The rule to determine the direction of a force experienced by a straight current carrying conductor placed in a magnetic field which is perpendicular to it is
- (a)Right-hand thumb rule
- (b)Fleming's left-hand rule
- (c)Fleming's right-hand rule
- (d)Hund's rule
Correct — B, Fleming's left-hand rule. Stretch the thumb, forefinger and middle finger of the left hand mutually at right angles: the forefinger points along the magnetic field, the middle finger along the current, and the thumb then gives the direction of the force (or motion) on the conductor. This is the 'motor rule', used whenever a current-carrying conductor sits in a magnetic field.
- (a)Right-hand thumb rule — The right-hand thumb rule (Maxwell's corkscrew rule) gives the direction of the magnetic field produced around a current-carrying wire — it tells you about the field, not the force experienced by the conductor.
- (c)Fleming's right-hand rule — Fleming's right-hand rule applies to electromagnetic induction — it gives the direction of the induced current in a conductor moving through a magnetic field (the generator rule), the reverse situation to a force-carrying conductor.
- (d)Hund's rule — Hund's rule is from chemistry — it governs how electrons fill degenerate atomic orbitals (singly, with parallel spins, before pairing). It has nothing to do with the force on a conductor.
A straight conductor carrying a current, placed in a magnetic field, experiences a force. When the current is perpendicular to the field, that force is perpendicular to both. Fleming's left-hand rule fixes its direction, and this is the working principle of the electric motor.
The trap is the mirror-image pair of rules. Fleming's LEFT hand is for the force on a current (the motor effect); Fleming's RIGHT hand is for the current induced by motion (the generator effect). The right-hand thumb rule is a different tool entirely — it finds the field around a wire, not a force.
- Fleming's left-hand rule: forefinger for field, middle finger for current, thumb for force/thrust.
- It gives the direction of the force on a current-carrying conductor in a magnetic field — the basis of the electric motor.
- Fleming's right-hand rule is the generator rule, giving the direction of an induced current.
- The right-hand thumb (Maxwell corkscrew) rule gives the direction of the magnetic field around a current-carrying wire.
- Left hand = motor (force on a current); right hand = generator (induced current). Do not swap them.
- The right-hand thumb rule gives the field around a wire, not the force on it.
Examiners test whether you can match the correct hand rule to the situation — force on a conductor versus induced current versus field direction.
No directly related past PYQ was found.
- practice — not a real PYQ
Fleming's right-hand rule is used to find the direction of
- (a)force on a current-carrying conductor
- (b)the induced current in a generator
- (c)the magnetic field around a wire
- (d)the drift of electrons in a wire
Answer(b) the induced current in a generator — the right-hand rule is the generator rule, the counterpart of the left-hand (motor) rule.
- practice — not a real PYQ
The device that uses the force on a current-carrying conductor in a magnetic field to produce rotation is the
- (a)electric generator
- (b)transformer
- (c)electric motor
- (d)galvanometer shunt
Answer(c) electric motor — it converts electrical energy into mechanical rotation via the force described by Fleming's left-hand rule.