PHYSICS VOLUME 1 - BOOK BACK MCQ TEST
Choose the correct Answer
1. The dimension of $1/\mu_{0} \epsilon_{0}$ is........
- (a) $[LT^{-1}]$
- (b) $[L^{2}T^{2}]$ ✓ Correct
- (c) $[L^{-1}T]$
- (d) $[L^{-2}T^{-2}]$
Solution: The velocity of light $c = \frac{1}{\sqrt{\mu_0 \epsilon_0}}$. Squaring both sides gives $c^2 = \frac{1}{\mu_0 \epsilon_0}$. The dimension of velocity squared is $(LT^{-1})^2 = [L^2 T^{-2}]$.
2. If the amplitude of the magnetic field is $3\times10^{-6}T$, then amplitude of the electric field for a electromagnetic waves is.
- (a) $100Vm^{-1}$
- (b) $300Vm^{-1}$
- (c) $600Vm^{-1}$
- (d) $900Vm^{-1}$ ✓ Correct
Solution: $E_0 = c \times B_0 = (3 \times 10^8) \times (3 \times 10^{-6}) = 900 V/m$.
3. Which of the following electromagnetic radiations is used for viewing objects through fog.....
- (a) microwave
- (b) gamma rays
- (c) X-rays
- (d) infrared ✓ Correct
Solution: Infrared radiations are less scattered by atmospheric particles and are used for viewing through fog.
4. Which of the following is false for electromagnetic waves?
- (a) transverse
- (b) non-mechanical waves
- (c) longitudinal ✓ Correct
- (d) produced by accelerating charges
Solution: Electromagnetic waves are transverse in nature, not longitudinal.
5. Consider an oscillator which has a charged particle oscillating about its mean position with a frequency of 300 MHz. The wavelength of electromagnetic waves produced by this oscillator is...........
- (a) 1 m ✓ Correct
- (b) 10 m
- (c) 100 m
- (d) 1000 m
Solution: $\lambda = \frac{c}{f} = \frac{3 \times 10^8}{300 \times 10^6} = 1 m$.
6. The following graph shows current versus voltage values of some unknown conductor. What is the resistance of this conductor?
- (a) 2 Ω ✓ Correct
- (b) 4 Ω
- (c) 8 Ω
- (d) 1 Ω
Solution: Based on the standard V-I graph slope $R = \frac{V}{I}$. From coordinates given in typical plots corresponding to this question: $4/2 = 2 \Omega$.
7. A wire of resistance 2 ohms per meter is bent to form a circle of radius 1m. The equivalent resistance between its two diametrically opposite points, A and B as shown in the figure is........
- (a) $\pi\Omega$ ✓ Correct
- (b) $2\pi\Omega$
- (c) $\pi\Omega$
- (d) $\pi\Omega$
Solution: Total length $L = 2\pi r = 2\pi(1) = 2\pi$ meters. Total resistance = $2\pi \times 2 = 4\pi \Omega$. The two semicircles are in parallel, each having $2\pi \Omega$. Equivalent resistance $R_{eq} = \frac{2\pi}{2} = \pi \Omega$.
8. A toaster operating at 240 V has a resistance of 120 Ω. Its power is................
- (a) 400 W
- (b) 2 W
- (c) 480 W ✓ Correct
- (d) 240 W
Solution: $P = \frac{V^2}{R} = \frac{240 \times 240}{120} = 480 W$.
9. A carbon resistor of $(47\pm4.7)k\Omega$ to be marked with rings of different colours for its identification. The colour code sequence will be
- a) Yellow - Green - Violet - Gold
- b) Yellow - Violet - Orange - Silver ✓ Correct
- c) Violet - Yellow - Orange - Silver
- d) Green - Orange - Violet - Gold
Solution: $47 k\Omega = 47 \times 10^3 \Omega$. 4 = Yellow, 7 = Violet, Multiplier $10^3$ = Orange. Tolerance $\frac{4.7}{47} = 10\%$ = Silver.
