Imagine a current-carrying straight conductor with magnetic field of lines in anti-clockwise direction. Then the direction of current is determined by
- A.the Right-Hand Thumb rule and it would be in the downward direction.
- B.the Left-Hand Thumb rule and it would be in the downward direction.
- C.the Right-Hand Thumb rule and it would be in the upward direction.
- D.the Left-Hand Thumb rule and it would be in the upward direction.
Correct answer
Explanation
The correct answer is C, the Right-Hand Thumb rule and it would be in the upward direction. For a straight wire, the right-hand thumb rule links the current to the circular field lines around it.
Hold the wire in the right hand with the thumb along the current; the curled fingers then show the direction of the magnetic field lines, which form concentric circles around the wire. If the field lines, seen from above, run anticlockwise, the curled fingers of the right hand go anticlockwise only when the thumb points up, towards the viewer. So the current flows upward. If the field were clockwise, the current would flow downward. This rule is also called Maxwell's corkscrew rule.
A is wrong because an anticlockwise field means an upward, not a downward, current. B and D are wrong because there is no left-hand thumb rule for this; Fleming's left-hand rule gives the force on a conductor in a magnetic field, as in a motor.
Exam tip: right-hand thumb rule for the field around a wire; Fleming's left-hand rule for motors, right-hand rule for generators.
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