NCERT Exemplar

Samples of two radioactive nuclides A and B are taken. \lambda_A and \lambda_B are the disintegration constants of A and B respectively. In which of the following cases, the two samples can simultaneously have the same decay rate at any time?
(a) Initial rate of decay of A is twice the initial rate of decay of B and \lambda_A=\lambda_B
(b) Initial rate of decay of A is twice the initial rate of decay of B and \lambda _A>\lambda_B
(c) Initial rate of decay of B is twice the initial rate of decay of A and \lambda_A>\lambda_B
(d) Initial rate of decay of B is the same as the rate of decay of A at t = 2h and \lambda_B<\lambda_A

The correct options are: (b) Initial rate of decay of A is twice the initial rate of decay of B and $\lambda _A>\lambda_B$ (d) Initial rate of decay of B is the same as the rate of decay of A at...

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In a nuclear reactor, moderators slow down the neutrons which come out in a fission process. The moderator used to have light nuclei. Heavy nuclei will not serve the purpose because
(a) they will break up
(b) elastic collision of neutrons with heavy nuclei will not slow them down
(c) the net weight of the reactor would be unbearably high
(d) substances with heavy nuclei do not occur in a liquid or gaseous state at room temperature

The correct option is: (b) elastic collision of neutrons with heavy nuclei will not slow them down

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Tritium is an isotope of hydrogen whose nucleus Triton contains 2 neutrons and 1 proton. Free neutrons decay into P+\bar{e}+\bar{v}. If one of the neutrons in Triton decays, it would transform into He3 nucleus. This does not happen. This is because
(a) Triton energy is less than that of a \mathrm{He}^{3} nucleus
(b) the electron created in the beta decay process cannot remain in the nucleus
(c) both the neutrons in triton have to decay simultaneously resulting in a nucleus with 3 protons, which is not a \mathrm{He}^{3} nucleus
(d) because free neutrons decay due to external perturbations which is absent in a triton nucleus

The correct option is: (a) Triton energy is less than that of a $\mathrm{He}^{3}$ nucleus

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Suppose we consider a large number of containers each containing initially 10000 atoms of a radioactive material with a half-life of 1 year. After 1 year
(a) all the containers will have 5000 atoms of the material
(b) all the containers will contain the same number of atoms of the material but that number will only be approximately 5000
(c) the containers will, in general, have different numbers of the atoms of the material but their average will be close to 5000
(d) none of the containers can have more than 5000 atoms

The correct option is: (c) the containers will, in general, have different numbers of the atoms of the material but their average will be close to 5000

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In a group of 50 students, the number of students studying French, English, Sanskrit were found to be as follows: French = 17, English = 13, Sanskrit = 15 French and English = 09, English and Sanskrit = 4 French and Sanskrit = 5, English, French and Sanskrit = 3. Find the number of students who study (i) at least one of the three languages (ii) none of the three languages

Solution: Given, $50 =$ Total number of students $13 =$ Number of students studying English $17 =$ Number of students studying French $15 =$ Number of students studying Sanskrit $9 =$ Number of...

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In a group of 50 students, the number of students studying French, English, Sanskrit were found to be as follows: French = 17, English = 13, Sanskrit = 15 French and English = 09, English and Sanskrit = 4 French and Sanskrit = 5, English, French and Sanskrit = 3. Find the number of students who study (i) French and Sanskrit but not English (ii) French and English but not Sanskrit

Solution: Given, $50 =$ Total number of students $13 =$ Number of students studying English $17 =$ Number of students studying French $15 =$ Number of students studying Sanskrit $9 =$ Number of...

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In a group of 50 students, the number of students studying French, English, Sanskrit were found to be as follows: French = 17, English = 13, Sanskrit = 15 French and English = 09, English and Sanskrit = 4 French and Sanskrit = 5, English, French and Sanskrit = 3. Find the number of students who study (i) Sanskrit only (ii) English and Sanskrit but not French

Solution: Given, $50 =$ Total number of students $13 =$ Number of students studying English $17 =$ Number of students studying French $15 =$ Number of students studying Sanskrit $9 =$ Number of...

