A telescope has an objective of focal length $$60 \mathrm{~cm}$$ and eyepiece of focal length $$5 \mathrm{~cm}$$. The telescope is focussed for least distance of distinct vision $$300 \mathrm{~cm}$$ away from the object. The magnification produced by the telescope at least distance of distinct vision is
A parallel plate capacitor having a dielectric constant 5 and dielectric strength $$10^6 \mathrm{~V} \mathrm{~m}^{-1}$$ is to be designed with voltage rating of $$2 \mathrm{~kV}$$. The field should never exceed $$10 \%$$ of its dielectric strength. To have the capacitance of $$60 \mathrm{~pF}$$ the minimum area of the plates should be
The coefficient of volume expansion of glycerine is $$49 \times 10^{-5} \mathrm{~K}^{-1}$$. The percentage change in its density for a $$50^{\circ} \mathrm{C}$$ rise in temperature is
A current I flows in an infinitely long wire with cross section in the form of semi-circular ring of radius $$1 \mathrm{~m}$$. The magnitude of the magnetic induction along its axis is