Tag: physics

Questions Related to physics

A metallic sphere cools from $50^{\circ}C$ to $40^{\circ}C$ in $300\ s$. If the room temperature is $20^{\circ}C$, then its temperature in the next $5$ min will be

physics transfer of heat applications of convection convection thermal energy transfers

  1. $38^{\circ}C$

  2. $33.3^{\circ}C$

  3. $30^{\circ}C$

  4. $36^{\circ}C$

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Correct Option: C

In natural convection, a heated portion of a liquid moves because

physics transfer of heat applications of convection convection thermal energy transfers

  1. Its molecular motion becomes aligned

  2. Of molecular collisions within it

  3. Its density is less than that of the
    surrounding fluid

  4. Of currents of the surrounding fluid

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Correct Option: C

A sphere of density d, statisfied heat s and radius r is hung by thermally insulating thread in an enclosure which is kept at a lower temperature than the sphere.The temperature of the sphere to drop at a rate which depends upon the temperature difference between the and the enclosure,If the temperature difference is $ \triangle T $ and surrounding temperature is $ T _o $ then rate of fall in temperature will be-

physics transfer of heat applications of convection convection thermal energy transfers

  1. $ \frac {4 \sigma T _o^2 \triangle T}{rdc} $

  2. $ \frac {12 \sigma T _o^2 \triangle T}{rdc} $

  3. $ \frac {12 \sigma T _o^4 \triangle T}{rdc} $

  4. $ \frac {12\sigma \triangle T}{rdc T _o^3 } $

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Correct Option: A

The correct statement of the following:
A) Water can be boiled inside the artificial satellite by convection
B) Heavy liquid can be boiled in artificial satellite by convection
Mark correct option/s

physics transfer of heat applications of convection convection thermal energy transfers

  1. Both statements are correct

  2. Both statements are wrong

  3. A is correct but B is wrong

  4. B is correct but A is wrong

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Correct Option: B
Explanation:

Conduction and Convection are non-existent in space, so both the statements are wrong. 

It is hotter at some distance over the flames than in front of it because :

physics transfer of heat applications of convection convection thermal energy transfers

  1. air conducts heat upwards only

  2. heat is radiated upwards only

  3. convection of heat occurs upwards only

  4. heat is radiated downwards only

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Correct Option: C
Explanation:

Near flames, heat transfer occurs by both convection and radiation (and some conduction too).
But convection occurs only in upward direction. So, above the flame, it is hotter rather than in front of the flame.

Which of the following process(es) occur(s) during the convective transfer of heat within a large volume of air? 

physics transfer of heat applications of convection convection thermal energy transfers

  1. A continuous flow of warmer and cooler parts of air is established.

  2. The flow of air molecules results in flow of heat.

  3. The cooler portion of the air surrounding a warmer part exerts a buoyant force on it.

  4. As the warmer part of the air moves, it is replaced by cooler air that is subsequently warmed up.

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Correct Option: A,B,C,D
Explanation:

A continuous flow of warmer and cooler parts of air is established in which the air molecules are responsible for flow of heat. As the warmer air molecules move up, they are replaced bu cooler air molecules.

The inductance of the oscillatory circuit of a radio station is 10 milli henry band its capacitance is $0.25 \mu F$. Taking the effect of the resistance negligible, wavelength of the broadcasted waves will be (velocity of light = $3.0  \ 10 ^4 \ m/s, \pi = 3.14$):

capacitance of an isolated spherical conductor capacitance of isolated bodies capacitance physics

  1. $9,42 \times 10^4 m$

  2. $18.8 \times 10^4 m$

  3. $4.5 \times 10^4 m$

  4. $none\ of\ these$

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Correct Option: C

The capacitance of an air filled parallel plate capacitor is $10\times {10}^{-12}F$. The separation between the plates is doubled and the space between the plates is then filled with wax giving the capacitance a new value of $40\times {10}^{-12}F$. The dielectric constant of wax is:

capacitance of an isolated spherical conductor capacitance of isolated bodies capacitance physics

  1. $12.0$

  2. $10.0$

  3. $8.0$

  4. $4.2$

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Correct Option: C
Explanation:

${C} _{1}=\cfrac{{ \varepsilon  } _{ 0 }A}{d}$ and ${C} _{2}=\cfrac{K{ \varepsilon  } _{ 0 }A}{2d}$
$\Rightarrow$ $\cfrac{{C} _{2}}{{C} _{1}}=\cfrac{K}{2}=\cfrac{40\times {10}^{-12}}{10\times {10}^{-12}}$
$\Rightarrow$ $K=8$

Each plate of parallel plate capacitor has a charge q on it. The capacitor is now connected to a battery. Now,

capacitance of an isolated spherical conductor capacitance of isolated bodies capacitance physics

  1. the facing surfaces of the capacitor have equal and opposite charges

  2. the two plates of the capacitor have equal and opposite charges

  3. the battery supplies equal and opposite charges to the two plates

  4. the outer surfaces of the plates have equal charges

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Correct Option: A,C,D

We assume that earth is at zero potential because capacitance of the earth is

capacitance of an isolated spherical conductor capacitance of isolated bodies capacitance physics

  1. infinite

  2. zero

  3. cannot say

  4. $10^6 farad$

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Correct Option: A
Explanation:

Because earth can store infinite charge so its capacitance is infinite, therefore it is at a zero potential. 

$\left (C=\dfrac {q}{V}\right )$.