Question

Charges $Q_1$ and $Q_2$ lies inside and outside respectively of a closed surface $S$. Let $E$ be the field at any point on $S$ and $\varphi$ be the flux of $E$ over $S$:

• A. If $Q_1$ changes, both $E$ and $\varphi$ will change
• B. If $Q_2$ changes, $E$ will change but $\varphi$ will not change
• C. If $Q_1=0$ and $q_2\ne 0$ then $E\ne 0$ but $\varphi =0$
• D. If $Q_1=0$ and $Q_2=0$ then $E=0$ but $\varphi \ne 0$

Answer 1

Answer: (A), (B), (C)

The correct options are
A If $Q_1$ changes, both $E$ and $\varphi$ will change
B If $Q_2$ changes, $E$ will change but $\varphi$ will not change
C If $Q_1=0$ and $q_2\ne 0$ then $E\ne 0$ but $\varphi =0$

According to Gauss's law, the flux depends only on the charge inside the surface and the net electric field is the vector sum of field due to all charges. Therefore, (A),(B) and (C) are the correct option.