Electric Motor and Electric Generator MCQs

Q1. Consider the following statements about motors and generators: (I) A motor converts electrical energy to mechanical energy. (II) A generator works on electromagnetic induction. (III) A DC motor uses slip rings. Which of the above statement(s) is/are correct?

• A.Only III
• B.None of these
• C.Only I and II
• D.Only I

Answer: C. Only I and II

Key chapter idea: Correct: I, II. Connecting concept to answer: Correct. Ruling out the other options: option A) 'Only III' doesn't hold - Re-check each statement individually; option B) 'None of these' is incorrect: Re-check each statement individually; option D) 'Only I' fails since Re-check each statement individually. Putting it together the correct option is C) Only I and II.

Q2. The rotating coil of a motor/generator is the (NCERT):

• A.commutator
• B.field magnet
• C.brush
• D.armature

Answer: D. armature

Eliminate first - option A) 'commutator' doesn't hold - That reverses current; the coil is the armature; option B) 'field magnet' doesn't hold - That provides the field; option C) 'brush' is wrong because Brushes are contacts. That leaves only option D). Confirm with the chapter rule: An electric motor exploits F = B*I*L on a current-carrying coil between magnet poles to produce a torque; the split-ring commutator reverses I every half rotation -> the coil keeps turning the same way; hence the listed function = armature (physics, chapter 'Magnetic Effects of Electric Current'). Answer: D) armature.

Q3. A step-down transformer has 1000 primary turns and primary voltage 220 V. The secondary voltage needed is 22 V. Compute the secondary turns Ns and the turns ratio (Np:Ns).

• A.Ns = 100, ratio = 10:1
• B.Ns = 1000, ratio = 1:1
• C.Ns = 100, ratio = 1:10
• D.Ns = 101, ratio = 11:1

Answer: A. Ns = 100, ratio = 10:1