A wire loop is rotated in a magnetic field. The frequency of change of direction of the induced emf is
once per revolution
twice per revolution
four times per revolution
six times per revolution
B.
twice per revolution
The knowledge based the correct choice is (b).
A coil of self-inductance L is connected in series with a bulb B and an AC source. Brightness of the bulb decreases when
frequency of the AC source is decreased
number of turns in the coil is reduced
a capacitance of reactance XC =XL is included in the same circuit
an iron rod is inserted in the coil
The condition under which a microwave oven heats up a food item containing water molecules most efficiently is
the frequency of the microwave must match the resonant frequency of the water molecules
the frequency of the microwave has no relation with natural frequency of water molecules
microwave are heat waves, so always produce heating
infrared waves produce heating in a microwave oven
The half-life of a radioactive isotope X is 20 yr. It decays to another element Y which is stable. The two elements X and Y were found to be in the ratio 1 : 7 in a sample of a given rock. The age of the rock is estimated to be
40 yr
60 yr
80 yr
100 yr
A certain mass of hydrogen is changed to helium by the process of fusion. The mass defect in fusion reaction is 0.02866 u. The energy liberated per u is (given 1 u = 931 MeV)
2.67 MeV
26.7 MeV
6.675 MeV
13.35 MeV
For photoelectric emission from certain metal the cut-off frequency is v. If radiation of frequency 2v impinges on the metal plate, the maximumpossible velocity of the emitted electron will be (m is the electron mass)
The wavelength λe of an electron and λp of a photon of same energy E are related by
λp ∝ λe2
λp ∝ λe
A plano-convex lens fits exactly into a plano-concave lens. Their plane surfaces are parallel to each other. If lenses are made of different materials of refractive indices µ1 and µ2 and R is the radius of curvature of the curved surface of the lenses, then the focal length of the combination is
For a normal eye, the cornea of eye provides a converging power of 40 D and the least converging power of the eye lens behind the cornea is 20 D. Using this information, the distance between the retina and the cornea-eye lens can be estimated to be
5 cm
2.5 cm
1.67 cm
1.5 cm