Discussion
The back EMF in a D.C. motor opposes the supply voltage. This is explained by...
Answer: B
The opposition of the back emf to the supply is explained by Lenz's law.
When the armature of a DC motor rotates in the field, its conductors cut the flux and an emf is induced in them. Lenz's law says that an induced emf always acts to oppose the change that produces it; here it opposes the current driven by the supply, so it acts against the supply voltage, which is why it is called the back emf.
Faraday's laws give the magnitude of the induced emf but not its direction. Fleming's left hand rule gives the direction of the force on a current-carrying conductor (motor action), and the right hand rule gives the direction of an induced current, not the reason it opposes the supply.
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