Que-Ans (Each of 2 Mark )

Question 12 Marks

An electron is regarded as a universal particle. Explain.

Answer

The values of charge (e) and mass ( m ) of the electron always remain the same whatever may be the source of their emission. In the discharge tube, the electrons may be emitted either from the cathode or from the gas enclosed in the discharge tube. Whatever may be the metal which forms cathode or the gas present in the discharge tube, these values remain the same. Therefore, electron is regarded as a universal particle.
or
The electron is an elementary particle. It cannot be further divided into more divisions. Also other particles are through to have condensed from combinations of these. Thus electron is called a universal particle.

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Question 22 Marks

A stone of $1 \ kg$ is thrown with a velocity of $20\ ms^{-1}$ across the frozen surface of a lake and comes to rest after travelling a distance of $50 m$. What is the force of friction between the stone and the ice?

Answer

Here,
Mass, $m=1 \ kg$
Initial velocity, $u =20\ ms^{-1}$
Final velocity,$v =0\ ms^{-1}$
Distance travelled, $S=50 m$
Force of friction, $F = ?$
From the third equation of motion,
we have, $v^2=u^2+2 a s$
$\Rightarrow a=\frac{v^2-u^2}{2 s}$
$=\frac{0-\left(20 ms^{-1}\right)^2}{2 \times 50 m}$
$=\frac{-400\ m^2 s^{-2}}{100 m}$
$=-4\ ms^{-2}$
From Newton's second law of motion, we have
$F=ma$
$=1 \ kg \times\left(-4\ ms^{-2}\right)$
$=-4 \ kgms^{-2}$
$=-4 N$
Negative sign indicates the force is acting opposite to the direction of motion.
Thus, the force of friction between the stone and ice is $-4 N$

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Question 32 Marks

A man throws a ball of mass 0.4 kg vertically upwards with a velocity of 10 m/s. What will be its initial momentum? What would be its momentum at the highest point of its reach?

Answer

Here, $m =0.4 kg, u =10 m / s$
Initial momentum of the ball $= mu =0.4 \times 10=4 kg m / s$
At the highest point, velocity of ball is zero,
Therefore,the momentum of the ball at the highest point of flight $=0 \times 4=0$.

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Question 42 Marks

How much water should be mixed with $12\ mL$ of alcohol so as to obtain a $12\%$ alcohol solution?

Answer

Volume of alcohol $($solute$) =12\ mL$
Let the volume of water $($ Solvent $)= x\ mL$
$\therefore$ Volume of solution $=(12+x)\ mL$
Concentration of solution $=\frac{\text { Volume of solute }}{\text { Volume of solution }} \times 100=\frac{\text { Volunnc of allohol }}{\text { Volume of solution }} \times 100$
$12=\frac{12}{12+x} \times 100$
$12+x=100$
$x=100-12=88\ mL$
So, 88 mL of water should be mixed with $12\ mL$ of alcohol to obtain $12 \%$ alcohol solution.

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Question 52 Marks

Explain how sound is produced by your school bell.

Answer

When the peon strikes the school bell with a hammer, the particles of bell metal start vibrating and those vibrations produce sound.

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Question 62 Marks

Alka was making tea in a kettle. Suddenly she felt intense heat from the puff of steam gushing out of the spout of the kettle. She wondered whether the temperature of the steam was higher than that of the water boiling in the kettle. Comment.

Answer

The temperature of both boiling water and steam is $100^{\circ} C$, but steam has more energy because of latent heat of vaporisation. Latent heat of vapourisation makes it much more hotter than hot boiling water.

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Question 72 Marks

A machine raises a load of $750 N$ through a height of $16 m$ in $5$ seconds. Calculate the power at which the machine works.

Answer

Given force$, F =750 N$, displacement $=$ height,
$h =16 m$ and time, $t =5$
Work done by machine,
$W=$ Force $\times$ Displacement $=F \times h$
$=750 N \times 16 \text { joule }$
Power of machine$, P \frac{W}{t}$
$\frac{750 N \times 15 . J}{5 s}$
$=2400$ watt .

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Question 82 Marks

A battery lights a bulb. Describe the energy changes involved in the process.

Answer

A battery has stored chemical energy. The chemical energy is converted into electrical energy during lighting of bulb. So a battery converts chemical energy into electrical energy. The filament in the bulb becomes white hot and gives out light. Electrical energy is converted into heat and light energy.
Thus, the energy changes involved in this process can be written as follows:
Chemical energy $\rightarrow$ Electrical energy $\rightarrow$ Heat energy + Light energy.

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Science Que-Ans (Each of 2 Mark )

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