Q.Consider a wire carrying a steady current, placed in a uniform magnetic field perpendicular to its length. Consider the charges inside the wire. It is known that magnetic forces do no work. This implies that,
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Start your 14-day free trial to unlock the full solution →The magnetic force is always perpendicular to a charge's total velocity, so it does zero work on any individual charge - even the lattice ions of a wire that is itself moving. This matches stem options (c) and (d).
Why the magnetic force can never do work
For any charge moving with velocity in a field :
The rate of work done is
because is, by construction, perpendicular to . This is true for every moving charge, always - it is a geometric fact about the cross product, not a special property of wires.
Applying this to the wire
The free electrons inside the wire have a small drift velocity, but the crucial point is the ions of the lattice (positively charged, effectively fixed within the wire). If the wire as a whole is set moving by the external field, those ions now have a nonzero velocity too - the wire's velocity. Since the magnetic force is perpendicular to whatever velocity a charge actually has, it does zero work on these ions as well, even while the wire moves:
This is exactly statement (d): if the wire moves under the influence of , no work is done by the magnetic force on the ions, assumed fixed within the wire.
And since this holds for the motion of the wire as a whole (the ions carry it), it also confirms (c): if the wire moves under the influence of , no work is done by the force - i.e. any kinetic energy the wire gains must come from elsewhere (the battery/agent maintaining the current), never from the magnetic field itself.
Why (a) is false …
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