The back emf in an electric motor
Correct answer: B. opposes the supply voltage and limits the current drawn
- A. adds to the supply voltage
- B. opposes the supply voltage and limits the current drawn
- C. has no effect on the current
- D. appears only when the motor is stationary
Explanation
As the armature spins it acts as a generator, inducing an emf that by Lenz's law opposes the supply, so the effective driving voltage and therefore the current fall as the motor speeds up. At the instant of starting there is no back emf, which is why the starting current is very large and why motors need starters. Overloading a motor slows it, reduces the back emf and can burn out the windings.
Report an error
The more specific you are, the faster it gets fixed. A source beats an opinion.
Prefer email? support@testustad.com
About Lenz's Law
Lenz's law states that the induced current flows in a direction that opposes the change in magnetic flux producing it. Questions connect this rule with Faraday's law, magnetic flux, induced emf and the direction of motion or current, while distinguishing opposition to flux change from opposition to the magnetic field itself.
Practise Electromagnetic Induction
288 free Electromagnetic Induction MCQs from Physics, each with the correct answer and an explanation. Unlimited attempts, no account needed.
Discussion
Stuck on an option, or know a faster way to get there? Ask or explain it here.
No comments yet. Be the first to explain this one.
Exams that ask Physics questions like this
Physics is on 16 papers prepared for on TestUstad, and all of them draw the same bank, so this question is worth knowing for every one of them.
More Lenz's Law questions
A magnet dropped down a long vertical copper tube falls unusually slowly because
The minus sign in the expression for Faraday's law expresses
Lenz's law states that the direction of an induced current is such that it
A metal ring is placed on the core of a coil carrying alternating current. The ring
The energy delivered by an induced current ultimately comes from
A loop moves toward a stationary magnet. The induced current will: