A simple motor is made in a school laboratory. A coil of wire is mounted on an axle between the poles of a horseshoe magnet, as illustrated.

Question 6 Choice 2.jpg

In the example above, coil ABCD is horizontal and the battery is connected as shown.

a. For this position, state the direction of the force on the arm AB.

b. Why does the current in the arm BC not contribute to the turning force on the coil?



For Arm AB

  • When battery is switched on, current flows through coil AB from A to B, and Magnetic Field is from North to South...
  • So, by Fleming's left hand rule, downward force is applied on AB.


For Arm BC

Current in arm BC does not contribute to the turning force of the coil because

  • Current in arm BC travels in the same direction as the magnetic field lines .
  • When the direction of magnetic field is parallel to the current , the resultant force on the current carrying object is zero .
  • Hence current in arm BC or DA does not contribute to the movement of the coil.
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CA Maninder Singh is a Chartered Accountant for the past 14 years and a teacher from the past 18 years. He teaches Science, Economics, Accounting and English at Teachoo