Energy — Class 5 Science (Samacheer Kalvi)
TN State Board (Samacheer Kalvi) Class 5 Science, Unit 3. Forms of energy, energy changes in daily life, and the law of conservation of energy. Taught in July and August.
1. What energy is
Energy is the capacity to do work. Its unit is the joule (J).
Nothing happens without it. A bus moves, a bulb glows, a plant grows — each needs energy, and each takes it from somewhere.
2. Mechanical energy
Energy possessed by an object due to its motion and position is called mechanical energy.
It comes in two kinds.
Kinetic energy is the energy possessed by a moving object. It is also called the energy of motion. Running water, a moving car, a falling apple, blowing wind, a running child — all have kinetic energy.
Potential energy is stored energy, held because of an object's position or condition. Water held behind a dam, a stretched rubber band, a book on a high shelf — none of them is moving, yet each is ready to do work.
Potential energy is not only about height. A stretched or compressed spring has it too, and so does the chemical arrangement inside a fuel.
3. Wind energy
Wind energy is the energy possessed by the wind. It is a renewable source, since the wind keeps blowing.
Tamil Nadu stands first in India in wind energy.
Its uses:
- Windmills use wind energy to generate electricity.
- Ships sail by the power of wind.
- Sports such as wind surfing, sailing and kite surfing use it.
- It can be used for pumping water.
4. Light energy
Light is a form of energy which travels as a wave. It contains a particle called the photon, which is a minute packet of energy.
Light has two properties worth remembering:
- It is the only form of energy visible to the human eye.
- It does not require any medium to travel, which is how sunlight crosses empty space to reach us.
Light travels at 3,00,000 km per second. Even at that speed, sunlight takes 8 minutes and 20 seconds to reach the Earth. The sunlight falling on you now left the Sun more than eight minutes ago.
Its uses:
- We see objects with the help of light energy.
- Plants use it to synthesise their food.
- Our skin uses it to synthesise Vitamin D.
- Electricity can be produced from it, in solar panels.
5. Electrical energy
Everything is made of tiny particles, and when charged particles flow through a wire we have an electric current. The energy it carries is electrical energy.
It is the most convenient form we have, because it travels along wires and turns easily into whatever else is needed.
6. Energy conversions
Energy changes from one form into another all the time.
A water dam. Water stored high in a dam has potential energy. When it falls, that potential energy is converted into kinetic energy. The kinetic energy of the moving water rotates the turbines, and electrical energy is generated.
Electrical appliances. As current flows into the coil of an electric device, the coil heats up.
| Device | Energy conversion |
|---|---|
| Electric iron box | Electrical energy to heat energy |
| Electric stove | Electrical energy to heat energy |
| Electric fan | Electrical energy to mechanical energy |
| Speaker | Electrical energy to sound energy |
| Generator | Mechanical energy to electrical energy |
| Burning diesel | Chemical energy to heat energy |
7. The law of conservation of energy
Energy can neither be created nor destroyed. It can only be converted from one form into another.
This is why saying "the energy was used up" is not quite right. When a battery goes flat, its chemical energy has not vanished — it has become light, heat and sound along the way.
Follow the chain far enough and it always closes. Sunlight becomes chemical energy in a plant; the plant becomes food; the food becomes movement in your muscles; the movement ends as heat.
8. Worked examples
Example 1. When diesel is burnt, chemical energy is converted into what?
Solution: Heat energy.
Example 2. Running water possesses which kind of energy?
Solution: Kinetic energy, the energy of motion.
Example 3. What is the unit of energy?
Solution: The joule.
Example 4. Name the energy conversion in an electric fan and in a speaker.
Solution: Fan: electrical to mechanical. Speaker: electrical to sound.
Example 5. State the law of conservation of energy.
Solution: Energy can neither be created nor destroyed; it can only be converted from one form into another.
Example 6. How long does sunlight take to reach the Earth, and how fast does light travel?
Solution: 8 minutes and 20 seconds, at 3,00,000 km per second.
Example 7. Trace the energy changes in a hydroelectric dam.
Solution: Stored water has potential energy. Falling water converts it into kinetic energy. The kinetic energy turns the turbines, producing electrical energy.
9. Practice
- Define energy and give its unit.
- What is mechanical energy, and what are its two kinds?
- Give two examples of objects with potential energy.
- Which state stands first in India in wind energy?
- Name three uses of wind energy.
- What is a photon?
- Why can light travel from the Sun to the Earth through empty space?
- Name two uses of light energy.
- What energy conversion happens in an electric iron box?
- Why is it wrong to say that energy is used up?
10. Answers
- Energy is the capacity to do work. Its unit is the joule.
- Energy possessed by an object due to its motion and position. Its two kinds are kinetic and potential energy.
- Water stored in a dam and a stretched rubber band (also a book on a high shelf).
- Tamil Nadu.
- Generating electricity through windmills, sailing ships, and pumping water.
- A minute packet of energy, the particle contained in light.
- Because light does not require any medium to travel.
- Seeing objects and letting plants make their food (also Vitamin D in skin, and solar electricity).
- Electrical energy to heat energy.
- Because energy is never destroyed, only converted into another form.
11. Summary
- Energy is the capacity to do work, measured in joules.
- Mechanical energy is due to motion and position, and splits into kinetic and potential energy.
- Kinetic energy belongs to moving things; potential energy is stored by position or condition.
- Tamil Nadu stands first in India in wind energy, used for electricity, sailing and pumping water.
- Light travels as a wave, contains photons, needs no medium, and moves at 3,00,000 km per second.
- Sunlight takes 8 minutes 20 seconds to reach the Earth.
- Light is the only form of energy visible to the human eye.
- In a dam, potential energy becomes kinetic energy, which turns turbines to make electricity.
- Electrical energy becomes heat in an iron box, motion in a fan, and sound in a speaker.
- Energy can neither be created nor destroyed, only converted from one form into another.
