Electrical Resistance

What are the differences between EMF and potential difference?

Define non-ohmic resistance?

An example of non-ohmic resistance is:

A lamp can work on a $50 \mathrm{volt}$mains taking $2$$\mathrm{amps}$. What value of the resistance must be connected in series with it so that it can be operated from$200 \mathrm{volt}$ mains giving the same power.

A copper wire with a diameter of $0.5 \mathrm{mm}$ and a resistivity of $1.6\times {10}^{-8} \mathrm{\Omega } \mathrm{m}$ . What is the length of this wire required to get a resistance of $10 \mathrm{\Omega }$? How much does the resistance change if the diameter is doubled?

Two metal wires of the same material, A and B, are joined in parallel. Wire A has a radius of $r$ and a length of $l$. Wire B has a radius of $2r$ and a length of $2l$. Calculate the ratio of total parallel combination and the resistance of wire A.

Give two examples of non-ohmic devices.