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In a group of 50 students, the number of students studying French, English, Sanskrit were found to be as follows: French = 17, English = 13, Sanskrit = 15 French and English = 09, English and Sanskrit = 4 French and Sanskrit = 5, English, French and Sanskrit = 3. Find the number of students who study (i) French only (ii) English only

Solution: Given, $50 =$ Total number of students $13 =$ Number of students studying English $17 =$ Number of students studying French $15 =$ Number of students studying Sanskrit $9 =$ Number of...

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In a town of 10,000 families it was found that 40% families buy newspaper A, 20% families buy newspaper B, 10% families buy newspaper C, 5% families buy A and B, 3% buy B and C and 4% buy A and C. If 2% families buy all the three newspapers. Find (a) The number of families which buy newspaper A only. (b) The number of families which buy none of A, B and C

Solution: As per the question, $=10,000$ = Total number of families $\mathrm{A}=\mathrm{n}(\mathrm{A})=40 \%$ = Number of families buying newspaper $B=n(B)=20 \%$ = Number of families buying...

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In a survey of 200 students of a school, it was found that 120 study Mathematics, 90 study Physics and 70 study Chemistry, 40 study Mathematics and Physics, 30 study Physics and Chemistry, 50 study Chemistry and Mathematics and 20 none of these subjects. Find the number of students who study all the three subjects.

Solution: As per the question, n(U) = 200 = Total number of students n(M) = 120 = Number of students who study Mathematics n(P) = 90 = Number of students who study Physics n(C) = 70 = Number of...

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Out of 100 students; 15 passed in English, 12 passed in Mathematics, 8 in Science, 6 in English and Mathematics, 7 in Mathematics and Science; 4 in English and Science; 4 in all the three. Find how many passed (iii) in Mathematics only (iv) in more than one subject only.

Solution: As per the question, 100 = Total number of students 15 = Number of students who passed in English 12 = Number of students who passed in Mathematics 8 = Number of students who passed in...

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Out of 100 students; 15 passed in English, 12 passed in Mathematics, 8 in Science, 6 in English and Mathematics, 7 in Mathematics and Science; 4 in English and Science; 4 in all the three. Find how many passed (i) in English and Mathematics but not in Science (ii) in Mathematics and Science but not in English

Solution: As per the question, 100 = Total number of students 15 = Number of students who passed in English 12 = Number of students who passed in Mathematics 8 = Number of students who passed in...

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A long straight cable of length I is placed symmetrically along the z-axis and has radius a. The cable consists of a thin wire and a co-axial conducting tube. An alternating current I(t)=I_{0} sin (2\pi vt) flows down the central thin wire and returns along the co-axial conducting tube. The induced electric field at a distance s from the wire inside the cable is \mathbf{E}(\mathrm{s}, \mathrm{t})=\mu_{0} \mathrm{l}_{0} \mathrm{~V} cos (2\pi vt). In \left(\frac{s}{a}\right) \hat{k},
compare the conduction current 10 with the displacement current I_{0}^{\mathrm{d}}

The displacement will be, $I_{0}^{\mathrm{d}} / \mathrm{I}_{0}=(\mathrm{am} / \lambda)^{2}$

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Seawater at frequency v=4 \times 10^8 \mathrm{~Hz} has permittivity \varepsilon=80 \varepsilon_{0}, permeability \mu=\mu_{0} and resistivity \rho= 0.25 \Omega \mathrm{m}. Imagine a parallel plate capacitor immersed in seawater and driven by an alternating voltage source V(t)=V_{0} \sin (2 \pi v t) . What fraction of the conduction current density is the displacement current density?

The separation between the plates of the capacitor is given as $V(t)=V_{0} \sin (2 \pi v t)$ Ohm's law for the conduction of current density is given as $\mathrm{J}_{0}{...

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Let U be the set of all boys and girls in a school, G be the set of all girls in the school, B be the set of all boys in the school, and S be the set of all students in the school who take swimming. Some, but not all, students in the school take swimming. Draw a Venn diagram showing one of the possible interrelationship among sets U, G, B and S.

Solution: As per the question, Four sets given here are U, G, B, S So here, A universal set containing set of all boys and girls in a school = U Set of all girls in the school = G Set of all boys in...

