When a current carrying conductor is placed in a magnetic field at right angles to it, it experiences a force as demonstrated by the following experiment.
Fix two iron nails into the opposite faces of a large size cork. The nails should be in line with each other and should not touch inside the cork. Clamp the cork on a stand so that the nails are horizontal. Now take a thick wire and bend it in the shape of U. make hooks at its two ends. With the help of these hooks, suspend the U shaped wire from the two nails so that the horizontal part of the wire passes through the space between the pole pieces of a horse shoe magnet (fig. 17.7). Connect the two nails with a battery and a switch. As the key is pressed, current starts flowing through the U shaped wire and simultaneously the wire moves inwards. This movement of the wire is due to the force which is acting on the side AB of the direction of current shown in Fig. 17. 7 is reversed, the wire moves outwards. The direction of the force acting on the side AB of the wire can be determined by Fleming’s left hand rule.
Fig. 17. 7
According to this rule, stretch the thumb, forefinger and the middle finger of the left hand mutually at right angles to each other. If the forefinger points in the direction of the magnetic field, the middle finger in the direction of the current, then the thumb would indicate the direction of the force acting on the conductor (Fig. 17.8)
It can be seen that the direction of the force acting on the conductor in Fig. 17.7 is in accordance with Fleming’s left hand rule. Note that the force acting on the current carrying conductor is at right angles to both the directions of current and magnetic field.
If instead of 90 degree the current carrying conductor makes an angle 0 with the magnetic field (Fig. 17.9), experiment has shown that the force acting upon the conductor in such a case is directly proportional to sin 0… As such the force acting upon the conductor is maximum only when it is placed at right angles to the field. If the conductor is placed parallel to the field, no force would act upon the conductor because the value of sin 0 is zero in this case.