A circular disc of radius R carries surface charge density
$$\sigma \left( r \right) = {\sigma _0}\left( {1 - {r \over R}} \right)$$, where $$\sigma $$₀ is a constant and r is the distance from the center of the disc. Electric flux through a large spherical surface that encloses the charged disc completely is $$\phi $$₀. Electric flux through another spherical surface of radius $${R \over 4}$$ and concentric with the disc is $$\phi $$. Then the ratio $${{{\phi _0}} \over \phi }$$ is ________.

In the following circuit, the current through the resistor R(=2$$\Omega$$) is I amperes. The value of I is :

A galvanometer gives full scale deflection with 0.006 A current. By connecting it to a 4990 $$\Omega$$ resistance, it can be converted into a voltmeter of range 0-30V. If connected to a $${{2n} \over {249}}\Omega $$ resistance, it becomes an ammeter of range 0-1.5 A. The value of n is

Two batteries of different emfs and different internal resistance are connected as shown. The voltage across AB in volts is __________.