Question

Set the following units of energies in increasing order.
i) Joule
ii) eV
iii) kWh
iv) erg

Answer 1

Joule is the SI out of energy. Erg is the CGS unit of energy. 1 eV is the equal to the work done in moving an electron between a potential difference of 1 V. 1 kWh is equal to energy consumed by an electric device in a time of 1 hour, which is working with a power of 1kW.

Complete answer:
Let us discuss each of the units of energies given.
i) Joule :- Joule is the SI out of energy. We know that energy is equivalent to work done on a body. Hence, we can see that joule is a unit of work done. The unit Joule is usually used for normal work done on bodies in daily lives.
ii) eV :- The full form of eV is electron-volt. This unit of energy is used in the case of analysation of microscopic elements like electrons. 1 eV is the equal to the work done in moving an electron between a potential difference of 1 V. Therefore, $1eV=1.6\times {{10}^{-19}}J$.
iii) kWh :- The full form of kWh is kilowatt-hour. This unit is used for commercial purposes of electric circuits. 1 kWh is equal to energy consumed by an electric device in a time of 1 hour, which is working with a power of 1kW.
And it is found that $1kWh=3.6\times {{10}^{6}}J$.
iv) erg :- It is the CGS unit of energy. And $1erg={{10}^{-7}}J$.

Now, if we compare the four units of energy with respect to the unit Joule, then we get that From the above four points, we get that 1 eV < 1 erg < 1 J < 1 kWh.

Note: Some students may get confused between kilowatt-hour (kWh) and watt W.As we already know that kWh is a unit of energy. This is because the product of power and time results in energy. However, watt is a unit of power.