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6. Electromagnetic induction — Physics STD 12 Science — Question

CBSE BoardEnglish MediumSTD 12 SciencePhysics6. Electromagnetic induction1 Mark
MCQ
Assertion : An electric motor will have maximum efficiency when back $emf$ becomes equal to half of applied $emf$ .

Reason : Efficiency of electric motor depends only on magnitude of back $emf$ .

  • A
    If both Assertion and Reason are correct and the Reason is a correct explanation of the Assertion.
  • B
    If both Assertion and Reason are correct but Reason is not a correct explanation of the Assertion.
  • ✓
    If the Assertion is correct but Reason is incorrect.
  • D
    If both the Assertion and Reason are incorrect.
✓

Answer

Correct option: C.
If the Assertion is correct but Reason is incorrect.
c
Efficiency of an electric motor is maximum when the back $emf$ setup in the armature is half the value of the applied battery $emf$

Efficiency, $\eta \, = \,\frac{{{P_{out}}}}{{{P_{in}}}} = \frac{e}{E} = \frac{{{\text{Back emf}}}}{{{\text{Supply voltage}}}}$

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