CBSE-XI-Physics
35: Magnetic Field due to a Current
- #12Consider a long, straight wire of cross-sectional area A carrying a current i. Let there be n free electrons per unit volume. An observer places himself on a trolley moving in the direction opposite to the current with a speed
v=inAeand separation from the wire by a distance r. The magnetic field seen by the observer is very nearly
(a)
μ0i2πr
(b) zero
(c)
μ0iπr
(d)
2μ0iπrdigAnsr: aAns : (a) `` \frac{{\,\mathrm{\,\mu \,}}_{o}i}{2\,\mathrm{\,\pi \,}r}``
Magnetic field will be independent of the motion of the observer because the velocity with which the observer is moving is comparable to drift velocity of electron which is very small as compared to the speed of flow of current from one end of wire to other end. So it can be neglected and hence, magnetic field due to the wire w.r.t the observer will be
B = `` \frac{{\,\mathrm{\,\mu \,}}_{o}i}{2\,\mathrm{\,\pi \,}r}``
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