Question

The direction of rotation of a DC motor can be determined by

• A. Fleming's right hand rule
• B. Fleming's left hand rule
• C. Lenz's law
• D. Ampere law

Hint:

Easy Mnemonic: Motor = Left hand (Mother-Law) and Generator = Right hand.

Answer 1

The Correct Option is B

Determining the direction of force (and thus rotation) in an electric motor requires understanding the interaction between a current-carrying conductor and a magnetic field. 1. Fleming's Left Hand Rule (Motor Rule): This rule is used for electric motors. When you hold your left hand with the thumb, forefinger, and middle finger mutually perpendicular:

• Forefinger: Points in the direction of the Magnetic Field ($B$).

• Middle Finger: Points in the direction of the Current ($I$).

• Thumb: Points in the direction of the Resulting Force ($F$) or Motion.

2. Why not the others?

• Fleming's Right Hand Rule: Used for

Generators to determine the direction of induced current.

• Lenz's Law: Used to determine the direction of induced EMF (it opposes the cause).

• Ampere Law: Relates the integrated magnetic field around a closed loop to the electric current passing through the loop.
Since a DC motor uses current and a field to produce motion, the Left Hand Rule is the correct tool.