Phy Chapter 5 Magnetic Effect Of An Electric Current Caltec S.4

CHAPTER ELEVEN MAGNETIC EFFECT OF AN ELECTRIC CURRENT When electric current flows in a wire, magnetic field is created around the wire. The direction of the magnetic field (flux) depends on the direction of the current in the wire. Ampere’s Swimming Rule Imagine an observer to be swimming along the wire in the direction of the current and facing the needle, and then the north pole of the needle will be deflected towards his left hand. Magnetic flux pattern due to a current in a long straight wire A large rectangular coil consisting of about twenty turns of insulated copper wire is set up vertically with one of its vertical sides passing through a hole in the centre of a piece of a cardboard supported horizontally as shown in the figure above. A current of about 3A may be passed though the coil from a 6V battery in series with a rheostat, an ammeter and a switch. A fine layer of iron fillings is then sprinkled on the cardboard; the current is switched on and the card tapped gently. The fillings set in series of concentric circles about the wire as center. A small plotting compass placed on the card indicates the direction of the flux. If the current is reversed by changing over the battery connections, the compass needle will swing round and point in the opposite direction, but the pattern of the flux remain unchanged. The direction of the magnetic field can be predicted by Maxwell’s screw rule and the right hand grip rule. Maxwell’s screw rule Imagine a screw being screwed along the wire in the direction of the current. The direction of rotation of the screw gives the direction of magnetic field. Right hand grip rule Imagine the wire to be grasped in the right hand …