The resistivity of a certain metal is p, using a wire drawing machine, a wire of uniform cross section is made out of volume V of the metal. The wire is shaped in a square loop, which is placed in a uniform magnetic field B whose direction is parallel to the axis of the loop. Find the charge flown through any crosssection of the wire, when the magnetic field is switched off.

(a) VB/16p (b) VB/a(pi)p (c) VB/p (d) VB/4p

If you hold a compass near a wire through which current is flowing, the needle on the compass will be deflected.

Since compasses work by pointing along magnetic field lines, this means that there must be a magnetic field near the wire through which the current is flowing.

The magnetic field produced by an electric current is always oriented perpendicular to the direction of the current flow. Below is a sketch of what the magnetic field around a wire looks like when the wire has a current flowing in it. We use B⃗ to denote a magnetic field and arrows on field lines to show the direction of the magnetic field. Note that if there is no current there will be no magnetic field.