A coil of area 100 cm² is kept at an angle of 30$$^\circ$$ with a magnetic field of 10^$$-$$1 T. The magnetic field is reduced to zero in 10^$$-$$4 s. The induced emf in the coil is -

A 15 $$\Omega$$ resistor, an 80 mH inductor and a capacitor of capacitance C are connected in series with a 50 Hz ac source. If the source voltage and current in the circuit are in phase, then the value of capacitance is

Four objects W, X, Y and Z, each with charge +q are held fixed at four points of a square of side d as shown in the figure. Objects X and Z are on the midpoints of the sides of the square. The electrostatic force exerted by object W on object X is F. Then the magnitude of the force exerted by object W on Z is

Two sources of equal emf are connected in series. This combination is, in turn connected to an external resistance R. The internal resistance of two sources are r₁ and r₂ (r₂ > r₁). If the potential difference across the source of internal resistance r₂ is zero, then R equals to -