A circuit is connected to an ac source of variable frequency. As the frequency of the source is increased, the current first increases and then decreases. Which of the following combinations of elements is likely to comprise the circuit?

If n, e, $$\tau$$ and m have their usual meanings, then the resistance of a wire of length l and cross-sectional area A is given by -

A proton and an alpha particle move in circular orbits in a uniform magnetic field. Their speeds are in the ratio of 9 : 4. The ratio of radii of their circular orbits $$\left( {{{{r_p}} \over {{r_{alpha}}}}} \right)$$ is

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 -