Units of energy Class 9 Physics

Units of energy

Hello students, we know that energy is capacity of doing work. Hence magnitude of energy is always equal to that of work. Therefore the SI unit of work and energy are same i.e. joule.

Whereas joule is small unit, so to represent energy in higher extent we need to use some commercial units of energy such as calories, kilowatt-hour etc.

Heat energy, mechanical energy is generally measured in calories and kilo calories.

Remember that 1 joule= 4.184 calorie

1 kilocalories = 1000 calories.

Whereas electrical energy which is used in large scale in industrial section is measured in kilowatt-hour. It is the energy spends by any electric device of power (P) in time (t).

We know that SI unit of power is watt, and that of time is second then SI unit of energy used here is watt-second (W-s). For large scale instruments, it is written as kilowatt-hour (kW-hr)

Remember that, 1 kilowatt= 1000 watt.

Hence if we consider an instrument of power 1000 watt is used for 1 hour then the energy spent/consumed is given as

1 kilowatt-hour= 1000 watt × 3600 sec= 3.6×106 joule


Ex:1) Heat energy of 120 joules is required to cook a food. Energy in calorie and kilocalories will be?

Ans: Here, E= 120 joule

We know that,

1 joule = 4.18 calorie

1 calorie = 1/4.184 joule

Hence 120 joule = 120/4.184 = 28.68 calorie

120 joule =120/4.184 = 0.02868 kilo calorie

Ex:2) Electric geyser of power rating 3200 watt works for 2 hours daily. Find the energy in kilowatt-hr and in joule.

Ans: Here P=3200 watt = 3.2 kW

t = 2 hr = 2×3600 sec = 7200 second

Energy in kilowatt – hr = 3.2×2=6.4 kW-hr

Energy in joule = 3200×7200 = 230, 40,000 = 2.304×107 joule

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