Energy—How Things Work - Sources of Energy: Fuel and Electricity

Energy is defined as the capacity or ability to do work. It is measured in Joules ($J$).

Fuel is a substance that releases energy, usually in the form of heat, when it is burned (combustion). Common examples include wood, coal, and LPG.

Non-renewable Energy Sources are those that exist in limited quantities and take millions of years to form, such as fossil fuels (coal, petroleum, and natural gas).

Renewable Energy Sources are inexhaustible and can be replenished naturally. Examples include Solar Energy ($E_{solar}$), Wind Energy ($E_{wind}$), and Hydroelectric Energy ($E_{hydro}$).

Electricity is a form of energy resulting from the flow of electric charge. It is often generated by converting mechanical energy into electrical energy using a generator.

Energy Transformation: Energy can change from one form to another. For example, in an electric bulb, Electrical Energy $\rightarrow$ Light Energy + Heat Energy.

Calorific Value is the amount of heat energy produced by the complete combustion of $1$ kg of a fuel. It is expressed in kilojoules per kilogram ($kJ/kg$).