Two point charges placed in air separated by distance $R$ exert a force $F$ on each other. Now the space between those charges is filled with dielectric constant $K$. At distance $R_1$ between the charges, the same force is exerted as $F$. Then

Figure shows a potentiometer wire AB having resistance of $5\ \Omega$ and length 10 m. An e.m.f. is $0.4$ V of battery, the balancing length AP is (internal resistance is negligible)

The currents in different parts of the electric circuit are shown in following figure. The value of current $i$ is

An electron is moving with initial velocity $\vec{v} = V_0\hat{j}$ in a magnetic field $\vec{B} = B_0\hat{i}$. Its de-Broglie wavelength will