What is the effect of light on the conductivity of a semiconductor used in solar cells?

Why should copper tubes be given a higher number of coils in a solar water heater?

Name two forms in which solar energy manifests itself in oceans.

Which of the following is a non-renewable source of energy?

The power generated in a windmill:

$(a)$ Magnetic field lines of two bar magnets $A$ and $B$ are as shown below. Name the poles of the magnets facing each other.
$(b)$ Two magnetic field lines never intersect each other. Why?
$(c)$ How does the strength of the magnetic field at the centre of a current-carrying circular coil depend on the
$(i)$ radius of the coil,
$(ii)$ number of turns in the coil, and
$(iii)$ strength of the current flowing in the coil?