Magnetic Field Around a Straight Current-Carrying Conductor and Circular Loop

Placing a compass at different points near a current-carrying conductor and noting the direction it settles in (it always aligns tangent to the local field) lets you trace out the full field-line pattern of the conductor. The two shapes considered here -- a long straight wire and a circular loop …

For a straight current-carrying conductor, the magnetic field lines form concentric circles whose common centre lies on the axis of the conductor -- consistent with the compass deflections traced out around it. The circles are clockwise or anticlockwise depending on the current's direction; increasing the current's magnitude increases the density (crowding) of these circles, and the field strength $B$ decreases as the distance $r$ from the wire increases (this …

Near a current-carrying circular loop, a compass placed close to the wire (points such as A and B just outside the coil) shows the field lines are still locally circular around the wire, but near the centre of the loop the lines become nearly parallel to each other, indicating the field there is close to uniform. The field's strength increases if the current, the number of turns, or both are increased, and the loop's polarity (which face behaves as a north pole and which as a south pole) depends on the sense of current flo …