Energy and its forms
Energy is the capacity to do work, measured in joules. It comes in many forms: mechanical, heat, light, sound, electrical, magnetic, chemical and nuclear.
Energy and work
After a long game or heavy housework, you feel tired and hungry. You have used energy in doing work. Food and rest give it back. Energy is the capacity to do work. A body with more energy can do more work. So energy has the same unit as work, the joule (J). In practice, not all energy turns into useful work; some is always wasted, often as heat.
Forms of energy
| Form | What it is | Example |
|---|---|---|
| Mechanical | Energy due to motion (kinetic) or position (potential) | A moving bus; water stored in a dam |
| Heat (thermal) | Energy that flows from a hotter body to a colder one | A hot tawa warming a roti |
| Light | The energy that lets us see | Sunlight; a glowing LED bulb |
| Sound | Energy of vibrations, carried through a medium to our ears | A temple bell; a loudspeaker |
| Electrical | Energy of moving charges | Current running a fan or a mixer |
| Magnetic | Energy with which a magnet can pull iron | A magnet pulling a pin |
| Chemical | Energy stored in substances, released in reactions | Food, LPG, petrol, a dry cell |
| Nuclear | Energy released in nuclear reactions when some mass changes into energy | Nuclear power plants |
Activity: inflating balloons in two ways
You need two identical long balloons and a friend.
- 1Blow into one balloon with your lips sealed tightly around the whole opening.
- 2Your friend blows into the other balloon with lips covering only part of the opening, so some air leaks out.
- 3Blow for the same time, with about the same effort.
- 4★Observe: the balloon blown with the whole opening sealed becomes bigger.
- 5Conclude: both of you spent energy, but the leaky way wasted part of it as escaping air. Not all the energy you spend becomes useful work.
★ key step
Key points about energy
- Energy is measured in joules. Large amounts are given in kilojoules: 1 kJ = 1000 J.
- Energy can be stored and released later, as in a stretched rubber band, a wound spring, food or a charged cell.
- A stretched or compressed spring has potential energy because of its changed shape.
- Energy does not vanish when it is used. It changes into other forms, as the topic on conservation of energy shows.
- India generates part of its electricity from nuclear energy.
The energy printed on a biscuit packet is chemical energy stored in the food, given in kilocalories or kilojoules. Your body turns it into the muscular energy you use to walk, climb and lift.
1 marks: “What is the SI unit of energy?” The joule (J), the same as the unit of work.
2 marks: “Define energy. Name the form of energy in (a) a stretched bow, (b) LPG.” Energy is the capacity to do work. (a) Potential energy, due to its changed shape. (b) Chemical energy, released when the gas burns.
2 marks: “Name four forms of energy, with one source of each found in a home.” Electrical: the wall socket. Chemical: food or LPG. Light: a bulb. Heat: the flame of a gas stove. (Sound: a radio, is another.)
Energy is the ability to do work, and it shares the work's unit, the joule.