Question

Briefly explain an activity to plot the magnetic field lines around a straight current carrying conductor. Sketch the field pattern for the same, specifying current and field directions.
What happens to the field,
(a) if the strength of the current is decreased?
(b) if the direction of the current is reversed?

Answer 1

The pattern of magnetic field lines around a straight conductor carrying current can be described by the following activity. 
(i) Insert vertically a long straight wire carrying an electric current so that it passes through the centre of a horizontal piece of plastic boardasshowninfigure. 
(ii) Take care that the plastic board is fixed and does not move up and down. Now, sprinkle some iron filings onto the plastic board to show the shape of the field. 
(iii) You will notice that the iron filings get arrangedround the wires in the shape of circles. This is due to the reason that the magnetic field lines around the current carrying straight conductor are circular. The iron filings align along these field lines.

(iv) On reversing the direction of flow of current, we observe that the iron filings arrange themselves in circles around the wire showing that the magnetic field lines are still circular in nature
The direction of the magnetic field can be obtained by using a compass. If the current direction is reversed, the direction of the magnetic field is also reversed. 
(a)When current through the wire is decreased, fieldalso gets reduced. 
(b) When the current is reversed, the field also gets reversed in direction.