Tag: physics

Questions Related to physics

 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 0? (a stands for Stefan's constant.)

stefan's law black body radiation heat transfer thermal properties physics

  1. $

    \left(\frac{4 \pi R^{2} Q}{\sigma}\right)^{1 / 4}

    $

  2. $

    \left(\frac{Q}{4 \pi R^{2} \sigma}\right)^{1 / 4}

    $

  3. $

    \frac{Q}{4 \pi R^{2} \sigma}

    $

  4. $

    \left(\frac{Q}{4 \pi R^{2} \sigma}\right)^{-1 / 2}

    $

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Correct Option: C

$\dfrac {watt} {kelvin}$ is the unit of 

stefan's law black body radiation heat transfer thermal properties physics

  1. Stefan's constant

  2. Wien's constant

  3. Cooling's constant

  4. Thermal constant

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Correct Option: A

Assuming the Sun to be a spherical body of radius $R$ at a temperature of $T\ K$. Evaluate the intensity of radiant power, incident on Earth, at a distance $r$ from the Sun where $r _{0}$ is the radius of the Earth and $\sigma$ is Stefan's constant :

stefan's law black body radiation heat transfer thermal properties physics

  1. $\dfrac{R^{2}\sigma T^{4} }{r^{2}}$

  2. $\dfrac{4\pi ^{2}R^{2}\sigma T^{4}}{r^{2}}$

  3. $\dfrac{\pi ^{2}R^{2}\sigma T^{4}}{r^{2}}$

  4. $\dfrac{\pi ^{2}R^{2}\sigma T^{4}}{4\pi r^{2}}$

Show answer
Correct Option: A
Explanation:
Total power radiated by the sun

 $=\sigma { T }^{ 4 }\times 4\pi { R }^{ 2 }$

The intensity of power at earth surface

$=\cfrac{\sigma { T }^{ 4 }\times 4\pi { R }^{ 2 }}{4\pi { r }^{ 2 }} \\=\cfrac{\sigma { T }^{ 4 } { R }^{ 2 }}{{ r }^{ 2 }}$

The rectangular surface of area $8 cm \times 4 cm$ of a black body at a temperature of $127^0C$ emits energy at the rate of $E$ per second. If both length and breadth of the surface are reduced to half of its initial value, and the temperature is raised to $327^0C$, then the rate of emission of energy will become :

stefan's law black body radiation heat transfer thermal properties physics

  1. $\dfrac{3}{8}E$

  2. $\dfrac{81}{16}E$

  3. $\dfrac{9}{16}E$

  4. $\dfrac{81}{64}E$

Show answer
Correct Option: D
Explanation:
Let $A _1=32$ as given.
Let $A _2$ be the area when length and breadth are reduced by half. Thus the area will be $\dfrac {1}{4}$th of $A _1$
$ \therefore A _2=\dfrac {1}{4} *32=8$
Given $T _1={127}^0C={400}^0K$
Given $T _2={327}^0C={600}^0K$
From Stefan's law $E=\sigma AT^4$
$ \therefore\dfrac{E _1}{E _2}= \dfrac {A _1{T _1}^4}{A _2{T _2}^4}=\dfrac {32*(400)^4}{8*(600)^4}=\dfrac{64}{81}$
$ \therefore \dfrac{E _2}{E _1}= \dfrac{81}{64}$

LEDs are extensively used to replace . bulbs because:

physics simple electric circuits bulbs bulb bulb connected to an electric cell

  1. it consumes less electricity

  2. have longer life

  3. has more power

  4. all of the above

Show answer
Correct Option: D
Explanation:

A light-emitting diode (LED) is a two-lead semiconductor light source. it is a p-n junction diode, which emits light when activated. LEDs have more power, more life time and consume less electricity than a bulb, thus these are extensively used to replace bulbs.

The glowing of electric bulb is based on _________ effect of current.

physics simple electric circuits bulbs bulb bulb connected to an electric cell

  1. heating

  2. magnetic

  3. luminous

  4. none of the above

Show answer
Correct Option: A
Explanation:

Answer is A.
The electric bulb has a filament called tungsten when electricity passes through this filament, it heats up and glows. This heat is generated due to the passage of electric current and the drift of electrons due to the current and the resistance it causes.
Hence, The glowing of electric bulb is based on the heating effect of electric current.

When an electric bulb breaks, there is a mild bang due to:

physics simple electric circuits bulbs bulb bulb connected to an electric cell

  1. The photo-chemical reaction between the enclosed gases.

  2. The compressed gases rushing out suddenly.

  3. The air rushing into fill the evacuated space.

  4. None of these

Show answer
Correct Option: C
Explanation:

Answer is C.

When an electric bulb breaks, there is a mild bang due to the sudden air rushing into fill the evacuated space.

In the bulb there is a -

physics simple electric circuits bulbs bulb bulb connected to an electric cell

  1. Long wire

  2. Short wire

  3. Thin wire

  4. Wide wire

Show answer
Correct Option: C
Explanation:

as we know $Resistance =Resistivity\dfrac{Length}{Area}$

therefore, for making resistance more the radius of the wire should be less to make area less and make resistance more. as resistance is inversely proportional to area. therefore to make bulb we need thin wire. which is capable of withstanding high temperature and have high resistance.

An electric bulb converts electrical energy into

physics simple electric circuits bulbs bulb bulb connected to an electric cell

  1. sound energy

  2. mechanical energy

  3. nuclear energy

  4. none of these

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Correct Option: D
Explanation:

Due to passage of electric current, the tungsten filament of the bulb gets heated up and glows. Thus an electric bulb converts electrical energy into heat energy as well as light energy.

Hence option D is correct.

The filament of an electric bulb is of tungston because

physics simple electric circuits bulbs bulb bulb connected to an electric cell

  1. Its resistance is negligible.

  2. It is cheaper.

  3. Its melting point is high.

  4. filament is easily made.

Show answer
Correct Option: C
Explanation:

Tungston has very high melting point. Thus it does not melt even when a large amount of heat is produced in the bulb due to heating effect of current. Hence it is used as a filament in the bulb.