Energy-Definition, Unit of energy, Relationship between different units of energy

Details of energy, the definition of energy, a unit of energy, and the relationship between them

Definition of energy: 

The ability or capability of a body for doing work is known as energy. There are several kinds of forms of energy, but the most important ones are – 1. electrical energy, 2. mechanical energy, and 3. thermal energy.

Now, remember these energies are all interchangeable.

Unit of energy:

Energy is a measurable quantity so we need to assign units to energies, and as the energies are interchangeable so their unit should also have a relation between them or the same unit can be assigned because the energies are just in different forms.

1. Electrical Energy: The SI unit of electrical energy is watt-second also known as joule. Suppose there are two points that have a potential difference of 1 volt now a current of 1 ampere is flowing from one point to another. 

So Power is equal to 1 volt * 1 ampere =1 watt. 

Now if this continues for 1 second, then the energy consumed will be 1 watt * 1 second which is equal to 1 watt-second. 

In case the system is AC then another factor called power factor comes into the picture, 

so, watt-second =1 volt * 1 ampere * power factor *1 second.

2. Mechanical Energy:

The unit of mechanical energy in the SI system is Newton’s meter also known as the joule. When a force of 1 Newton makes a body move by a 1-meter distance then the work done on the body is 1 joule or 1 Newton meter.

3. Heat Energy:

The unit of heat energy in the SI unit is the joule. There are also other units that as calories, British thermal units, or B. Th. U etc. Calories stand for the amount of heat energy that is required to increase the temperature of 1 gram of water by 1 degree centigrade.

similarly, the British thermal unit can be expressed as the heat energy required to increase the temperature of one pound of water by one degree Fahrenheit.

Relationship between the unit of energy

All three energies as mentioned above are convertible from one form to another, so their units also have a relationship between them, and another thing is there to remember that all three energies can do some work so they should have a common unit, and we can see that all the three energies have a common unit that is “Joule” now let us see the relationship between them.

Relationship between kWhr and Joule

1 kWhr (Unit)

= 1000 watt * 1 hr

=1*10^3 watt*3.6*10^3 sec.

=3.6*10^6 watt- sec= 3.6*10^6 Joule

Relationship between kWhr and N-m

1 N-m = 1 Joule

1 kWhr= 3.6*10^6 Joule

= 3.6*10^6 N-m

Relationship between Calorie and kWhr

1 kWhr=3.6*10^6 Joule


1 Calorie=4.18 Joule (Experimental value)


1 Joule= 1/4.18 Cal


1 kWhr= 3.6*10^6/4.18 Cal

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