Two circular coils 1 and 2 are made from the same wire but the radius of the 1^st coil is twice that of the 2^nd coil. What is the ratio of potential difference in volts should be applied across them so that the magnetic field at their centres is the same?

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