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NCERT Exemplar · Q7

Q.Consider a wire carrying a steady current, II placed in a uniform magnetic field B⃗\vec{B} perpendicular to its length. Consider the charges inside the wire. It is known that magnetic forces do no work. This implies that,

(a) motion of charges inside the conductor is unaffected by B since they do not absorb energy.
(b) some charges inside the wire move to the surface as a result of B.
(c) if the wire moves under the influence of B, no work is done by the force.
(d) if the wire moves under the influence of B, no work is done by the magnetic force on the ions, assumed fixed within the wire.
Chhattisgarh CgbseMCQ· 1mImportance★★★★★
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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 qq moving with velocity v⃗\vec{v} in a field B⃗\vec{B}:

F⃗=q(v⃗×B⃗).\vec{F} = q(\vec{v} \times \vec{B}).

The rate of work done is

dWdt=F⃗⋅v⃗=q(v⃗×B⃗)⋅v⃗=0,\frac{dW}{dt} = \vec{F} \cdot \vec{v} = q(\vec{v}\times\vec{B})\cdot\vec{v} = 0,

because v⃗×B⃗\vec{v} \times \vec{B} is, by construction, perpendicular to v⃗\vec{v}. 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:

dWion=q(v⃗wire×B⃗)⋅v⃗wire dt=0.dW_{\text{ion}} = q(\vec{v}_{\text{wire}}\times\vec{B})\cdot \vec{v}_{\text{wire}}\,dt = 0.

This is exactly statement (d): if the wire moves under the influence of B⃗\vec{B}, 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 B⃗\vec{B}, 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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