CBSE Class 10 Answered

The Kicking Wire Experiment: A thick copper wire AB is suspended vertically from a support T by means of a flexible joint J (Belo figure). The lower end B of this wire is free to move between the poles of a U–shaped magnet M. The lower end B of the wire just touches the surface of mercury kept in a shallow vessel V so that it can move when a force acts on it. The positive terminal of a battery is connected to end A of the wire. The circuit is completed by dipping another wire from the negative terminal of the battery into the mercury pool. We know that mercury is a liquid which is a good conductor of electricity, so the circuit is completed through mercury contained in the vessel V. 
 
On pressing the switch, a current flows in the wire AB in the vertically downward direction. The wire AB is kicked in the forward direction (towards south) and its lower end B reaches the position B', so that the wire comes to the new position AB' . When the lower end B of the hanging wire comes forward to B', its contact with the mercury surface is broken due to which the circuit breaks and current stops flowing in the wire AB. Since no current flows in the wire, no force acts on the wire in this position and it falls back to its original position. As soon as the wire falls back, its lower end again touches the mercury surface, current starts flowing in the wire and it is kicked again. This action is repeated as long as the current is passed in wire AB. It should be noted that the current carrying wire is kicked forward because a force is exerted on it by the magnetic field of the U–shaped magnet. From this experiment we conclude that when a current carrying conductor is placed in a magnetic field, a mechanical force is exerted on the conductor which makes it move.

The current is flowing in the vertically downward direction and the direction of magnetic field is from left to right directed towards east, thus, the current carrying conductor is at right angles to the magnetic field. Now, we have just seen that the motion of the conductor is in the forward direction (towards south) which is at right angles to both, the direction of current and the direction of magnetic field. Since the direction of motion of the wire represents the direction of force acting on it, we can say that : The direction of force acting on a current carrying wire placed in a magnetic field is (i) perpendicular to the direction of current, and (ii) perpendicular to the direction of magnetic field. In other words, we can say that the current, the magnetic field and the force, are at right angles to one another. It should be noted that the maximum force is exerted on a current carrying conductor only when it is perpendicular to the direction of magnetic field. No force acts on a current carrying conductor when it is parallel to the magnetic field.

If we reverse the direction of current in the wire AB so that it flows in the vertically upward direction from B to A, then the wire swings in the backward direction (towards north). This means that the direction of force on the current carrying wire has been reversed. From this we conclude that the direction of force on a current carrying conductor placed in a magnetic field can be reversed by reversing the direction of current flowing in the conductor. Keeping the direction of current unchanged, if we reverse the direction of magnetic field applied in Figure 16 by turning the magnet M so that its poles are reversed, even then "the wire swings backwards showing that the direction of force acting on it has been reversed. Thus, the direction of force on a current carrying conductor placed in a magnetic field can also be reversed by reversing the direction of magnetic field.