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Physics NEET Questions — Free MCQ Practice

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Two metal wires of identical dimensions are connected in series. If $σ$ ₁ and $σ$ ₂ are the conductivities of the metal wires respectively, the effective conductivity of the combination is

A.

_{$\frac{2 σ_{1} σ_{2}}{σ_{1} + σ_{2}}$}

B.

$\frac{σ_{1} σ_{2}}{2 σ_{1} σ_{2}}$

C.

$\frac{σ_{1} + σ_{2}}{σ_{1} σ_{2}}$

D.

$\frac{σ_{1} σ_{2}}{σ_{1} + σ_{2}}$

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The value of coefficient of volume expansion of glycerin is 5x10^-4 K^-1. The fractional change in the density of glycerin for a rise of 40°C in its temperature is -

A.

0.015

B.

0.020

C.

0.025

D.

0.010

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Explanation

$ρ_{T} = ρ_{0} (1 - γ ∆ T) \frac{ρ_{T}}{ρ_{0}} = (1 - γ ∆ T) 1 - \frac{ρ_{T}}{ρ_{0}} = γ ∆ T \frac{ρ_{0} - ρ_{T}}{ρ_{0}} = γ ∆ T = 5 \times 10^{- 4} \times 40 = 0.020$

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Steam at 100°C is passed into 20 g of water at 10°C. When water acquires a temperature of 80°C, the mass of water present will be (Take specific heat of water=1 cal g^-1 °C^-1 and latent heat of steam = 540 cal g^-1)

A.

24 g

B.

31.5g

C.

42.5 g

D.

22.5 g

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Explanation

Heat lost by steam = Heat gained by water

Let m' amount of steam converts into water.

m' x L +m's(100-80) =msΔt

m' x 540 + m' X 20 = 20 x 1 x(80-10)

m'=20 x 70/560=2.5g

Now, net water=20+2.5=22.5g

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Certain quantity of water cools from 70°C to 60°C in the first 5 min and to 54°C in the next 5 min. The temperature of the surroundings is

A.

45°C

B.

20°C

C.

42°C

D.

10°C

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Explanation

Let the temperature of the surrounding is t°C

For first case, (70-60)/5min=K(65°-t°C)

(65° is average of 70°C and 60°C)

10/5min=K(65°C-t°C) ...(i)

For second case,

(60-54)/5min=K(57-t) ...(ii)

(57°C is average of 60°C and 54°C)

From Eqs. (i)/(ii), 10/6=(65-t)/(57-t)

Solving, we get t=45° C

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A piece of iron is heated in a flame. If first becomes dull red then becomes reddish yellow and finally turns to white hot. The correct explanation for the above observation is possible by using

A.

Stefan's law

B.

Wien's displacement law

C.

Kirchoff's law

D.

Newton's law of cooling

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Explanation

(b) The equation of Wien's displacement law, i.e., λ mT=constant

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If the radius of a star is R and it acts as a black body, what would be the temperature of the star, in which the rate of energy production is Q?

A.

$Q / 4 {πR}^{2} σ$

B.

${(Q / 4 {πR}^{2} σ)}^{- 1 / 2}$

C.

${(4 {πR}^{2} Q / σ)}^{1 / 4}$

D.

${(Q / 4 {πR}^{2} σ)}^{1 / 4}$

$(σ s \tan d s f o r S t e f a n' s c o n s \tan t .)$

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Explanation

From Stefan' law,

$E = σ T^{4}$

So, the rate energy production

$Q = E \times A$

$Q = σ T^{4} \times 4 {πR}^{2}$

Temperature of star

$T = {(\frac{Q}{4 {πR}^{2} σ})}^{1 / 4}$

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A slab of stone of area of 0.36 $m^{2}$ and

thickness 0.1 m is exposed on the lower

surface to steam at $100 ° C$ . A block of ice

at $0 ° C$ rests on the upper surface of the slab.

