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What is Energy?
- Energy is the capacity to do work.
- Anything that can do work has energy.
What are the different forms of energy?
| Form of energy | What it is | Example |
|---|---|---|
| Mechanical energy | Energy due to motion or position of objects (energy of motion is called kinetic energy, energy of position is called potential energy — we will learn these in the next section) | A moving bicycle, a rolling ball, a flowerpot at a height |
| Thermal energy | Energy that makes things warm or hot | Heat from a stove, hot water |
| Light energy | Energy that allows us to see | Light from the Sun, from a bulb |
| Sound energy | Energy of vibrations of air or other molecules | A ringing bell, our voice |
| Electrical energy | Energy related to position or motion of charges | Current in wires running a fan |
| Nuclear energy | Energy stored in the nuclei of atoms | Energy of the Sun, nuclear power plant |
| Chemical energy | Energy stored in fuels and food in the form of chemical bonds between atoms | Petrol, food we eat, a battery |
Can energy change from one form to another?
- Yes. It is possible to change energy from one form to another.
| Device / Activity | Energy change |
|---|---|
| Electric bulb | Electrical energy → Light energy |
| Electric water heater | Electrical energy → Thermal energy |
| Food powering our muscles | Chemical energy → Mechanical energy |
| Ringing bell | Mechanical energy → Sound energy |
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🤔 Pause and Ponder 3 (from the book)
When you pedal a bicycle on a flat road, your muscles supply energy. In what forms does this muscular energy appear as you ride?View answer
Answer- Our muscles use chemical energy from food to do work on the pedals.
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This energy appears in different forms as we ride:
- Mechanical energy — motion of the bicycle and our body
- Thermal energy — heat in our body, and heat due to friction between the tyres, road and moving parts
- Sound energy — sound of the wheels, chain and air
- So chemical energy of food changes into mechanical, thermal and sound energy.
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📋 Revise, Reflect, Refine, Q4 (Page 137)
For each of the following situations, identify the energy transformation that takes place:View answer
AnswerSituation Energy transformation (i) A truck moving uphill Chemical energy of fuel → Mechanical energy (motion + height) (ii) Unwinding of a watch spring Potential energy of spring → Mechanical energy of moving parts (iii) Photosynthesis in green leaves Light energy → Chemical energy (stored in food) (iv) Water flowing from a dam Potential energy → Kinetic energy (v) Burning of a matchstick Chemical energy → Thermal + Light energy (vi) Explosion of a fire cracker Chemical energy → Thermal + Light + Sound + Mechanical energy (vii) Speaking into a microphone Sound energy → Electrical energy (viii) A glowing electric bulb Electrical energy → Light + Thermal energy (ix) A solar panel Light energy → Electrical energy
Which form will we study in this chapter?
- Among all these, we will now look more closely at mechanical energy .
- Why? Because it is directly connected to the forces and motions we studied earlier.
- Mechanical energy has two types — kinetic energy (motion) and potential energy (position).
- Lets start with Kinetic Energy in the next section.
📝 Important points — 7.3 Forms of Energy
| Point | Detail |
|---|---|
| Energy | Capacity to do work |
| 7 forms | Mechanical, thermal, light, sound, electrical, nuclear, chemical |
| Energy conversion | Energy can change from one form to another |
| This chapter's focus | Mechanical energy — kinetic + potential |
💡 Worth remembering
- Every device around us is an energy converter — bulb, heater, bell, our own body.
- Energy is never "used up" — it only changes form.
- Mechanical energy = kinetic (motion) + potential (position). That's our next topic.
✅ Quick self-check
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Which form of energy is stored in food?
View Answer
- Chemical energy — stored in the chemical bonds between atoms.
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What energy change happens in a fan?
View Answer
- Electrical energy → Mechanical energy (rotation of the blades).
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A firecracker explodes. Name any 3 forms of energy produced.
View Answer
- Thermal, light and sound energy (mechanical energy is also produced).
Key terms and units
| Term | Meaning |
|---|---|
| Mechanical energy | Energy due to motion or position of objects |
| Chemical energy | Energy stored in chemical bonds of fuels and food |
| Thermal energy | Energy that makes things warm or hot |
