NDA (II) 2021 General Ability Test · Question 126 of 149

Correct — C, the Right-Hand Thumb rule and it would be in the upward direction. Maxwell's right-hand thumb rule, also called the right-hand grip rule, relates the current in a straight conductor to its magnetic field: if the right hand grips the wire so that the curled fingers point along the circular field lines, the outstretched thumb points in the direction of the current. For field lines running anti-clockwise, as seen looking along the wire, the thumb, and hence the current, points upward, out towards the viewer. The left-hand rule applies to the force on a current in a field, a different situation.

• (a)the Right-Hand Thumb rule and it would be in the downward direction. — The rule is right, but anti-clockwise field lines give an upward current, not downward; a downward current would produce clockwise field lines.
• (b)the Left-Hand Thumb rule and it would be in the downward direction. — There is no left-hand thumb rule for this; the current-and-field relation uses the right hand, and the current here points upward.
• (d)the Left-Hand Thumb rule and it would be in the upward direction. — The direction, upward, is right, but the rule is wrong; Fleming's left-hand rule gives the force on a current-carrying conductor in a field, not the current from its own field.

A straight current-carrying conductor sets up a magnetic field in concentric circles around it. The right-hand thumb rule links the two directions: with the right hand gripping the wire, the fingers curl along the field while the thumb points along the current.

Separate the two hand rules. The right-hand thumb (grip) rule links a current to the magnetic field it creates, while Fleming's left-hand rule gives the force on a current placed in an external field. Here we go from field to current, so use the right hand, and anti-clockwise field lines mean the current comes upward toward the viewer.

• In the right-hand thumb (grip) rule, the fingers curl along the magnetic field and the thumb points along the current.
• A straight current-carrying wire produces circular magnetic field lines around it.
• Anti-clockwise field lines, viewed along the wire, correspond to a current flowing upward, out of the page.
• Fleming's left-hand rule instead gives the direction of force on a current in a magnetic field.

• Using the left hand for a current-and-its-own-field problem.
• Reversing the current direction; an anti-clockwise field means an upward current, not downward.

Asked to name the rule and the current direction for a given field pattern around a straight wire; it is the right-hand thumb rule, giving an upward current here.

No directly related past PYQ was found.

• practice — not a real PYQ

  The magnetic field lines around a straight current-carrying conductor are

  • (a)straight and parallel to the wire
  • (b)concentric circles around the wire
  • (c)radial lines from the wire
  • (d)absent

  Answer(b) concentric circles around the wire.

• practice — not a real PYQ

  Fleming's left-hand rule is used to find the direction of

  • (a)the magnetic field of a current
  • (b)the force on a current-carrying conductor in a field
  • (c)an induced current
  • (d)a current from a field

  Answer(b) the force on a current-carrying conductor in a field.