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In drilling world’s deepest hole it was found that the temperature T in degree Celsius, x km below the earth’s surface was given by T = 30 + 25 (x – 3), 3 ≤ x ≤ 15. At what depth will the temperature be between 155°C and 205°C?

\[T\text{ }=\text{ }30\text{ }+\text{ }25\left( x\text{ }\text{ }3 \right),\text{ }3\text{ }\le \text{ }x\text{ }\le \text{ }15;\] where, T = temperature and x = depth inside the earth The...

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A solution of 9% acid is to be diluted by adding 3% acid solution to it. The resulting mixture is to be more than 5% but less than 7% acid. If there is 460 litres of the 9% solution, how many litres of 3% solution will have to be added?

According to the question, Let x litres of 3% solution is to be added to 460 liters of the 9% of solution Then, we get, Total solution = \[\left( 460\text{ }+\text{ }x \right)\text{ }litres\] Total...

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The water acidity in a pool is considered normal when the average pH reading of three daily measurements is between 8.2 and 8.5. If the first two pH readings are 8.48 and 8.35, find the range of pH value for the third reading that will result in the acidity level being normal.

According to the question, First reading = 8.48 Second reading = 8.35 Now, let the third reading be ‘x’ Average pH should be between 8.2 and 8.5 Average \[pH~=\text{ }\left( 8.48\text{ }+\text{...

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An EM wave of intensity I falls on a surface kept in vacuum and exerts radiation pressure p on it. Which of the following are true?
a) radiation pressure is \mathrm{I} / \mathrm{c} if the wave is totally absorbed
b) radiation pressure is I / c if the wave is totally reflected
c) radiation pressure is 2 \mathrm{I} / \mathrm{c} if the wave is totally reflected
d) radiation pressure is in the range I / c<p<2 I / c for real surface

The correct options are: a) radiation pressure is $\mathrm{l} / \mathrm{c}$ if the wave is totally absorbed c) radiation pressure is $2 \mathrm{l} / \mathrm{c}$ if the wave is totally reflected d)...

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An electromagnetic wave travelling along the z-axis is given as: E=E_{0} \cos (k z-\omega t). Choose the correct options from the following a) the associated magnetic field is given as B=\frac{1}{c} \hat{k} \times E=\frac{1}{\omega}(\hat{k} \times E)
b) the electromagnetic field can be written in terms of the associated magnetic field asE=c(B \times \hat{k})
c)\hat{k} \cdot E=0, \hat{k} \cdot B=0
d)\hat{k} \times E=0, \hat{k} \times B=0

a) the associated magnetic field is given as $B=\frac{1}{c} \hat{k} \times E=\frac{1}{\omega}(\hat{k} \times E)$ b) the electromagnetic field can be written in terms of the associated magnetic field...

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In the LCR circuit the ac driving voltage is v = vm sin ωt.
(i) Write down the equation of motion for q (t).
(ii) At t=t_{0}, the voltage source stops and R is short-circuited. Now write down how much energy is stored in each of L and C.

i) The equation for charge motion variation with respect to time is as follows: $\mathrm{L} d^{2} q(\mathrm{t}) / \mathrm{dt}+\mathrm{R} \mathrm{dq}(\mathrm{t}) / \mathrm{dt}+\mathrm{q}(\mathrm{t})...

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1MW power is to be delivered from a power station to a town 10 km away. One uses a pair of Cu wires of radius 0.5 cm for this purpose. Calculate the fraction of ohmic losses to power transmitted if
(i) power is transmitted at 220V. Comment on the feasibility of doing this.
(ii) a step-up transformer is used to boost the voltage to 11000 V, power transmitted, then a step-down transformer is used to bring the voltage to 220 V.

i) It is given that the power station is $10 \mathrm{~km}$ away from the town Length of the $\mathrm{Cu}$ wire is given as $\mathrm{L}=20 \mathrm{~km}=20000 \mathrm{~m}$ Resistance of $\mathrm{Cu}$...

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Electrical energy is transmitted over large distances at high alternating voltages. Which of the following statements is (are) correct?
(a) For a given power level, there is a lower current.(b) Lower current implies less power loss.
(c) Transmission lines can be made thinner.
(d) It is easy to reduce the voltage at the receiving end using step-down transformers.