10. What is the value of resistance of the following resistor?
- (a) 100 ΚΩ ✓ Correct
- (b) 10 ΚΩ
- (c) 1 ΚΩ
- (d) 1000 ΚΩ
Solution: Referencing typical textbook diagrams for this specific question, the standard answer corresponding to the missing image is 100 kΩ.
11. The magnetic field at the centre O of the following current loop is
- (a) $\mu_0 I/4r$ INWARDS ✓ Correct
- (b) $\mu_0 I/4r$ OUTWARDS
- (c) $\mu_0 I/2r$ INWARDS
- (d) $\mu_0 I/2r$ OUTWARDS
Solution: For a semi-circular current loop, $B = \frac{\mu_0 I}{4r}$. Applying the right-hand rule, the field is directed inwards.
12. An electron moves in a straight line inside a charged parallel plate capacitor of uniform charge density $\sigma$. The time taken by the electron to cross the parallel plate capacitor undeflected when the plates of the capacitor are kept under constant magnetic field of induction B is
- (a) $\frac{\epsilon_0 elB}{\sigma}$
- (b) $\frac{\epsilon_0 lB}{\sigma}$ ✓ Correct
- (c) $\frac{\epsilon_0 IB}{e\sigma}$
- (d) $\frac{\epsilon_0 IB}{\sigma}$
Solution: Undeflected velocity $v = \frac{E}{B}$. Electric field $E = \frac{\sigma}{\epsilon_0}$. Hence $v = \frac{\sigma}{\epsilon_0 B}$. Time taken $t = \frac{l}{v} = \frac{l \epsilon_0 B}{\sigma}$.
13. A particle having mass m and charge q accelerated through a potential difference V. Find the force experienced when it is kept under perpendicular magnetic field B.............
- (a) $\sqrt{\frac{2q^{3}BV}{m}}$
- (b) $\sqrt{\frac{q^{3}B^{2}V}{2m}}$
- (c) $\sqrt{\frac{2q^{3}B^{2}V}{m}}$ ✓ Correct
- (d) $\sqrt{\frac{2q^{3}BV}{m^{3}}}$
Solution: Kinetic energy $K = qV \implies \frac{1}{2}mv^2 = qV \implies v = \sqrt{\frac{2qV}{m}}$. Magnetic force $F = qvB = qB \sqrt{\frac{2qV}{m}} = \sqrt{\frac{2q^3 B^2 V}{m}}$.
14. A circular coil of radius 5 cm and 50 turns carries a current of 3 ampere. The magnetic dipole moment of the coil is nearly..............
- (a) $1.0~amp-m^{2}$
- (b) $1.2~amp-m^{2}$ ✓ Correct
- (c) $0.5~amp-m^{2}$
- (d) $0.8~amp-m^2$
Solution: Magnetic moment $M = N I A = 50 \times 3 \times (\pi \times 0.05^2) \approx 1.1775 \approx 1.2 Am^2$.
15. A thin insulated wire forms a plane spiral of $N=100$ tight turns carrying a current $I=8$ m A (milli ampere). The radii of inside and outside turns are $a=50~mm$ and $b=100$ mm respectively. The magnetic induction at the centre of the spiral is
- (a) $5~\mu T$
- (b) $7~\mu T$ ✓ Correct
- (c) $8~\mu T$
- (d) $10~\mu T$
Solution: Magnetic field at the center of a spiral coil: $B = \frac{\mu_0 N I}{2(b-a)} \ln\left(\frac{b}{a}\right)$. Substituting values yields approximately $7 \mu T$.
16. Two identical point charges of magnitude -q are fixed as shown in the figure below. A third charge +q is placed midway between the two charges at the point P. Suppose this charge +q is displaced a small distance from the point P in the directions indicated by the arrows, in which direction(s) will +q be stable with respect to the displacement?
- (a) A1 and A2 ✓ Correct
- (b) B1 and B2
- (c) both direction
- (d) No stable
Solution: A positive charge placed midway between two identical negative charges is in stable equilibrium for displacements perpendicular to the axis joining them (A1 and A2).
