Tag: physics

Questions Related to physics

A red hot brick is placed on an iron tripod stand which stands on a large block of copper. The brick loses heat by:

physics temperature and heat modes of heat transfer - conduction conduction heat and modes of heat transfer

  1. conduction

  2. conduction and radiation

  3. conduction and convection

  4. conduction, convection, and radiation

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Correct Option: B
Explanation:

The brick loses heat (i) by conduction through iron tripod stand to the copper block which is then radiated away and convection currents are set up in the surrounding air, and (ii) by radiation.

A constant voltage is applied between the two ends of a uniform metallic wire. Some heat is
developed in it. The heat developed is doubled if

physics temperature and heat modes of heat transfer - conduction conduction heat and modes of heat transfer

  1. both the length and the radius of the wire are halved.

  2. both the length and the radius of the wire are doubled

  3. the radius of the wire is doubled

  4. the length of the wire is doubled

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Correct Option: B

In the Arctic region hemispherical houses called Igloos are made of ice. It is possible to maintain a temperature inside an Igloo as high as $20^o$C because.

physics temperature and heat modes of heat transfer - conduction conduction heat and modes of heat transfer

  1. Ice has high thermal conductivity

  2. Ice has low thermal conductivity

  3. Ice has high specific heat

  4. Ice has higher density than water

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Correct Option: B
Explanation:

ice is good at trapping heat. It is a  good insulator and low thermal conductivity .

A long silver tea spoon is placed in a cup filled with hot tea. After some time, the exposed end (the end which is not dipped in tea) of the spoon becomes hot even without a direct contact with the tea. This phenomenon can be explained mainly by_______

physics temperature and heat modes of heat transfer - conduction conduction heat and modes of heat transfer

  1. conduction

  2. reflection

  3. radiation

  4. thermal expansion

Show answer
Correct Option: A
Explanation:

As atoms in the spoon vibrates about their equilibrium positions and transfer energy form one end to other end. This process is called conduction.

A piece of metal is heated to increase its temperature from $5^{\circ}C$ to $15^{\circ}C$. The increase in temperature expressed in $K$ and $^{\circ}F$ are respectively.

physics temperature and heat modes of heat transfer - conduction conduction heat and modes of heat transfer

  1. $10\ K, 18^{\circ}F$

  2. $283\ K, 50^{\circ}F$

  3. $18\ K, 10^{\circ}F$

  4. $50\ K, 283^{\circ}F$

Show answer
Correct Option: A
Explanation:

$K=273.16+C\F=\cfrac{9C}{5}+32\quad dK=dC \quad dF=\cfrac{9}{5}dC$

A/Q, $dC=15-5=10\ \therefore dK=10K,\quad dF=\cfrac{9}{5}\times10=18°F$

A slab of stone area $3500{cm}^{2}$ and thickness $10cm$ is exposed on the lower surface to steam at ${100}^{o}C$. A block of ice at ${0}^{o}C$ rests on upper surface of the slab. In one hour $4.8kg$ of ice of melted. The thermal conductivity of the stone is $J{s}^{-1}$ ${m}^{-1}$ ${k} _{-1}$ is
(Latent heat of ice $=3.36\times { 10 }^{5 }J/kg$)

physics temperature and heat modes of heat transfer - conduction conduction heat and modes of heat transfer

  1. $12.0$

  2. $10.5$

  3. $1.02$

  4. $1.24$

Show answer
Correct Option: D
Explanation:

Given :  $A = 3500 \ cm^2 = 0.35 \ m^2$         $l = 10 \ cm = 0.1 \ m$             $\Delta T =100-0 = 100^o C$
Mass of ice melted  $m = 4.8 \ kg$
Time taken  $t = 1 \ hr = 3600 \ s$
Latent heat of ice  $L = 3.36\times 10^{5} \ J/kg$
Heat absorbed by ice = Heat conducted by slab
$\therefore$   $mL = \dfrac{KA t\Delta T}{l}$
Or    $4.8\times 3.36\times 10^{5} = \dfrac{K(0.35) (3600)(100)}{0.1}$
$\implies \ K = 1.24 \ Js^{-1} m^{-2} k^{-1}$

Which of the following minimizes the transference of heat in a thermos flask?
$1$. Conduction
$2$. Convection
$3$. Radiation

physics temperature and heat modes of heat transfer - conduction conduction heat and modes of heat transfer

  1. $2$ and $3$

  2. $1$ and $2$

  3. $1, 2$ and $3$

  4. $1$ and $3$

Show answer
Correct Option: C

The number of quanta of radiation of frequency $4.98 \times {10^{14}}{s^{ - 1}}$ required to melt 100 g of ice are (latent heat of melting of ice is 33 joule per g):

physics temperature and heat modes of heat transfer - conduction conduction heat and modes of heat transfer

  1. ${10^{20}}$

  2. ${10^{22}}$

  3. ${10^{24}}$

  4. $6.023 \times {10^{21}}$

Show answer
Correct Option: B
Explanation:

$1g$ requires $33J$. So, $100g$ will require $3300J$ heat.

Total quanta, 
E=hv
$=6.62\times { 10 }^{ -34 }\times 4.98\times { 10 }^{ 14 }\ =32.97\times { 10 }^{ -20 }$
$100g$ of ice will require=$\dfrac { 3300 }{ 32.97\times { 10 }^{ -20 } } =100.09$
$100$quantal=${ 10 }^{ 22 }J$

Two rods of the same length and diameter having thermal conductivities ${K _1}\,{K _2}$ are joined in parallel. The equivalent thermal conductivity of the combination is:

physics temperature and heat modes of heat transfer - conduction conduction heat and modes of heat transfer

  1. $\dfrac{{{K _1}{K _2}}}{{{K _1} + {K _2}}}$

  2. ${{K _1} + {K _2}}$

  3. $\dfrac{{{K _1} + {K _2}}}{2}$

  4. $\sqrt {{K _1}{K _2}} $

Show answer
Correct Option: A
Explanation:

$\dfrac{1}{{{K _{eq}}}} = \dfrac{1}{{{K _1}}} + \dfrac{1}{{{K _2}}}$

$\boxed{{K _{eq}} = \dfrac{{{K _1}{K _2}}}{{{K _1} + {K _2}}}}$

A cylinder of radius $R$ made of a material of thermal conductivity $K _1$ is surrounded by a cylindrical shell of inner radius $R$ and outer radius $2R$ made of a material of thermal conductivity $K _2$. The two ends of the combined system are maintained at two different temperatures. There is no loss of heat across the cylindrical surface and the system is in steady state. The effective thermal conductivity of the system is?

physics temperature and heat modes of heat transfer - conduction conduction heat and modes of heat transfer

  1. $K _1+K _2$

  2. $\dfrac{K _{1}+3K _{2}}{4}$

  3. $\dfrac{K _{1}+8K _{2}}{9}$

  4. $\dfrac{8K _{1}+K _{2}}{9}$

Show answer
Correct Option: A,C