. . Right-Hand Thumb Rule A convenient way of finding the direction of magnetic field associated with a current-carrying conductor is given in Fig. .
. Connect the copper wire vertically between the points X and Y, as shown in Fig. . (a), in series with the battery, a plug and key.
Sprinkle some iron filings uniformly on the cardboard. (You may use a salt sprinkler for this purpose.) Keep the variable of the rheostat at a fixed position and note the current through the ammeter. Close the key so that a current flows through the wire. Ensure that the copper wire placed between the points X and Y remains vertically straight.
Gently tap the cardboard a few times. Observe the pattern of the iron filings. You would find that the iron filings align themselves showing a pattern of concentric circles around the copper wire (Fig. .
). What do these concentric circles represent? They represent the magnetic field lines. How can the direction of the magnetic field be found?
Place a compass at a point (say P) over a circle. Observe the direction of the needle. The direction of the north pole of the compass needle would give the direction of the field lines produced by the electric current through the straight wire at point P. Show the direction by an arrow.
Does the direction of magnetic field lines get reversed if the direction of current through the straight copper wire is reversed? Check it. (a) A pattern of concentric circles indicating the field lines of a magnetic field around a straight conducting wire. The arrows in the circles show the direction of the field lines.
(b) A close up of the pattern obtained. Variable resistance Imagine that you are holding a current-carrying straight conductor in your right hand such that the thumb points towards the direction of current. Then your fingers will wrap around the conductor in the direction of the field lines of the magnetic field, as shown in Fig. .
. This is known as the right-hand thumb rule * . Example . A current through a horizontal power line flows in east to west direction.
What is the direction of magnetic field at a point directly below it and at a point directly above it? Solution The current is in the east-west direction. Applying the right-hand thumb rule, we get that the magnetic field (at any point below or above the wire) turns clockwise in a plane perpendicular to the wire, when viewed from the east end, and anti-clockwise, when viewed from the west end. Right-hand thumb rule * This rule is also called Maxwell’s corkscrew rule.
If we consider ourselves driving a corkscrew in the direction of the current, then the direction of the rotation of corkscrew is the direction of the magnetic field.