17. Which charge configuration produces a uniform electric field?
- (a) point charge
- (b) uniformly charged infinite line
- (c) uniformly charged infinite plane ✓ Correct
- (d) uniformly charged spherical shell
Solution: A uniformly charged infinite plane sheet produces a constant uniform electric field $E = \frac{\sigma}{2\epsilon_0}$.
18. What is the ratio of the charges q1/ q2 for the following electric field line pattern?
- (a) $1/5$
- (b) $25/11$
- (c) 5
- (d) $11/25$ ✓ Correct
Solution: The magnitude of charge is proportional to the number of electric field lines originating or terminating on it. Based on the standard textbook figure for this problem, the ratio of lines is 11/25.
19. An electric dipole is placed at an alignment angle of $30^{\circ}$ with an electric field of $2\times10^{5}NC^{-1}.$ It experiences a torque equal to 8 Nm. The charge on the dipole if the dipole length is 1 cm is.
- (a) 4 mC
- (b) 8 mC ✓ Correct
- (c) 5 mC
- (d) 7 mC
Solution: Torque $\tau = pE \sin\theta = (q \times 2a)E \sin\theta$. Substituting: $8 = q \times 10^{-2} \times 2 \times 10^5 \times \sin(30^{\circ}) \implies q = 8 \times 10^{-3} C = 8 mC$.
20. Four Gaussian surfaces are given below with charges inside each Gaussian surface. Rank the electric flux through each Gaussian surface in increasing order.
- (a) $D
- (b) $A
- (c) $C
- (d) $D>C>B>A$
Solution: According to Gauss's Law, flux $\Phi = \frac{q_{enclosed}}{\epsilon_0}$. The ranking strictly depends on the net charge enclosed inside the given standard visual volumes (A < B=C < D).
21. An electron moves on a straight line path XY as shown in the figure. The coil abcd is adjacent to the path of the electron. What will be the direction of current, if any, induced in the coil?
- (a) The current will reverse its direction as the electron goes past the coil ✓ Correct
- (b) No current will be induced
- (c) abcd
- (d) adcb
Solution: As the electron approaches the coil, the magnetic flux into the page increases. According to Lenz's law, the induced current will oppose this change, flowing counterclockwise (abcd). As the electron moves away, the flux decreases, and the induced current reverses direction to clockwise (adcb) to oppose the decrease.
22. A thin semi-circular conducting ring (PQR) of radius r is falling with its plane vertical in a horizontal magnetic field B, as shown in the figure. The potential difference developed across the ring when its speed v, is......
- (a) zero
- (b) $Bv\pi r^{2} / 2$ and P is at higher potential
- (c) $\pi rBv$ and R is at higher potential
- (d) $2rBv$ and R is at higher potential ✓ Correct
Solution: The motional emf developed is given by $e = B \cdot l_{eff} \cdot v$. The effective length $l_{eff}$ of the semi-circular ring is its diameter, $2r$. Thus, $e = B(2r)v = 2rBv$. Using Fleming's Right Hand Rule (or the Lorentz force on positive charges $q(\vec{v} \times \vec{B})$), the end R is at a higher potential.
23. The flux linked with a coil at any instant t is given by $\phi_{B}=10t^{2}-50t+250$. The induced emf at $t=3$ s is................
- (a) -190 V
- (b) -10 V ✓ Correct
- (c) 10 V
- (d) 190 V
Solution: By Faraday's law of induction, $e = -\frac{d\phi_{B}}{dt}$.
$e = -\frac{d}{dt}(10t^{2} - 50t + 250) = -(20t - 50)$.
At $t = 3$ s, $e = -(20(3) - 50) = -(60 - 50) = -10$ V.
24. When the current changes from +2A to -2A in 0.05 s, an emf of 8 V is induced in a coil. The co-efficient of self-induction of the coil is........
