31.A solenoid 30 cm long is made up of winding 2000 loops of wire on an iron rod whose cross-section is $1.5 cm^{2}$. If the relative permeability of the iron is 6000. What is the self-inductance of the solenoid? A) 1.5 H B) 2.5H C) 3.5 H D) 0.5H View Answer Report DiscussAnswer: Option AExplanation:As we know , self-inductance of the solenoid $L= \frac{\mu_{r}.\mu_{0} N^{2}A}{I}$ $= \frac{600 \times 4 \pi \times 10^{-7} \times (2000)^{2} \times (1.5)\times 10^{-4}}{0.3}$ $=1.5H$
32.An electron moves at right angle to a magnetic field of $5 \times 10^{-2}T$ wirth speed of $6 \times 10^{7}m/s$ .If the specific charge orf the electrons is 1.7$\times 10^{11}$C/kg .The radius of the circular path will be A) 2.9 cm B) 3.9 cm C) 2.35 cm D) 2 cm View Answer Report DiscussAnswer: Option CExplanation:Radius of circular path $r=\frac{mV}{B}=\frac{V}{\left(\frac{e}{m}\right)B}$ $r=\frac{6\times 10^{7}}{1.7\times 10^{11}\times15\times 10^{-2}}$ $=2.35 \times 10^{-2}$m=2.35 m
33.A metallic surface ejects electrons .when exposed to green light of intensity I but no photoelectrons are emitted when exposed to the yellow light of intensity I. It is possible to eject electrons from the same surface by A) yellow light of same intensity which is more than I B) green light of any intensity C) red light of any intensity D) none of above View Answer Report DiscussAnswer: Option AExplanation:$\lambda_{red}$ > $\lambda_{violet}$ As wavelength, $\lambda$ less than that of yellow colour and hence can initiate photoelectric effect irrespective of intensity
34.Beyond which frequency, the ionosphere bands any incident electromagnetic radiation but do not reflect it back towards the earth? A) 50MHz B) 40MHz C) 30MHz D) 20MHz View Answer Report DiscussAnswer: Option BExplanation: The ionosphere can reflect electromagnetic waves of frequency less than 40MHz but not of frequency more than 40 MHz
35.A potential difference across the terminals of a battery is 50V when 11A current is drawn and 60V when 1A current is drawn. The emf and the internal resistance of the battery are A) $6 V , 2\Omega$ B) $63 V , 1\Omega$ C) $61V , 1\Omega$ D) $64 V , 2\Omega$ View Answer Report DiscussAnswer: Option CExplanation:For a closed-circuit cell supplies a constant current in the circuit For cell E=V+Ir For V=50V E=50+11r similarly, for V=60V E=60+r From Eqs.(i) and (ii), we get E=61V