The correct options are: (a) For a given power level, there is a lower current. (b) Lower current implies less power loss. (d) It is easy to reduce the voltage at the receiving end using step-down...

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Earth’s orbit is an ellipse with eccentricity 0.0167. Thus, the earth’s distance from the sun and speed as it moves around the sun varies from day to day. This means that the length of the solar day is not constant throughout the year. Assume that earth’s spin axis is normal to its orbital plane and find out the length of the shortest and the longest day. A day should be taken from noon to noon. Does this explain the variation of the length of the day during the year?

Solution: Velocity of the earth at perigee is given as $v_{p}$ Velocity of the earth at apogee is given as $v_{a}$ Angular velocity of the earth at perihelion is given as $\omega_{p}$ Angular...

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A star like the sun has several bodies moving around it at different distances. Consider that all of them are moving in circular orbits. Let r be the distance of the body from the centre of the star and let its linear velocity be v, angular velocity ω, kinetic energy K, gravitational potential energy U, total energy E, and angular momentum l. As the radius r of the orbit increases, determine which of the above quantities increase and which ones decrease.

When a body moves around a star in equilibrium, the gravitational attraction produces a centripetal force. Consider a body of mass $m$ revolving in a circular path of radius $r$ around the star S of...

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Mean solar day is the time interval between two successive noon when the sun passes through zenith point. The sidereal day is the time interval between two successive transits of a distant star through the zenith point. By drawing the appropriate diagram showing earth’s spin and orbital motion, show that mean solar day is four minutes longer than the sidereal day. In other words, distant stars would rise 4 minutes early every successive day.

The polar axis of the earth and its movement are E and E’ respectively. Translational motion is P’ After every 24 hours, earth's orbit is approximately advanced by $1^{o}$ As a result, time taken...

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There have been suggestions that the value of the gravitational constant G becomes smaller when considered over a very large time period in the future. If that happens for our earth,
a) nothing will change
b) we will become hotter after billions of years
c) we will be going around but not strictly in closed orbits
d) after a sufficiently long time we will leave the solar system

The correct options are c) we will be going around but not strictly in closed orbits d) after a sufficiently long time we will leave the solar system

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If the mass of sun were ten times smaller and gravitational constant G were ten times larger in magnitudes
a) walking on ground would become more difficult
b) the acceleration due to gravity on earth will not change
c) raindrops will fall much faster
d) aeroplanes will have to travel much faster

The correct options are a) walking on ground would become more difficult c) raindrops will fall much faster d) aeroplanes will have to travel much faster

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In our solar system, the inter-planetary region has chunks of matter called asteroids. They
a) will not move around the sun since they have very small masses compared to the sun
b) will move in an irregular way because of their small masses and will drift away outer space
c) will move around the sun in closed orbits but not obey Kepler’s laws
d) will move in orbits like planets and obey Kepler’s laws

The correct option is d) will move in orbits like planets and obey Kepler’s laws

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Satellites orbiting the earth have a finite life and sometimes debris of satellites fall to the earth. This is because
a) the solar cells and batteries in satellites run out
b) the laws of gravitation predict a trajectory spiralling inwards
c) of viscous forces causing the speed of the satellite and hence height to gradually decrease
d) of collisions with other satellites

The correct option is c) of viscous forces causing the speed of the satellite and hence height to gradually decrease

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As observed from earth, the sun appears to move in an approximately circular orbit. For the motion of another planet like mercury as observed from earth, this would
a) be similarly true
b) not be true because the force between earth and mercury is not inverse square law
c) not be true because the major gravitational force on mercury is due to sun
d) not be true because mercury is influenced by forces other than gravitational forces

The correct option is c) not be true because the major gravitational force on mercury is due to sun

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The earth is an approximate sphere. If the interior contained matter which is not of the same density everywhere, then on the surface of the earth, the acceleration due to gravity
a) will be directed towards the centre but not the same everywhere
b) will have the same value everywhere but not directed towards the centre
c) will be same everywhere in magnitude directed towards the centre
d) cannot be zero at any point

The correct option is d) cannot be zero at any point

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Choose the correct statements from the following. (i) Beryllium is not readily attacked by acids because of the presence of an oxide film on the surface of the metal. (ii) Beryllium sulphate is readily soluble in water as the greater hydration enthalpy of Be2+ overcomes the lattice enthalpy factor. (iii) Beryllium exhibits coordination number more than four. (iv) Beryllium oxide is purely acidic.

