NCERT Exemplar

A swimming pool is to be drained for cleaning. If L represents the number of litres of water in the pool t seconds after the pool has been plugged off to drain and L = 200 (10 – t)2. How fast is the water running out at the end of 5 seconds? What is the average rate at which the water flows out during the first 5 seconds?

Given, \[L\text{ }=\text{ }200\left( 10\text{ }\text{ }t \right)2\] where L addresses the quantity of liters of water in the pool. On separating both the sides w.r.t, t, we get \[dL/dt\text{...

read more

A man, 2m tall, walks at the rate of m/s towards a street light which is m above the ground. At what rate is the tip of his shadow moving? At what rate is the length of the shadow changing when he is m from the base of the light?

Let AB is the stature of streetlamp post and CD is the tallness of the man with the end goal that \[AB\text{ }=\text{ }5\left( 1/3 \right)\text{ }=\text{ }16/3\text{ }m\text{ }and\text{ }CD\text{...

read more

Consider the non-empty set consisting of children in a family and a relation R defined as aRb if a is brother of b. Then R is (A) symmetric but not transitive (B) transitive but not symmetric (C) neither symmetric nor transitive (D) both symmetric and transitive

The correct option is (B) transitive but not symmetric Given aRb ⇒ a is brother of b. This does not mean b is also a brother of a as b can be a sister of a. Therefore, R is not symmetric. aRb ⇒ a is...

read more

Let T be the set of all triangles in the Euclidean plane, and let a relation R on T be defined as aRb if a is congruent to b ∀ a, b ∈ T. Then R is (A) reflexive but not transitive (B) transitive but not symmetric (C) equivalence (D) none of these

The correct option is (C) equivalence Given aRb, if a is congruent to b, ∀ a, b ∈ T. Then, we have aRa ⇒ a is congruent to a; which is always true. So, R is reflexive. Let aRb ⇒ a ~ b b ~ a bRa So,...

read more

Let * be binary operation defined on R by

    \[\mathbf{a}\text{ }*\text{ }\mathbf{b}\text{ }=\text{ }\mathbf{1}\text{ }+\text{ }\mathbf{ab},\forall \mathbf{a},\text{ }\mathbf{b}\in \mathbf{R}\]

. Then the operation * is (i) commutative but not associative (ii) associative but not commutative (iii) neither commutative nor associative (iv) both commutative and associative

(i) Given that * is a binary operation defined on R by \[\mathbf{a}\text{ }*\text{ }\mathbf{b}\text{ }=\text{ }\mathbf{1}\text{ }+\text{ }\mathbf{ab},\forall \mathbf{a},\text{ }\mathbf{b}\in...

read more

Each of the following defines a relation on N: (i) x is greater than y,

    \[\mathbf{x},\text{ }\mathbf{y}\in \mathbf{N}\]

(ii)

    \[\mathbf{x}\text{ }+\text{ }\mathbf{y}\text{ }=\text{ }\mathbf{10},\text{ }\mathbf{x},\text{ }\mathbf{y}\in \mathbf{N}\]

(iii) x y is square of an integer

    \[\mathbf{x},\text{ }\mathbf{y}\in \mathbf{N}\]

(iv)

    \[\mathbf{x}\text{ }+\text{ }\mathbf{4y}\text{ }=\text{ }\mathbf{10}\text{ }\mathbf{x},\text{ }\mathbf{y}\in \mathbf{N}\]

. Determine which of the above relations are reflexive, symmetric and transitive.

(i) Given, x is greater than y; \[\mathbf{x},\text{ }\mathbf{y}\in \mathbf{N}\] If \[\left( x,\text{ }x \right)\in R\], then \[x\text{ }>\text{ }x\], which is not true for any \[x\in N\]. Thus, R...

read more

A kite is moving horizontally at a height of 151.5 meters. If the speed of kite is 10 m/s, how fast is the string being let out; when the kite is 250 m away from the boy who is flying the kite? The height of boy is 1.5 m.

