CAPF AC 2020 Q3: Science & Technology – Physics
- (a) Fleming's left hand rule gives direction of force on a current-carrying conductor in a magnetic field
- (b) Fleming's right hand rule gives direction of force on a current-carrying conductor in a magnetic field
- (c) Both the left-hand and right-hand rule can be used for finding direction of force on a current-carrying conductor in a magnetic field
- (d) Fleming's rules has nothing to do with magnetic field
Answer: (a) Fleming's left hand rule gives direction of force on a current-carrying conductor in a magnetic field
Explanation
Fleming's two rules
How to think: a motor needs a force to turn, and the motor rule is the left-hand rule.
So force goes with the left hand.
Both rules use the thumb, forefinger and middle finger held at right angles to each other.
They answer different questions.
| Rule | What it gives | Used for |
|---|---|---|
| Fleming's left-hand rule | Direction of force on a current-carrying conductor in a magnetic field | Electric motor |
| Fleming's right-hand rule | Direction of induced current when a conductor moves in a magnetic field | Electric generator |
What each finger shows:
| Finger | Left-hand rule | Right-hand rule |
|---|---|---|
| Forefinger | Magnetic field | Magnetic field |
| Middle finger | Current | Induced current |
| Thumb | Force (motion) | Motion of the conductor |
Answer: (a) Fleming's left hand rule gives direction of force on a current-carrying conductor in a magnetic field.
Why each option is right or wrong
- (a) CORRECT. The left-hand rule gives the direction of force on a current-carrying conductor.
- (b) Incorrect. The right-hand rule gives the direction of induced current, not force.
- (c) Incorrect. Only the left-hand rule is used for force; the two rules are not interchangeable.
- (d) Incorrect. Both rules are about a conductor in a magnetic field.
Key point
Fleming's left-hand rule = force on a current-carrying conductor (motor). Fleming's right-hand rule = induced current (generator).
Remember
Left hand = force (motor); right hand = induced current (generator).
Common mistakes
- Swapping the two rules: using the right hand for the force on a conductor.
- Forgetting that the three fingers must be mutually perpendicular.
- Confusing Fleming's rules with the right-hand thumb rule, which gives the field around a wire.
Exam tip
Left = motor, right = generator. A motor needs force, so force goes with the left hand.
Sources
- NCERT Class X Science — Magnetic Effects of Electric Current
- NCERT Class XII Physics — Moving Charges and Magnetism