The Magnetic field lines around a straight conductor carrying current are concentric circles whose centers lie on the wire. Smooth cardboard with iron filings spread over it is fixed in a horizontal plane with the help of a clamp. The magnetic effect of electric current is one of the major effects of electric current in use, without the applications of which we cannot have motors in the existing world. The strength of the magnetic field created depends on the current through the conductor. A straight wire passes through a hole made at the center of the cardboard (Figure).
Fig: Magnetic field around a straight conductor carrying current
It states that if you holding the current-carrying conductor in your right hand such that the thumb points in direction of the current, then the direction in which the fingers encircle, gives the direction of magnetic lines. This rule is also called Maxwell’s Corkscrew Rule. The nature of the magnetic field lines around a straight current-carrying conductor is concentric circles with the center at the axis of the conductor.
A current is passed through the wire by connecting its ends to a battery. When the cardboard is gently tapped, it is found that the iron filings arrange themselves along with concentric circles. A current-carrying conductor creates a magnetic field around it, which can be comprehended by using magnetic lines of force or magnetic field lines.
This clearly shows that the magnetic field is developed around a current-carrying conductor.
To find the direction of the magnetic field, let us imagine, a straight wire passes through the plane of the paper and perpendicular to it. When a compass needle is placed, it comes to rest in such a way that its axis is always tangential to a circular field around the conductor. When the current is inwards (Fig: a) the direction of the magnetic field around the conductor looks clockwise.
When the direction of the current is reversed, that it is outwards, (Fig: b) the direction of the magnetic pole of the compass needle also changes showing the reversal of the direction of the magnetic field. Now, it is anticlockwise around the conductor. The direction of the magnetic field lines of force around a conductor is given by Maxwell’s right-hand grip rule or the right-handed corkscrew rule. This proves that the direction of the magnetic field also depends on the direction of the current in the conductor. This is given by Maxwell’s rule. Imagine that you are holding a current-carrying straight conductor in your right hand such that the thumb points towards the direction of the current. Then your fingers will wrap around the conductor in the direction of the field lines of the magnetic field. This is known as a right-hand thumb rule.