A uniformly charged ring of radius 3a and total charge q is placed in xy-plane centred at origin. A point charge q is moving towards the ring along the z-axis and has speed u at z = 4a. The minimum value of u such that it crosses the origin is :

In the given circuit, an ideal voltmeter connected across the 10$$\Omega $$ resistance reads 2V. The internal resistance r, of each cell is:

A moving coil galvanometer allows a full scale current of 10^–4 A. A series resistance of 2 M$$\Omega $$ is required to convert the above galvanometer into a voltmeter of range 0-5 V. Therefore the value of shunt resistance required to convert the above galvanometer into an ammeter of range 0.10 mA is :