & Conservation

- **Crisis:** Overuse of non-renewable resources → shortages & environmental damage.
- **Conservation:** 
  - Use energy-saving devices.
  - Switch to renewable sources.
  - Practice efficient consumption.

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## 9. Common Exam Questions

1. Define energy and its SI unit.
2. Explain the law of conservation of energy with an example.
3. List differences between renewable and non-renewable energy.
4. A 2kg mass is lifted to a height of 5m. Calculate PE. (Ans: 98J)
5. What is the efficiency if a machine gives 600J of output from 1000J input? (Ans: 60%)

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## 10. Important Formulas Summary Table

| Formula            | Meaning                |
|--------------------|------------------------|
| KE = ½ mv²         | Kinetic Energy         |
| PE = mgh           | Potential Energy       |
| W = Fd             | Work Done              |
| P = W/t            | Power                  |
| Efficiency = (Output/Input) x 100 | Efficiency |

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## 11. Easy Memory Aids

- **KE:** “Moving Mass, Velocity Squared!”
- **PE:** “Mass x Gravity x Height!”
- **Law of Conservation:** “Energy isn't made or lost, only swapped!”

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