Que-Ans (Each of 2 Mark )

Question 12 Marks

Summarise the rules for writing of distribution of electrons in various shells for the first eighteen elements.

Answer

The following rules are followed for writing the number of electrons in different energy levels or shells:
(i) The maximum number of electrons present in a shell is given by the formula $2 n^2$, where ' $n$ ' is the orbit number or the energy level $(1,2,3, \ldots)$
Hence, the maximum number of electrons in different shells are as follows:-
Maximum number of electrons in the first orbit or K -shell will be $=2 \times 1^2=2$
Maximum number of electrons in the second orbit or L-shell will be $=2 \times 2^2=8$
Maximum number of electrons in the third orbit or M-shell will be $=2 \times 3^2=18$
Maximum number of electrons in the fourth orbit or N -shell will be $=2 \times 4^2=32$ and so on.
(ii) The maximum number of electrons that can be accommodated in the outermost (valence) orbit is 8 .
(iii) The shells are built up in a step-wise manner. Electrons are not accommodated in a given shell, unless the inner shells have been filled.

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Question 22 Marks

A truck of mass M is moved under a force F. If the truck is then loaded with an object equal to the mass of the truck and the driving force is halved, then how does the acceleration change?

Answer

Acceleration can be given as follow:
$
F=\text { ma or } a=\frac{\delta}{m} \text { or } a_1=\frac{F}{M}
$
When mass is doubled and force is halved;
$a_2=\frac{F}{4 M} \quad$ or $\quad \frac{a_2}{a_1}=\frac{F}{4 M} \div \frac{F}{M}=\frac{1}{4}$
So, acceleration becomes one-fourth.

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Question 32 Marks

Two balls of the same size of different materials, rubber and iron are kept on the smooth floow of a moving train.The brakes are applied suddenly to stop the train. Will the balls start rolling? If so, in which direction? Will they move with the same speed? Give reasons for yours answer.

Answer

Yes, the balls will start rolling in the direction in which the train was moving. Due to the application of the brakes, the train comes to rest but due to inertia the balls try to remain in motion, therefore, they begin to roll. Since the masses of the balls are not the same, therefore, the inertial force are not same on both the balls. Thus, the balls will move with different speeds.

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Question 42 Marks

Why the temperature of boiling water does not rise even when heat is continuously supplied to it?

Answer

The heat which is taken by the particles of water in liquid state is used to overcome the forces of attraction which are holding these particles together in the liquid state. When these particles change their state from liquid (at $100^{\circ} C$ ) to vapour state (at $100^{\circ} C$ ), they only absorb latent heat which is only $22.5 \times 10^5 J / kg$. This latent heat does not increase the kinetic energy of water particles in gaseous state and hence no rise in temperature takes place during the boiling of water.

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Question 52 Marks

A sound wave has a frequency 2 kHz and wavelength 40 cm. How long will it take to travel 1.6 km?

Answer

Given frequency, $v =2 kHz =2 \times 10^3 Hz$
Wavelength, $\lambda=40 cm=0.40 m$
Speed of sound $=$ Frequency $\times$ Wavelength
i. e., $v=v \lambda$
$=\left(2 \times 10^3 Hz\right) \times(0.40 m)$
$=0.80 \times 10 Hz=800 ms^{-1}$
Time, $t =\frac{\text { Distance }}{\text { Speed }}$
$
t=\frac{8}{v}
$
Given distance, $s=1.6 km=1.6 \times 10^3 m$
Time, $t =\frac{1.6 \times 10^3 m}{800 ms^{-1}}$
$=\frac{16000}{800} s=2 s$

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Question 62 Marks

Kelvin scale of temperature is regarded as better than Celsius scale. Give reason.

Answer

In the Celsius scale of temperature we often come across a negative sign for the temperature (e.g., $-4{ }^0 C$ ). Since the sign is always positive in the Kelvin scale, it is regarded as better.

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Question 72 Marks

A rocket is moving up with a velocity v. If the velocity of this rocket is suddenly tripled, what will be the ratio of two kinetic energies?

Answer

Initial velocity $=v$, then final velocity, $v=3 v$

Initial kinetic energy $=\frac{1}{2} m v^2$

Final kinetic energy (KE) $=\frac{1}{2} m v^{\prime 2}=\frac{1}{2} m(3 v)^2=9\left(\frac{1}{2} m v^2\right)$

$( KE )_{\text {initial }}:( KE )_{\text {final }}=1: 9$.

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Question 82 Marks

Calculate the work done against the gravity.

Answer

Suppose a body of mass $m$ is lifted vertically upwards against the gravity ( g ) through a distance h .
Here, the force ( F ) required to lift the body = weight of the body, mg (where m is mass and g is acceleration due to gravity).
Now, Work done (W) in lifting a body $=$ Force $\times$ displacement $=$ Weight of body $\times$ Vertical distance
$
W=mg \times h=mgh .$
Therefore, Work done against the gravity = mgh

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Science Que-Ans (Each of 2 Mark )

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