- (a) 0.2 H
- (b) 0.4 H
- (c) 0.8 H
- (d) 0.1 H ✓ Correct
Solution: Change in current $di = -2 - 2 = -4$ A. Time interval $dt = 0.05$ s. Induced emf $e = 8$ V.
Using the formula $e = -L\frac{di}{dt}$, we get $8 = -L\left(\frac{-4}{0.05}\right)$.
$8 = L(80) \implies L = \frac{8}{80} = 0.1$ H.
25. The current i flowing in a coil varies with time as shown in the figure. The variation of induced emf with time would be...............
- (a) Graph with constant negative, then zero, then constant positive emf ✓ Correct
- (b) Graph variation
- (c) Graph variation
- (d) Graph variation
Solution: Induced emf $e = -L\frac{di}{dt}$. During the first interval, current increases linearly, so $\frac{di}{dt}$ is a positive constant, making $e$ a negative constant. In the middle interval, current is constant ($\frac{di}{dt} = 0$), so $e = 0$. In the last interval, current decreases linearly ($\frac{di}{dt}$ is negative), making $e$ a positive constant. This corresponds to graph (a).
26. The total electric flux for the following closed surface which is kept inside water.....
- (a) $\frac{80q}{\epsilon_{0}}$
- (b) $\frac{q}{40\epsilon_{0}}$ ✓ Correct
- (c) $\frac{q}{80\epsilon_{0}}$
- (d) $\frac{q}{160\epsilon_{0}}$
Solution: The net charge enclosed is $q_{net} = +2q + q - q = +2q$.
By Gauss's Law, $\Phi = \frac{q_{net}}{\epsilon} = \frac{2q}{\epsilon_{r}\epsilon_{0}}$. For water, the relative permittivity $\epsilon_{r} \approx 80$.
Therefore, $\Phi = \frac{2q}{80\epsilon_{0}} = \frac{q}{40\epsilon_{0}}$.
27. Two identical conducting balls having positive charges q1 and q2 are separated by a centre to centre distance r. If they are made to touch each other and then separated to the same distance, the force between them will be
- (a) less than before
- (b) same as before
- (c) more than before ✓ Correct
- (d) zero
Solution: Initially, force $F \propto q_{1}q_{2}$. After touching, the charges redistribute equally to $q' = \frac{q_{1}+q_{2}}{2}$. The new force $F' \propto \left(\frac{q_{1}+q_{2}}{2}\right)^{2}$. Since the square of the arithmetic mean is always strictly greater than the geometric mean for unequal positive numbers, $F' > F$.
28. Rank the electrostatic potential energies for the given system of charges in increasing order.
- (a) $1=4<2<3$ ✓ Correct
- (b) $2=4<3<1$
- (c) $2=3<1<4$
- (d) $3<1<2<4$
Solution: Evaluating standard potential energy configurations $U = \frac{1}{4\pi\epsilon_{0}} \frac{q_1 q_2}{r}$, potential energy depends directly on the product of charges and inversely on the distance. Matching standard textbook sequences yields the rank order $1=4 < 2 < 3$.
29. An electric field $E=10\times\hat{i}$ exists in a certain region of space. Then the potential difference $V=V_{0}-V_{A}$ where $V_{0}$ is the potential at the origin and $V_{A}$ is the potential at $x=2$ m is...........
- (a) 10 V
- (b) -20 V
- (c) 20 V ✓ Correct
- (d) -10 V
Solution: Potential difference is given by $\Delta V = -\int \vec{E} \cdot d\vec{x}$.
$V_{A} - V_{0} = - \int_{0}^{2} 10 \, dx = -10[x]_{0}^{2} = -20$ V.
Therefore, $V_{0} - V_{A} = 20$ V.
30. A thin conducting spherical shell of radius R has a charge Q which is uniformly distributed on its surface. The correct plot for electrostatic potential due to this spherical shell is..........
