Tag: conduction

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

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

  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
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Heat developed H = V^2 * t / R. Resistance R = rho * L / A = rho * L / (pi * r^2). So H is proportional to A / L = r^2 / L. If both length and radius are doubled, H is proportional to (2r)^2 / (2L) = 4r^2 / 2L = 2 * (r^2 / L). Thus, the heat is doubled.

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

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.

  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
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

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

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

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_______

  1. conduction

  2. reflection

  3. radiation

  4. thermal expansion
Reveal answer Fill a bubble to check yourself
A Correct answer
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.

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

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.

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

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

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

  4. $50\ K, 283^{\circ}F$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

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

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

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

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 $=3.36\times { 10 }^{5 }J/kg$)

  1. $12.0$

  2. $10.5$

  3. $1.02$

  4. $1.24$
Reveal answer Fill a bubble to check yourself
D Correct answer
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    $4.8\times 3.36\times 10^{5} = \dfrac{K(0.35) (3600)(100)}{0.1}$
$\implies \ K = 1.24 \ Js^{-1} m^{-2} k^{-1}$

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

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

  1. $2$ and $3$

  2. $1$ and $2$

  3. $1, 2$ and $3$

  4. $1$ and $3$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

A thermos flask uses a vacuum (to stop conduction and convection) and silvered surfaces (to stop radiation) to minimize all three modes of heat transfer.

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

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):

  1. ${10^{20}}$

  2. ${10^{22}}$

  3. ${10^{24}}$

  4. $6.023 \times {10^{21}}$
Reveal answer Fill a bubble to check yourself
B Correct answer
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=${ 10 }^{ 22 }J$

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

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:

  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}} $
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

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

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

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

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?

  1. $K _1+K _2$

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

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

  4. $\dfrac{8K _{1}+K _{2}}{9}$
Reveal answer Fill a bubble to check yourself
A,C Correct answer