Who first observed the magnetic effect of an electric current?

$(a)$ What is the direction of magnetic field lines inside a bar magnet and outside of it?
$(b)$ What does the degree of closeness of the field lines represent?

$A$ constant current flows in a horizontal wire in the plane of the paper from east to west as shown in the figure. The direction of the magnetic field at a point will be North to South:

Name four appliances wherein an electric motor,a rotating device that converts electrical energy to mechanical energy,is used as an important component. In what respect are motors different from generators?

How does the strength of the magnetic field at the centre of a circular coil of wire depend on: $(i)$ the radius of the coil $(ii)$ the number of turns of wire in the coil $(iii)$ the strength of the current flowing in the coil?

$(a)$ Write three differences between $AC$ and $DC$.
$(b)$ State the frequency of $AC$ supply in India and mention the potential difference between the neutral and live wire in a domestic circuit.
How many times does $AC$ change its direction in one second?