In a very long straight conductor the current varies with time according to the relation

PHYSICS · Multi-part · A Level · Physics · Fields and their consequences · Fields and their consequences

Problem

I(t)=kt

, where

k

is a positive constant. (i) Explain how the magnitude of the magnetic field at a fixed distance

r

from the conductor changes with time. (ii) If the magnetic field at distance

r=5\,\text{cm}

B_1

t=4\,\text{s}

and

B_2

t=9\,\text{s}

, find the ratio

\dfrac{B_2}{B_1}

. (iii) At the same point, how does the energy density of the magnetic field at

t=9\,\text{s}

compare with that at

t=4\,\text{s}

? State the factor by which it changes.