Speed of the kite(V) \[=\text{ }10\text{ }m/s\] Leave FD alone the tallness of the kite and AB be the stature of the kite and AB be the tallness of the kid. Presently, let AF \[=\text{ }x\text{ }m\]...

read more

A Zener of power rating 1 W is to be used as a voltage regulator. If Zener has a breakdown of 5V and it has to regulate voltage which fluctuated between 3V and 7V, what should be the value of Rs for safe operation in the figure?

Answer: According to the question, Power = 1W and Zener breakdown voltage = 5V We are given that the minimum voltage = 3V and the maximum voltage = 7V $ {{I}_{\max }}=\frac{P}{{{V}_{z}}}=\frac{1}{5}...

read more

Two car garages have a common gate which needs to open automatically when a car enters either of the garages or cars enter both. Devise a circuit that resembles this situation using diodes for this situation.

Answer: When an automobile approaches the gate, one or both gates are open. As a result, the OR gate produces the required result. The following is a truth table for the same: A B Y = A + B 0 0 0 0...

read more

In a CE transistor amplifier there is a current and voltage gain associated with the circuit. In other words, there is a power gain. Considering power a measure of energy, does the circuit violate conservation of energy?

Answer: The DC supply is connected to the CE transistor amplifier to provide energy to the signal. As a result, the CE configuration amplifier has a significant power gain. The extra power necessary...

read more

The amplifiers X, Y and Z are connected in series. If the voltage gains of X, Y and Z are 10, 20 and 30, respectively and the input signal is 1 mV peak value, then what is the output signal voltage (peak value)

(i) if dc supply voltage is 10V? (ii) if dc supply voltage is 5V? Answer: According to the question, we can write Voltage gain in X = vx = 10 Voltage gain in Y = vy = 20 Voltage gain in Z = vz = 30...

read more

Are the following set of ordered pairs functions? If so, examine whether the mapping is injective or surjective. (i) {(x, y): x is a person, y is the mother of x}. (ii){(a, b): a is a person, b is an ancestor of a}.

(i) from the question, {(x, y): x is a person, y is the mother of x} So we can say that each person ‘x’ has only one biological mother. Therefore, the above set of ordered pairs make a function. If...

read more

The Bohr model for the H-atom relies on the Coulomb’s law of electrostatics. Coulomb’s law has not directly been verified for very short distances of the order of angstroms. Supposing Coulomb’s law between two opposite charge +q 1,-q 2 is modified to =\frac{q_{1} q_{2}}{\left(4 \pi \epsilon_{0}\right)} \frac{1}{R_{0}^{2}}\left(\frac{R_{0}}{r}\right)^{\epsilon}, r \leq R_{0} \quad Calculate in such a case, the ground state energy of an \mathrm{H}- atom, if \varepsilon

Case 1: When $\varepsilon=0.1, \mathrm{R_O}=1 \mathrm{~A}$ $\mathrm R_1=8 \times 10^{-11}$ $\mathrm{M}=0.08 \mathrm{~nm}$ Velocity at ground level is given as $\mathrm v_1=1.44 \times 10^{6}...

read more

In the Auger process, an atom makes a transition to a lower state without emitting a photon. The excess energy is transferred to an outer electron which may be ejected by the atom. (This is called an Auger electron). Assuming the nucleus to be massive, calculate the kinetic energy of an \mathrm{n}=4 Auger electron emitted by Chromium by absorbing the energy from an =2 to n=1 transition.

The energy in the nth state can be calculated as, $-\operatorname{Rch} Z^{2} / n^{2}=-13.6 z^{2} / n^{2} e V$ Where $R=$ Rydberg constant and $\mathrm{Z}=24$ Energy released is given as $\left(13.6...

read more

Deuterium was discovered in 1932 by Harold Urey by measuring the small change in wavelength for a particular transition in 1 \mathrm{H} and 2 \mathrm{H}. This is because the wavelength of transition depends to a certain extent on the nuclear mass. If nuclear motion is taken into account then the electrons and nucleus revolve around their common centre of mass. Such a system is equivalent to a single particle with a reduced mass \mu, revolving around the nucleus at a distance equal to the electron-nucleus separation. Here \mu=\mathrm{me} \mathrm{M} /(\mathrm{me}+\mathrm{M}) where \mathrm{M} is the nuclear mass and \mathrm{me} is the electronic mass. Estimate the percentage difference in wavelength for the 1 st line of the Lyman series in 1 \mathrm{H} and 2 \mathrm{H}. (Mass of 1 \mathrm{H} nucleus is 1.6725 \times 10^{-27} \mathrm{~kg}, Mass of 2 \mathrm{H} nucleus is 3.3374 \times 10^{-27} \mathrm{~kg}, Mass of electron =9.109 \times 10^{-31} kg.)

