In the Right-Hand Thumb Rule, the thumb is directed towards the direction of:

Correct — A. Rule: thumb = current, curled fingers = magnetic field.

Grip the straight conductor in your right hand with the thumb pointing along the direction of conventional current. The fingers then curl the way the circular magnetic field lines run around the wire.

So the thumb gives the current and the field is what you read off the fingers. The two are not interchangeable.

Why the others are wrong

• (b)
  magnetic field — The magnetic field is what the curled fingers show, not the thumb. Swapping the two reverses the field direction you predict around the wire.
• (c)
  electric field — No electric field enters this rule. The right-hand thumb rule is about the magnetic effect of a steady current, a magnetostatic result.
• (d)
  motion of the conductor — Motion of the conductor is what the thumb stands for in Fleming's right-hand rule, the generator rule for induced current. Different rule, different thumb.

A straight current-carrying conductor sets up a magnetic field in concentric circles around itself, in the plane perpendicular to the wire. The right-hand thumb rule, also called Maxwell's corkscrew rule, is the memory device for the sense of those circles.

Hold the wire in the right hand with the thumb along the conventional current, and the curl of the fingers is the field's sense. Reverse the current and the circles reverse.

The rule assigns the thumb to the cause, current, and the fingers to the effect, field.

Key facts

• In the right-hand thumb rule the thumb points along the conventional current and the curled fingers give the direction of the magnetic field.
• The magnetic field around a straight current-carrying wire consists of concentric circles centred on the wire.
• Fleming's left-hand rule gives the direction of force on a current-carrying conductor placed in a magnetic field.
• Fleming's right-hand rule gives the direction of the current induced in a conductor moving in a magnetic field.

Common traps

• Pointing the thumb along the magnetic field instead of along the current.
• Applying the right-hand thumb rule to an induced-current question, where Fleming's right-hand rule is the one that applies.

No directly related past PYQ was found.