Name the process by which energy is produced in a nuclear reactor.

In a hydro power plant,

$(a)$ $A$ hydropower plant converts one form of energy into another. Name these two forms of energy.
$(b)$ State two advantages of generating hydroelectricity compared to the generation of electricity in a thermal power plant.

Explain how the energy of fossil fuels is ultimately derived from the sun.

$(i)$ Name the four gases commonly present in biogas.
$(ii)$ List two advantages of using biogas over fossil fuels.

What is hydroelectricity? Explain the basic principle involved in the generation of this energy.