CAPF AC 2022 Q24: Science & Technology – Physics
The electrical device used for converting mechanical energy into electrical energy is called
- (a) voltmeter
- (b) ammeter
- (c) motor
- (d) generator
Answer: (d) generator
Explanation
Generator and motor
How to think: these two machines are opposites.
A generator takes in motion and gives out electricity.
A motor takes in electricity and gives out motion.
| Device | Energy change | Principle |
|---|---|---|
| Generator (dynamo) | Mechanical → electrical | Electromagnetic induction (Faraday) |
| Motor | Electrical → mechanical | Force on a current-carrying conductor in a magnetic field |
| Voltmeter | Measures potential difference | Connected in parallel |
| Ammeter | Measures current | Connected in series |
Rules to remember: Fleming's right-hand rule is for the generator.
Fleming's left-hand rule is for the motor.
Answer: (d) generator.
Why each option is right or wrong
- (a) Incorrect. A voltmeter only measures potential difference.
- (b) Incorrect. An ammeter only measures electric current.
- (c) Incorrect. A motor does the reverse: electrical energy into mechanical energy.
- (d) CORRECT. A generator converts mechanical energy into electrical energy.
Key point
Generator: mechanical → electrical, electromagnetic induction, Fleming's right-hand rule. Motor: electrical → mechanical, Fleming's left-hand rule. Power stations use turbines to turn generators.
Remember
Generator: motion in, electricity out.
Common mistakes
- Reversing the two and marking the motor.
- Mixing up Fleming's left-hand rule (motor) with the right-hand rule (generator).
- Thinking a voltmeter or ammeter produces electricity.
Exam tip
Educated guess: a generator generates electricity; the name says it. A motor makes things move, like a fan or a mixer.
Sources
- NCERT Class X Science — Magnetic Effects of Electric Current (electric motor and electric generator)
- NCERT Class XII Physics — Electromagnetic Induction (AC generator)