Answer: Option i) & ii) Be mimics Al (diagonal relationship), and together they create a protective film of oxide that is resistant to acid assault. Because of the high hydration enthalpy of...

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Metallic elements are described by their standard electrode potential, fusion enthalpy, atomic size, etc. The alkali metals are characterised by which of the following properties? (i) High boiling point (ii) High negative standard electrode potential (iii) High density (iv) Large atomic size

Answer: Option ii) & iv) Periods begin with alkali metals. For their period, alkali metals have the biggest atomic radius. They have low density due to their huge size and low bulk. Alkali...

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Dehydration of hydrates of halides of calcium, barium and strontium i.e., CaCl26H2O, BaCl2.2H2O, SrCl2.2H2O, can be achieved by heating. These become wet on keeping in air. Which of the following statements is correct about these halides? (i) act as dehydrating agent (ii) can absorb moisture from the air (iii) The tendency to form hydrate decreases from calcium to barium (iv) All of the above

Answer: Option iv) Because they are hygroscopic in nature, the calcium, barium, and strontium halides operate as a dehydrating agent in the body. They are capable of absorbing moisture....

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A chemical A is used for the preparation of washing soda to recover ammonia. When CO2 is bubbled through an aqueous solution of A, the solution turns milky. It is used in whitewashing due to disinfectant nature. What is the the chemical formula of A? (i) Ca (HCO3)2 (ii) Cao (iii) Ca(OH)2 (iv) CaCO3

Answer: option iii) Ca(OH)2 is the chemical A that is used in the manufacturing of washing soda Na2CO3 in order to recover ammonia. It is a reaction that occurs during the Solvay process, which is...

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The solubility of metal halides depends on their nature, lattice enthalpy and hydration enthalpy of the individual ions. Amongst fluorides of alkali metals, the lowest solubility of LiF in water is due to (i) Ionic nature of lithium fluoride (ii) High lattice enthalpy (iii) High hydration enthalpy for lithium-ion. (iv) Low ionisation enthalpy of the lithium atom

Answer: Option ii) Lattice and hydration enthalpies encourage the dissolving of alkali metal halides in water. Fluorides are soluble in this order: LiF, NaF, KF, RbF, CsF. The high lattice energy of...

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The reducing power of a metal depends on various factors. Suggest the factor which makes Li, the strongest reducing agent in aqueous solution. (i) Sublimation enthalpy (ii) Ionisation enthalpy (iii) Hydration enthalpy (iv) Electron-gain enthalpy

Answer: option iii) Li's hydration enthalpy is likewise high (highly exothermic). Li atom has the highest hydration enthalpy, making it the strongest reducing agent in aqueous media. - Li atom's...

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Assertion (A): Beryllium carbonate is kept in the atmosphere of carbon dioxide. Reason (R): Beryllium carbonate is unstable and decomposes to give beryllium oxide and carbon dioxide. (i) Both A and R are correct and R is the correct explanation of A. (ii) Both A and R are correct but R is not the correct explanation of A. (iii) Both A and R are not correct. (iv) A is not correct but R is correct.

Answer: Option (i) is correct. Beryllium carbonate is stable and decomposes to give beryllium oxide and carbon dioxide. The concentration of carbon dioxide grows in the right side, causing the...

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Assertion (A): The carbonate of lithium decomposes easily on heating to form lithium oxide and CO2. Reason (R): Lithium being very small in size polarises large carbonate ion leading to the formation of more stable Li2O and CO2. (i) Both A and R are correct and R is the correct explanation of A. (ii) Both A and R are correct but R is not the correct explanation of A. (iii) Both A and R are not correct (iv) A is not correct but R is correct.

Answer: Option i) is correct Lithium carbonate readily decomposes into lithium oxide and carbon dioxide. Others do not decompose. $Li _{2} CO _{3} \stackrel{\Delta}{\longrightarrow} Li _{2} O + CO...

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