MAGNETIC FIELD DUE TO A STRAIGHT CURRENT CARRYING WIRE:
When a current is passed through a conducting wire, a magnetic field is produced around it. The direction of magnetic field due to a straight current carrying wire can be mapped by means of a small compass needle or by iron fillings.
Take a sheet of smooth cardboard with a hole at the centre. Place it horizontally and pass a wire vertically through the hole, Sprinkle some iron fillings on the cardboard and pass an electric current through the wire. Gently tap the cardboard. We find that the iron filling arrange themselves in concentric circles around the wire as shown in figure.
If a small compass needle is kept anywhere on the board near the wire, the direction in which the north pole of the needle points gives the direction of the magnetic the magnetic field (i.e., magnetic lines of force) at that point.
The magnetic lines of force form concentric circles near the wire, with their plane perpendicular to the straight conductor and with their centers lying on its axis. if the direction of current in the wire is reversed, the direction of lines of force is also reversed.
On increasing the strength of current in the wire, the lines of force becomes denser and iron fillings are arranged in circles upto a larger distance from the wire, showing that the magnetic field strength has increased.
(i) Magnitude of magnetic field produced by a straight current-carrying conductor:
The magnitude of magnetic field (or strength of magnetic field ) B produced by an infinitely long conductor in vacuum at a distance r from it, it given by :-
B = Magnetic field strength (unit of B is Tesla or T=N/mA)
m˳= Permeability of vacuum (a constant m˳= 4p×10-7 Tm/A)
I = Current flowing in conductor (in Amperes) and
r = Distance from the conductor where magnetic field is measured (in meter).
T= The unit of magnetic field B is tesla which is denoted by the symbol T (1 tesla is equal to 1 Newton per ampere per meter).
(ii) Direction of magnetic field:
Detailed experiments showed that the magnetic field produced by an electric current is always oriented perpendicular to the direction of flow. A simple method of showing this relationship is called the left-hand rule. Simply stated, the left-hand rule says that the magnetic flux lines produced by a current-carrying wire will be oriented the same direction as the curled fingers of a person's left hand (in the "hitchhiking" position), with the thumb pointing in the direction of electron flow:
The magnetic field encircles this straight piece of current-carrying wire, the magnetic flux lines having no definite "north" or "south' poles. While the magnetic field surrounding a current-carrying wire is indeed interesting, it is quite weak for common amounts of current, able to defect a compass needle and not much more.
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