Electric Motor: Principle and Working - UNSOLVED PRACTICE SET
Chapter: Magnetic Effects of Current | Topic: Electric Motor Principle and Working
ELECTRIC MOTOR: PRINCIPLE AND WORKING - UNSOLVED PRACTICE SET
Topic: Electric Motor Principle and Working
Multiple Choice Questions
Q1. An electric motor converts:
- Mechanical energy into electrical energy
- Electrical energy into mechanical energy
- Heat energy into electrical energy
- Light energy into electrical energy
Q2. The basic principle of an electric motor is based on:
- Electromagnetic induction
- The force experienced by a current-carrying conductor in a magnetic field
- The heating effect of current
- The chemical effect of current
Q3. In a simple DC motor, the split-ring commutator's main function is to:
- Increase the voltage supplied to the coil
- Reverse the direction of current in the coil every half rotation
- Convert AC to DC
- Reduce friction in the motor
Q4. In a DC motor, the coil rotates continuously because:
- The magnetic field constantly changes direction on its own
- The split-ring commutator reverses current direction every half-turn, maintaining the torque direction
- The brushes generate their own magnetic field
- The coil has no resistance
Q5. The brushes in a DC motor are typically made of:
- Rubber
- Plastic
- Carbon (graphite)
- Wood
Q6. Which of the following appliances commonly used in Indian homes contains an electric motor?
- Electric iron
- Tube light
- Ceiling fan
- Electric heater
Short Answer Questions
Q7. State the principle on which an electric motor works. Which earlier concept (rule) explains the direction of the force involved?
Q8. List the main parts of a simple DC electric motor and state the function of each:
(a) armature coil,
(b) magnets,
(c) split-ring commutator,
(d) brushes.
Q9. Why is a split-ring commutator necessary in a DC motor? What would happen if it were replaced with a simple continuous ring (no split)?
Q10. Explain why the forces on the two sides of the coil in a motor act in opposite directions, causing rotation rather than simple linear motion.
Q11. What is the role of brushes in an electric motor? Why must they make continuous contact with the rotating commutator?
Q12. Name three household appliances that use electric motors and briefly state what type of motion (rotational, oscillating, etc.) the motor produces in each.
Long Answer Questions
Q13. Describe the construction and working of a simple DC electric motor in detail. Your answer must cover:
(a) the main parts (armature/coil, permanent magnets, split-ring commutator, brushes, and battery),
(b) the principle behind the rotation (force on current-carrying conductor in a magnetic field, using Fleming's Left Hand Rule),
(c) why the forces on the two arms of the coil cause rotation,
(d) the role of the split-ring commutator at the half-rotation point, and
(e) how this cycle repeats to produce continuous rotation.
Q14. A ceiling fan in an Indian home contains an electric motor.
(a) Explain how electrical energy from the 220 V mains is converted into the rotational motion of the fan blades.
(b) The fan has a regulator (speed control) — explain (conceptually) how changing the regulator setting could affect the motor's speed (relate to current and force).
(c) Why does the fan motor get warm during long use? (Connect to a concept you studied in the Electricity chapter.)
(d) Real fan motors are usually AC induction motors, not simple DC motors with commutators — but the basic principle (force on current in magnetic field) still applies. Why do you think DC motors with commutators are less common in large appliances?
Q15. Compare the construction and working of an electric motor with that of a simple loudspeaker (which you considered in the previous topic).
(a) What is the input and output (energy form) of each device?
(b) What is common between them (the basic physical principle)?
(c) What is different — in a motor, the coil rotates continuously; in a speaker, the coil moves back and forth. What design feature (commutator vs no commutator) accounts for this difference?
(d) Both devices are described as 'electromechanical' — explain what this means.
(e) Give one more electromechanical device used in daily Indian life and briefly describe its function.
Numerical / Application-Based Problems
Q16. A simple DC motor's coil makes one complete rotation in 0.02 seconds (i.e., it rotates at 50 revolutions per second).
(a) The split-ring commutator must reverse the current direction at specific points during each rotation — how many times does the current reverse direction during ONE complete rotation?
(b) Calculate how many times per second the current direction reverses.
(c) If the motor's speed is doubled (100 revolutions per second), how does the frequency of current reversal change?
Q17. A student builds a simple electric motor model for a science exhibition using a battery, a coil of wire, two magnets, and a commutator made from stripped wire ends. The motor spins but very slowly and weakly. Suggest and explain THREE specific modifications (based on what affects force on a current-carrying conductor and field strength) that would make the motor spin faster and with more torque.
Q18. An electric motor in a small toy car is rated to draw a current of 0.5 A from a 3 V battery.
(a) Calculate the electrical power input to the motor (P = VI).
(b) If the motor is only 60% efficient (60% of electrical power converts to useful mechanical power), calculate the mechanical power output.
(c) What happens to the remaining 40% of the input power?
(d) If the toy car's motor is replaced with a more efficient one (80% efficient) at the same electrical power input, how does the mechanical power output change?