# Concept of Energy

• Energy is the capacity to do work
• Relationship between work and energy
• Units of energy

## Notes

COMMERCIAL UNIT OF ENERGY

The unit joule is too small and hence is inconvenient to express large quantities of energy. We use a bigger unit of energy called kilowatt hour (kW h).

Let us say we have a machine that uses 1000 J of energy every second. If this machine is used continuously for one hour, it will consume 1 kW h of energy. Thus, 1 kW h is the energy used in one hour at the rate of 1000 Js–1 (or 1 kW).

1 kW h = 1 kW ×1 h

= 1000 W × 3600 s

= 3600000 J 1 kW h

= 3.6 × 106 J.

The energy used in households, industries and commercial establishments are usually expressed in kilowatt hour. For example, electrical energy used during a month is expressed in terms of ‘units’. Here, 1 ‘unit’ means 1 kilowatt hour.

## Notes

Energy and Work:-

We, in our daily lives, use energy from various sources for doing work.

We use diesel to run our trains. We use electricity to light our street-lamps. Or we use energy in our muscles to cycle to school.

The muscular energy for carrying out physical work, electrical energy for running various appliances, chemical energy for cooking food or running a vehicle all come from some source.

Kinetic energy of the wind can be used to do work. This energy was harnessed by windmills in the past to do mechanical work. For example, in a water-lifting pump, the rotatory motion of windmill is utilised to lift water from a well.

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