Question

A conveyor belt of very wide conducting strap is moving at speed of $v$ as shown in figure. It has uniform surface charge density of $\delta$. a and c are points just above and below belt. Belt is very long.

• A. Magnetic field at point a is $\frac{{\mu }_0\delta v}{2}$
• B. Magnetic field at point b is ${\mu }_0\delta v$
• C. Electric field at a is $\frac{\delta }{2{\epsilon }_0}$
• D. Electric field at b is zero

Answer 1

Answer: (B), (D)

The correct options are
B Magnetic field at point b is ${\mu }_0\delta v$
D Electric field at b is zero

By ampere's law, taking loop passing through a and c $\oint B.dl={\mu }_0{I}_{in}\Rightarrow B\times 2l={\mu }_0\times 0\Rightarrow B=0$ B = 0 at a and c.
For loop passing through a and b $\oint B.dl={\mu }_0{I}_{in}B\times l={\mu }_0\frac{\Delta q}{\Delta t}={\mu }_{0}l\frac{x\delta }{\Delta t}\Rightarrow Bl={\mu }_{0}lv\delta B={\mu }_0\delta v$
b is point inside conductor so E = 0 and $E_a=E_c=\frac{\delta }{\epsilon }$