First slide

Photo Electric Effect

Question

P and Q are two points at distances 10 cm and 20 cm from the axis of a long thin straight current carrying wire. Then the ratio of magnetic energy densities at P and Q is

Easy

A

2 : 1
B

4 : 1
C

8 : 1
D

16 : 1

Solution

(2)
At distance r from the axis of a current carrying wire,
$\begin{array}{l} B = \frac{μ_{0} i}{2 π r} a n d m a g n e t i c \\ e n e r g y d e n s i t y u = \frac{B^{2}}{2 μ_{0}} = \frac{μ_{0} i^{2}}{8 π^{2} . r^{2}} \Rightarrow u \propto \frac{1}{r^{2}} \\ ∴ \frac{u_{P}}{u_{Q}} = {(\frac{20}{10})}^{2} = 4 \end{array}$

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