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Consider a long conducting wire carrying a current. Which of the following statements is NOT true?
The strength of the magnetic field increases if we increase the current.
The magnetic field decreases as we move away from the wire.
The magnetic field will reverse the direction if we reverse the current.
We can use Fleming's left-hand rule to determine the direction of magnetic field.
We can use Fleming's left-hand rule to determine the direction of magnetic field.
The statement 'We can use Fleming's left-hand rule to determine the direction of magnetic field' is false. Fleming's left-hand rule is used to determine the direction of the force acting on a current-carrying conductor in a magnetic field, whereas the direction of the magnetic field around a straight current-carrying wire is determined by the Right-Hand Thumb Rule.
The statement 'We can use Fleming's left-hand rule to determine the direction of magnetic field' is false. Fleming's left-hand rule is used to determine the direction of the force acting on a current-carrying conductor in a magnetic field, whereas the direction of the magnetic field around a straight current-carrying wire is determined by the Right-Hand Thumb Rule.
Think of the Right-Hand Thumb Rule like unscrewing a bottle cap: as you point your thumb in the direction of the 'flow' (current), your wrist turns in the direction of the circular 'swirl' of the magnetic field.
Right-Hand = Field; Left-Hand = Force (Motor effect).
B=2╧Аr╬╝0тАЛIтАЛ тАФ Magnitude of magnetic field due to a long straight wire
F=I(L├ЧB) тАФ Force on a current-carrying conductor (Fleming's Left-Hand Rule context)
According to the Right-Hand Thumb Rule, if you hold a current-carrying wire in your right hand such that your thumb points in the direction of the current, your curled fingers indicate the direction of the magnetic field lines. The magnetic field strength B at a distance r from a long straight wire is given by Ampere's Law as B=2╧Аr╬╝0тАЛIтАЛ, which shows that the field strength is directly proportional to the current I and inversely proportional to the distance r.
Magnetic field lines around a long straight current-carrying conductor are concentric circles.
Increasing the current I increases the magnetic field strength B.
Magnetic field strength B decreases as distance r from the wire increases.
Reversing the current I reverses the direction of the magnetic field B.
Fleming's Left-Hand Rule is for motor force, not for finding field direction.
Electromagnets
Solenoids
Electric motors
The Right-Hand Grip Rule (or Maxwell's Corkscrew Rule) is the correct rule for finding the field direction.
Option D is the incorrect statement because Fleming's Left-Hand Rule is exclusively for the motor effect.
D is correct тАФ Fleming's left-hand rule is used to find the direction of force on a current-carrying conductor in a magnetic field, not the direction of the magnetic field itself.
Always distinguish between 'cause' (current producing a field) and 'effect' (field acting on current); field direction needs the Thumb Rule, motor force needs the Left-Hand Rule.
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