Units of Measurement of Energy
Energy is defined as the ability to do work i.e. energy produces work. As per the second law of thermodynamics, whenever the work is done by absorbing the energy (heat) from the reservoir, some heat is always rejected to the sink. The work and heat are also forms of energy; hence the units of measurement of energy, work and heat are same.
A number energy measurement systems exist at present but most commonly used system is SI. Here are the units of measurement of energy, work and heat in various systems:
MKS system (Metric system): Calories or cal: This unit is very commonly used to indicate the energy content of the fuel and food, capacity of refrigeration and air-conditioning system, etc. One calorie is the amount of heat required to raise the temperature of one gram of water by one degree Celsius. As such calorie is a very small unit hence larger unit Kilocalories or Kcal is more commonly used. 1Kcal = 1000 cal
SI unit system: Joule: This system is most commonly used now. Joule unit has been named after the famous scientist Joule who has made very important contributions in the field of thermodynamics especially in work and energy. Even Joule is a small unit; hence kilo Joule or KJ is used commonly. 1KJ = 1000J
British thermal unit: Btu: One Btu is the amount of energy needed to raise the temperature of one pound of water by one degree Fahrenheit.
Conversion from One Unit to the other
1Btu = 1055J or 252cal
1cal = 4.187J or 0.003969 Btu
1J = 0.2388cal or 0.0009481 Btu.
Units of Measurement of Power
The rate at which the work is produced from the energy is called as power. For instance let us suppose that there are two engines, both of which consume two gallons of gasoline and produce same amount of work i.e. 5KJ. Let us suppose that the first one produces this work in five seconds and the seconds one produces this work in 10 seconds. Accordingly the power produced by the first engine is 5/5 = 1KJ/s = 1KW and the power produced by second engine is 5/10 = 0.5KW.
Here are various units of power:
- SI unit: The SI unit of power is W. It is the amount of power produced by consumption of one Joule of energy in one second. The unit W is very small, larger unit like KW and MW are used commonly. 1KW = 1000W and 1MW = 1000KW. SI unit is most commonly used now.
- Horsepower (hp): This unit is more commonly used to denote the power produced by the engine of the automobiles. 1hp = 746W or 0.746KW
What is Energy?