A very long straight wire carries a current $$I$$. At the instant when a charge +Q at point P has velocity $$\overrightarrow v $$, as shown, the force on the charge is

An electron moves in a circular orbit with a uniform speed v. It producess a magnetic field B at the centre of the circle. The radius of the circle is proportional to

To convert a galvanometer into a voltmeter one should connect a

A galvanometer of 50 ohm resistance has 25 divisions. A current of 4 $$ \times $$ 10^$$-$$4 ampere gives a deflection of one division. To convert this galvanometer into a voltmeter having a range of 25 volts, it should be connected with a resistance of