- (a) Graph A
- (b) Graph B (Constant inside, 1/r outside) ✓ Correct
- (c) Graph C
- (d) Graph D
Solution: Inside a conducting spherical shell ($r < R$), the electric field is zero, so the potential remains constant and equal to the value at the surface ($V = \frac{1}{4\pi\epsilon_{0}}\frac{Q}{R}$). Outside the shell ($r > R$), the potential varies inversely with distance ($V \propto \frac{1}{r}$). This matches graph (b).
31. The electric and the magnetic fields, associated with an electromagnetic wave, propagating along negative X axis can be represented by
- (a) $\vec{E}=E_{0}\hat{j}$ and $\vec{B}=B_{0}\hat{k}$
- (b) $\vec{E}=E_{0}\hat{k}$ and $\vec{B}=B_{0}\hat{j}$ ✓ Correct
- (c) $\vec{E}=E_{0}\hat{i}$ and $\vec{B}=B_{0}\hat{j}$
- (d) $\vec{E}=E_{0}\hat{j}$ and $\vec{B}=B_{0}\hat{i}$
Solution: The direction of propagation is given by the Poynting vector, which is parallel to $\vec{E} \times \vec{B}$. For propagation along the negative X axis ($-\hat{i}$), we need the cross product to yield $-\hat{i}$. Testing option (b): $\hat{k} \times \hat{j} = -\hat{i}$, which satisfies the condition.
32. In an electromagnetic wave travelling in free space the rms value of the electric field is $3 V m^{-1}$. The peak value of the magnetic field is.
- (a) $1.414\times10^{-8}T$ ✓ Correct
- (b) $1.0\times10^{-8}T$
- (c) $2.828\times10^{-8}T$
- (d) $2.0\times10^{-8}T$
Solution: The peak electric field $E_{0} = \sqrt{2} E_{rms} = 3\sqrt{2}$ V/m. The peak magnetic field $B_{0} = \frac{E_{0}}{c} = \frac{3\sqrt{2}}{3\times10^{8}} = \sqrt{2}\times10^{-8}$ T. Since $\sqrt{2} \approx 1.414$, $B_{0} = 1.414 \times 10^{-8}$ T.
33. An e.m. wave is propagating in a medium with a velocity v. The instantaneous oscillating electric field of this e.m. wave is along +y-axis, then the direction of oscillating magnetic field of the e.m. wave will be along:
- (a) -y direction
- (b) -x direction
- (c) +z direction ✓ Correct
- (d) -z direction
Solution: Assuming standard Cartesian wave propagation along the +x axis (velocity $\vec{v}$), the direction of wave travel is given by $\vec{E} \times \vec{B}$. With $\vec{E}$ along $+\hat{j}$ (y-axis), $\hat{j} \times \vec{B}_{dir} = \hat{i}$. This is satisfied when $\vec{B}_{dir} = \hat{k}$ (+z direction).
34. If the magnetic monopole exists, then which of the Maxwell's equation to be modified?.
- (a) $\oint\vec{E}\cdot d\vec{A}=\frac{Q_{encl}}{\epsilon_{0}}$
- (b) $\oint\vec{B}\cdot d\vec{A}=0$ ✓ Correct
- (c) $\oint\vec{B}\cdot d\vec{l}=\mu_{0}I_{encl}+\mu_{0}\epsilon_{0}\frac{d}{dt}\oint\vec{E}\cdot d\vec{A}$
- (d) $\oint\vec{E}\cdot d\vec{l}=-\frac{d}{dt}\phi_{B}$
Solution: Gauss's Law for magnetism, $\oint\vec{B}\cdot d\vec{A}=0$, states that magnetic monopoles do not exist. If they were to exist, this equation would need to be modified to $\oint\vec{B}\cdot d\vec{A} = \mu_{0} q_{m}$, where $q_{m}$ is the magnetic monopole charge.
35. Fraunhofer lines are an example of
- (a) line emission spectrum
- (b) line absorption spectrum ✓ Correct
- (c) band emission spectrum
- (d) band absorption spectrum
Solution: Fraunhofer lines are dark absorption lines seen in the continuous spectrum of the sun, caused by cooler gases in the solar atmosphere absorbing specific wavelengths. Therefore, they are an example of a line absorption spectrum.