In one hour 4.8 kg of ice is melted. The thermal

conductivity of slab is

(Given latent heat of fusion of ice

=3.36x $10^{5} J {kg}^{- 1}$ )

A.

$1.24 J / m / s / ° C$

B.

$1.29 J / m / s / ° C$

C.

$2.05 J / m / s / ° C$

D.

$1.02 J / m / s / ° C$

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Explanation

$\frac{δQ}{δt} = \frac{KA}{L} (T_{1} - T_{2}) Q = \frac{KA}{L} (T_{1} - T_{2}) t Q = {mL}_{f} \frac{KA}{L} (T_{1} - T_{2}) t = {mL}_{f} K = \frac{{mL}_{f} L}{A (T_{1} - T_{2}) t} K = \frac{4.8 \times 3.36 \times 10^{5} \times 0.1}{0.36 \times 100 \times 3600} = \frac{4.8 \times 3.36}{0.36 \times 36} = 1.24 J / m / s / ° C$

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When 1 kg of ice at $0 ° C$ melts to water at $0 ° C$ , the resulting change in its entropy, taking latent heat of ice to be $80 cal / ° C,$ is

A.

$8 \times 10^{4} cal / K$

B.

$80 cal / K$

C.

$293 cal / K$

D.

$273 cal / K$

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Explanation

Change in entropy

$∆ s = \frac{m L}{T} = \frac{1000 \times 80}{273} = 293 cal K^{- 1}$

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A cylindrical metallic rod in thermal contact with two reservoirs of heat at its two ends conducts an amount of heat Q in time t. The metallic rod is melted and the material is formed into a rod of half the radius of the original rod. What is the amount of heat conducted by the new rod when placed in thermal contact with the two reserviors in time t ?

A.

Q/4

B.

Q/16

C.

2Q

D.

Q/2

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Explanation

In steady state the amount of heat flowing from one face to the other face in time t is given by $Q = \frac{K A (θ_{1} - θ_{2}) t}{l}$ where K is coefficient of thermal conductivity of material of rod

$\Rightarrow \frac{Q}{t} \propto \frac{A}{l} \propto \frac{r^{2}}{l}$ ...(i)

As the metallic rod is melted and the material is formed into of half the radius

$V_{1} = V_{2}$

${πr}_{1}^{2} l_{1} = {πr}_{2}^{2} l_{2}$

$\Rightarrow l_{1} = \frac{l_{2}}{4}$ ...(ii)

Now, from Eqs. (i) and (ii)

$\frac{Q_{1}}{Q_{2}} = \frac{r_{1}^{2}}{l_{1}} \times \frac{l_{2}}{r_{2}^{2}} = \frac{r_{1}^{2}}{l_{1}} \times \frac{4 l_{1}}{{(r_{1} / 2)}^{2}}$

$\Rightarrow Q_{1} = 16 Q_{2}$

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The total radiant energy per unit area per unit time, normal to the direction of incidence, received at a distance R from the centre of a star of radius r,whose outer surface radiates as a black body at a temperature TK is given by

A.

$σ r^{2} T^{4} / R^{2}$

B.

$σ r^{2} T^{4} / 4 {πr}^{2}$

C.

$σ r^{4} T^{4} / r^{4}$

D.

$4 π σ r^{2} T^{4} / R^{2}$

(where $σ$ is Stefan's constant)

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Explanation

If r is the radius of the star and T its temperature, then the energy emitted by the star per second through radiation in accordance with Stefan's law will be given by

$P = A σ T^{4} = 4 {πr}^{2} {σT}^{4}$

In reaching a distance R this energy will spread over a sphere of radius R; so the intensity of radiation will be given by- $S = \frac{P}{A} = \frac{P}{4 {πR}^{2}} = \frac{4 {πr}^{2} {σT}^{4}}{4 {πR}^{2}}$

Physics MCQs for NEET — Practice Questions with Answers

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