• Can you draw the direction of magnetic force on sides AB and CD? Apply right hand rule to get the direction of magnetic force. At AB, the magnetic force acts inward perpendicular (F ) to field of the magnet and on CD, it acts outward (F).
The force on the sides BC and DA varies because they make different angles at different positions of the coil in the field.
• What are the directions of forces on BC and DA?
In the direction of currents in BC and DA are parallel to the field. magnetic force does’t act on them. If the direction of currents in BC and DA are perpendicular to the field at BC, magnetic force pulls the coil up and at DA magnetic force pulls it down.
• What is the net force on the rectangular coil?
The force on AB is equal and opposite to the force on CD due to external magnetic field because they carry equal currents in the opposite direction. Sum of these forces is zero; similarly the sum of the forces on sides BC and DA is also zero for the same reason. So, net force on the coil is zero. But there is rotation of the coil. How is it possible?
• Why does the coil rotate?
Let us consider opening a cap of the bottle as an example where two equal and opposite forces act on the cap. Two forces equal in magnitude but opposite in direction must act on the either side of cap of the bottle as shown in figure 12 (b). These forces bring the cap into rotation. Similarly the rectangular coil comes into rotation in clockwise direction because of equal and opposite pair of forces acting on the two sides of the coil.
• What happens to the rotation of the coil if the direction of current in the coil remains unchanged?
If the direction of current in the coil is unchanged, it rotates up to a vertical position then due to its inertia it rotates further in clockwise direction. But now the sides of the coil experience forces which are in the opposite direction to the previous case. Hence these forces try to rotate it F fig-12(b) Couple action on bottle cap in anti clockwise direction. As a result, this coil comes to halt and rotates in anti clock wise direction, this will go on if the direction of current remains unchanged.
• How could you make the coil rotate continuously?
If the direction of current in the coil, after the first half rotation, is reversed, the coil will continue to rotate in the same direction. Thus if the direction of current through the coil is reversed every half rotation, the coil will rotate continuously in one and the same direction.
• How can we achieve this?
To achieve this, brushes B1 and B2 are used, as shown in figure 12 (c). These brushes are connected to the battery. The ends of the coil
Why does the coil rotate?