The field in between north and south pole of horse shoe magnet is shown in the figure 11 (a). Let us imagine a wire passing perpendicular to the paper. Let the current pass through it (into the page). It produces a magnetic field as shown in figure 11(b). Now let us try to sketch the resultant field by observing the field lines. We can see that the direction of the field lines due to the wire in upper part (of circular lines) coincides with the direction of the field lines of horse shoe magnet. The direction of field lines by wire in lower part (of circular lines) is opposite to the direction of the field lines of horse shoe magnet. So that the net field in upper part is strong and in lower part it is weak. Hence a non-uniform field is created around the wire. This non uniform field is shown in figure 11 (c). Therefore the wire tries to move to the weaker field region.
• Does this deflection fit with the direction of magnetic force found by right hand rule?
• What happens when a current carrying coil is placed in a uniform magnetic field?
• Can we use this knowledge to construct an electric motor? Let us try to answer.
Electric motor
To understand the working of an electric motor we need to understand the behaviour of a current carrying coil kept in a uniform magnetic field.
Consider a rectangular coil kept in a uniform magnetic field as shown in figure 12(a). Switch on the circuit so that the current flows through the rectangular coil. The direction of current in the coil is shown in figure 12(a).
• What is the angle made by AB and CD with magnetic field?
You will notice that they are always at right angles to the magnetic field.
