21.An ammeter and a voltmeter of resistance R are connected in series to an electric cell of negligible internal resistance. Their readings are A and V respectively. If another resistance R is connected in parallel with the voltmeter, then A) both A and V will increases B) both A and V will decrease C) A will decrease and V will increase D) A will increase and V will decrease View Answer Report DiscussAnswer: Option DExplanation: The effective resistance will decrease when resistance R is connected in parallel with a voltmeter According to Ohm's law , $V=IR$, or $R=\frac{V}{I}$ Here, as R decreases, so V decreases and I should increase
22.A proton and an $\alpha$-particle accelerated through the same potential difference, enter a region of uniform magnetic field normally. if the radius of the proton orbit is 10 cm , the radius of $\alpha$-particle is A) 10 cm B) $10 \sqrt{2}$cm C) 20 cm D) $5 \sqrt{2}$cm View Answer Report DiscussAnswer: Option BExplanation:Radius of path $r_{time}=\frac{1}{\beta}\sqrt{\frac{2mv}{q}}$ $\therefore$ $\frac{r_{\alpha}}{r_{p}}=\sqrt{\frac{m_{\alpha}}{m_{p}}}\sqrt{\frac{q_{p}}{q_{\alpha}}}$ or $\frac{r_{\alpha}}{10}=\sqrt{\frac{4}{2}}\Rightarrow r_{\alpha}=10\sqrt{2}cm$
23.If we add impurity to a metal those atoms also deflect electrons, therefore A) the electrical and thermal conductivites both increases B) the electrical and thermal conductivites both decreases C) the electrical conductivity increases but thermal conductivity decreases D) the electrical conductivity decreases but thermal conductivity increases View Answer Report DiscussAnswer: Option BExplanation:If the number of electrons increases, their number of collision, increasing the thermal and electrical resistance So, electrical and thermal conductivities both decrease
24.A resistor of 6k$\Omega$ with a tolerance 10% and another resistance of 4$k \Omega$ with a tolerance 10% are connected in series. The tolerance of the combination is about A) 5% B) 10% C) 12% D) 15% View Answer Report DiscussAnswer: Option BExplanation:In series combination equivalent resistance $R=R_{1}+R_{2}$ =6+4=$10k \Omega$ Error in combinbation $\triangle R = \triangle R_{1}+\triangle R_{2}$ $=\frac{10}{100} \times 6+ \frac{10}{100} \times 4$ =$0.6+0.4=1$ $\frac{\triangle R}{R}=\frac{1}{10}=10$%
25.The output Y of the logic circuit shown in the figure is best represented as A) $\overline{A}+\overline {B.C}$ B) $\overline{A}+\overline {B}+C$ C) $\overline{A+B.C}$ D) $\overline{A+ \overline {B}.C}$ View Answer Report DiscussAnswer: Option DExplanation:Boolean expression for Logic gate-I B.C=Y' Boolean expression for Logic gate-II $A+(\overrightarrow{B}.\overrightarrow{C})=Y"$ Boolean expression for Logic gate -III $A+\overrightarrow{B}.\overrightarrow{C}=Y$
