Which one of the following statements regarding Fleming's Rule is correct?

Correct — A, Fleming's left hand rule gives direction of force on a current-carrying conductor in a magnetic field. The left hand serves the motor effect. Hold the thumb, forefinger and central finger of the left hand mutually at right angles: the forefinger points along the field, the central finger along the current, and the thumb then gives the direction of the force, which is the direction the conductor moves. The right hand rule answers the reverse question — a conductor is moved through a field, and it tells you which way the induced current flows.

• (b)Fleming's right hand rule gives direction of force on a current-carrying conductor in a magnetic field — The right hand rule belongs to electromagnetic induction, in a generator. There the motion is the input and the induced current is the output, the opposite of what this option claims.
• (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 — The two rules are not interchangeable. Each has a fixed assignment of hand to situation — the left hand for a motor, the right hand for a generator — and swapping them reverses the answer.
• (d)Fleming's rules has nothing to do with magnetic field — Both rules are about a conductor in a magnetic field. Neither can be stated without one.

A current-carrying conductor placed in a magnetic field experiences a force perpendicular to both the current and the field. That force is what turns an electric motor. The reverse process, a conductor moved across a field so that a current appears in it, is electromagnetic induction and is what a generator does. Fleming supplied one hand rule for each direction of the exchange between electrical and mechanical energy.

The reliable memory hook is the first letter. Left hand goes with the moving part of a motor; right hand goes with the generated current. Some textbooks assign the fingers differently between the two rules, so the safer habit is to remember the physical situation each rule serves rather than a finger-by-finger recitation. For the field around a current rather than the force on one, neither Fleming rule applies — that is the right-hand thumb rule, also called the corkscrew rule.

• Fleming's left hand rule gives the direction of the force on a current-carrying conductor placed in a magnetic field, which is the motor effect.
• Fleming's right hand rule gives the direction of the current induced when a conductor is moved through a magnetic field, which is the generator effect.
• In both rules the thumb, forefinger and central finger are held mutually perpendicular.
• The force, the field and the current are mutually at right angles, and the force vanishes when the conductor lies along the field.
• The right-hand thumb rule is a separate rule again — it gives the direction of the magnetic field encircling a straight current-carrying wire.

• Swapping the two rules, which reverses every answer that depends on them.
• Applying a Fleming rule to find the field around a wire, where the right-hand thumb rule is needed.
• Forgetting that the force disappears when the current runs parallel to the field.

Either as a direct which-rule-does-what item like this one, or by describing the finger assignment and asking what the remaining finger indicates.

• practice — not a real PYQ

  An electric motor works on the principle described by

  • (a)Fleming's right hand rule
  • (b)Fleming's left hand rule
  • (c)Lenz's law
  • (d)Coulomb's law

  Answer(b) Fleming's left hand rule — a current in a field produces a force, and that force is what turns the armature.

• practice — not a real PYQ

  A straight conductor carrying current is placed parallel to a uniform magnetic field. The force on the conductor is

  • (a)maximum
  • (b)zero
  • (c)half of the maximum
  • (d)directed along the current

  Answer(b) zero — the force needs a component of current perpendicular to the field, and a parallel conductor has none.