The energy of an electron in the nth state is given by the expression, $E_ n=-\mu Z^{2} \mathrm{e}^{4} / 8 \varepsilon_ 0^{2} h^{2}\left(1 / n^{2}\right)$ For hydrogen atom we have, $\mu...

read more

Let E_{n}=\frac{-1}{8 \epsilon_{0}^{2}} \frac{m e^{4}}{n^{2} h^{2}} \quad be the energy of the nth level of H-atom. If all the H-atoms are in the ground state and radiation of frequency (E_2-E_1)/h falls on it
(a) it will not be absorbed at all
(b) some of the atoms will move to the first excited state
(c) all atoms will be excited to the n=2 state
(d) no atoms will make a transition to the \mathrm{n}=3 state

The correct options are: (b) some of the atoms will move to the first excited state (d) no atoms will make a transition to the $n=3$ state

read more

The Balmer series for the H-atom can be observed
(a) if we measure the frequencies of light emitted when an excited atom falls to the ground state
(b) if we measure the frequencies of light emitted due to transitions between excited states and the first excited state
(c) in any transition in a \mathrm{H}-atom
(d) as a sequence of frequencies with the higher frequencies getting closely packed

The correct options are: (b) if we measure the frequencies of light emitted due to transitions between excited states and the first excited state (d) as a sequence of frequencies with the higher...

read more

\mathrm{O}_{2} molecule consists of two oxygen atoms. In the molecule, nuclear force between the nuclei of the two atoms
(a) is not important because nuclear forces are short-ranged
(b) is as important as an electrostatic force for binding the two atoms
(c) cancels the repulsive electrostatic force between the nuclei
(d) is not important because the oxygen nucleus have an equal number of neutrons and protons

The correct option is: (a) is not important because nuclear forces are short-ranged

read more

For the ground state, the electron in the H-atom has an angular momentum =h, according to the simple Bohr model. Angular momentum is a vector and hence there will be infinitely many orbits with the vector pointing in all possible directions. In actuality, this is not true,
(a) because Bohr model gives incorrect values of angular momentum
(b) because only one of these would have a minimum energy
(c) angular momentum must be in the direction of spin of electron
(d) because electrons go around only in horizontal orbits

The correct option is: (a) because Bohr model gives incorrect values of angular momentum

read more

The activity \mathrm{R} of an unknown radioactive nuclide is measured at hourly intervals. The results found are tabulated as follows:

    \[\begin{tabular}{|l|l|l|l|l|l|} \hline$t(\mathrm{~h})$ & 0 & 1 & 2 & 3 & 4 \\ \hline R(MBq) & 100 & $35.36$ & $12.51$ & $4.42$ & $1.56$ \\ \hline \end{tabular}\]


(i) Plot the graph of R versus t and calculate half-life from the graph.
(ii) Plot the graph of \ln \left(\mathrm{R} / \mathrm{R}_{0}\right) versus \mathrm{t} and obtain the value of half-life from the graph.

(i) Graph between $\mathrm{R}$ versus $\mathrm{t}$ will be an exponential curve. From the graph at slightly more than $\mathrm{t}=\frac{1}{2} \mathrm{~h}$ the $\mathrm{R}$ should be $50 \%$ so at...

read more

Fusion processes, like combining two deuterons to form a He nucleus are impossible at ordinary temperatures and pressure. The reasons for this can be traced to the fact:
(a) nuclear forces have short-range
(b) nuclei are positively charged
(c) the original nuclei must be completely ionized before fusion can take place
(d) the original nuclei must first break up before combining with each other

The correct options are: (a) nuclear forces have short-range (b) nuclei are positively charged

read more