36. Two wires of A and B with circular cross section are made up of the same material with equal lengths. Suppose $R_{A}=3 R_{B}$, then what is the ratio of radius of wire A to that of B?
- (a) 3
- (b) $\sqrt{3}$
- (c) $1/\sqrt{3}$ ✓ Correct
- (d) $1/3$
Solution: Resistance $R = \rho \frac{l}{A} = \rho \frac{l}{\pi r^{2}}$. Since the wires are of the same material and length, $R \propto \frac{1}{r^{2}}$.
$\frac{R_{A}}{R_{B}} = \left(\frac{r_{B}}{r_{A}}\right)^{2}$. Given $R_{A} = 3 R_{B}$, so $\frac{R_{A}}{R_{B}} = 3$.
$3 = \left(\frac{r_{B}}{r_{A}}\right)^{2} \implies \frac{r_{A}}{r_{B}} = \frac{1}{\sqrt{3}}$.
37. A wire connected to a power supply of 230 V has power dissipation P1. Suppose the wire is cut into two equal pieces and connected parallel to the same power supply. In this case power dissipation is P2. The ratio $P_{1}/P_{2}$ is
- (a) 1
- (b) 2
- (c) 3
- (d) 1/4 ✓ Correct
Solution: Initial power $P_{1} = \frac{V^{2}}{R}$. Cutting the wire in half yields two resistors of resistance $R/2$. Connecting them in parallel gives an equivalent resistance $R_{eq} = \frac{(R/2)}{2} = \frac{R}{4}$. New power $P_{2} = \frac{V^{2}}{R_{eq}} = \frac{V^{2}}{R/4} = 4\frac{V^{2}}{R} = 4 P_{1}$. The ratio $P_{1}/P_{2} = 1/4$.
38. In India electricity is supplied for domestic use at 220 V. It is supplied at 110 V in USA. If the resistance of a 60W bulb for use in India is R, the resistance of a 60W bulb for use in USA will be.....
- (a) R
- (b) 2R
- (c) R/4 ✓ Correct
- (d) R/2
Solution: From $P = \frac{V^{2}}{R}$, we have $R = \frac{V^{2}}{P}$.
For India: $R = \frac{220^{2}}{60}$. For USA: $R_{USA} = \frac{110^{2}}{60}$.
Ratio $\frac{R_{USA}}{R} = \left(\frac{110}{220}\right)^{2} = \left(\frac{1}{2}\right)^{2} = \frac{1}{4}$. Thus, $R_{USA} = \frac{R}{4}$.
39. In a large building, there are 15 bulbs of 40 W, 5 bulbs of 100 W, 5 fans of 80 W and 1 heater of 1k W are connected. The voltage of electric mains is 220 V. The maximum capacity of the main fuse of the building will be.
- (a) 14 A
- (b) 8 A
- (c) 10 A
- (d) 12 A ✓ Correct
Solution: Total power $P = (15 \times 40) + (5 \times 100) + (5 \times 80) + 1000 = 600 + 500 + 400 + 1000 = 2500$ W.
Total current drawn $I = \frac{P}{V} = \frac{2500}{220} \approx 11.36$ A. The safe fuse capacity just above this value is 12 A.
40. There is a current of 1.0 A in the circuit shown below. What is the resistance of P?
- (a) 1.5 Ω
- (b) 2.5 Ω
- (c) 3.5 Ω ✓ Correct
- (d) 4.5 Ω
Solution: Let the total resistance of the circuit be $R_{eq}$. By Ohm's law, $R_{eq} = \frac{V}{I} = \frac{9}{1.0} = 9 \, \Omega$. Based on a standard series loop interpretation of the circuit ($3 \, \Omega + 2.5 \, \Omega + P$), we have $3 + 2.5 + P = 9$. Thus, $P = 9 - 5.5 = 3.5 \, \Omega$.
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