Modes of heat transfer - conduction - class-XI

Yes, it's true that ebonite are bad conductors of heat so they are used to handle hot utensis.

Using the steady-state heat flow condition (sum of heat currents at junction C is zero), the heat currents from A to C and B to C must balance. Given the geometry and thermal resistance, the calculation leads to the ratio 3/(sqrt(2)+1).

Both the bodies are at same temperature, so there will be no transfer of heat from one body to other body.

Thermal conductivity of a gas is independent of the pressure as compared to solid and liquid. So the thermal conductivity of gas will remain constant.

The Units of the respective quantities in SI are:

• $[K] = J(mKs)^{-1}$
• $[\rho] = kg(m)^{-3}$
• $[c] = J(kgK)^{-1}$

Using the formula Q/t = kA(dT/dx), where Q/t = 6000, k = 200, A = 0.75, and dx = 1. Rearranging gives dT = (Q/t * dx) / (kA) = (6000 * 1) / (200 * 0.75) = 6000 / 150 = 40 degrees C.

Conduction is only possible when the bodies in contact have a temperature difference.

Conduction is defined as the process of transferring heat from molecule to molecule, without the actual movement of molecules.

Thermal conductivity of any object depends solely on the material of the object.
It is an intrinsic property and is independent on dimensions/mass/etc of the body.
For example, all iron rods will have same thermal conductivity.
All copper rods will have same thermal conductivity.
But, thermal conductivity of iron rod will be different from that of copper rod.

High specific heat means high energy required to increase temperature.
Hence for a given amount of energy, temperature would be less for high specific heat.
Low conductivity means heat is not conveyed to all portions of the conductor easily.
Hence, handling the instrument would be easy.

Conductivity of gas does not depend on pressure.

Thermal conductivity is characteristic to the material and doesn't depend on any of the factors given.
In fact, heat flow depends on the factors given in options list.

Conduction is defined as the process of transferring heat from molecule to molecule, without the actual movement of molecules.

Coefficient of $Thermal$ $Conductivity$ is the internal property of the metal and is independent of all the $external$ properties of the material.

Conduction is the process by which heat or electricity is directly transmitted through the material of a substance without movement of the material.

Air is bad conductor of heat, so it does not allow the heat to pass through, while the single thick blanket is a better conductor of heat so it is less effective.

A solid medium is required for conduction to take place. Since, vaccum does not have a medium, conduction does not take place.

False, air do not conducts heat.

Birds puff up their feathers in winter because by doing so they trap a
large amount of air in their body which in turn acts as an insulator and
does not allow the heat from their bodies to flow out

The power transported per unit area is given as

The thermometer whose bulb is wrapped by wet hanky; undergoes evaporation process; and we know that evaporation results in cooling, so the wet bulb, thermometer will record less temperature than that of dry bulb thermometer.

The cooling of body P follows Newton's Law of Cooling, where the rate of heat loss dQ/dt = -C(dT/dt) = (kA/L)(T - Ta). Integrating this leads to the natural log form ln((T1 - Ta)/(T2 - Ta)) = (kA/LC)t.

The dimension of coefficient of thermal conductivity is ${M^1}{L^1}{T^{ - 3}}{K^{ - 1}}$ 

Heat flow H = kA(dT/dx). Given H = 360 cal/min = 6 cal/sec, k = 0.5, A = 4. 6 = 0.5 * 4 * (dT/dx). 6 = 2 * (dT/dx), so dT/dx = 3 C/cm.

Heat flux H/A = k * (dT/dx). Given H/A = 10, k = 0.5. 10 = 0.5 * (dT/dx), so dT/dx = 20 C/cm.

Using H = kA(dT/L), 6000 = 200 * 0.75 * (dT/1). 6000 = 150 * dT, so dT = 40 C.

The for the coefficient of thermal conductivity is

Conduction requires a material medium with particles that can transfer kinetic energy through collisions. Vacuum contains no particles, so conduction cannot occur.

Cooking utensils need to transfer heat from the stove to the food efficiently, so they must be made of materials with high thermal conductivity.

$\begin{array}{l} Let, , q, , be, , temperatrure, , middle, , po{ { int } }, , C, and, , in, , series, , rate, , of, , heat, , flow, , is, , same \ K\left( { 2A } \right) \left( { 100-\theta  } \right) =KA\left( { \theta -70 } \right)  \ 200-2\theta =\theta -70 \ 3\theta =270 \ \theta ={ 90^{ 0 } }C \end{array}$

Thermal conductivity is a material property.
It does not depend on area of cross section, length and mass of the rod.
option (D) is the correct answer.

$\begin{array}{l} Under, steady, state, heat, flux, per, unit, area, k\left( { \frac { { dT } }{ { dx } }  } \right)  \ is, same, across, two, walls., hence,, we, have \ { K _{ 1 } }\dfrac { { { T _{ 1 } }-{ T _{ c } } } }{ { { d _{ 1 } } } } ={ K _{ 2 } }\dfrac { { { T _{ c } }-{ T _{ 2 } } } }{ { { d _{ 2 } } } }  \ where, { T _{ c } }, is, common, wall, temperature., , solving, for, { T _{ c } }, we, will, get \ { T _{ c } }=\dfrac { { { T _{ 1 } }+\alpha { T _{ 2 } } } }{ { \alpha +1 } }  \ Where, \alpha =\dfrac { { { d _{ 1 } } } }{ { { d _{ 2 } } } } , \dfrac { { { k _{ 2 } } } }{ { { k _{ 2 } } } }  \end{array}$

When objects are at the same temperature as the surroundings, they feel different because of their thermal conductivity. Metal conducts heat away from the hand faster than wood, making it feel colder, even though they are at the same temperature.

In a variable state (non-steady state), the temperature at any point in the rod changes with time as heat is absorbed by the material to change its internal energy. Therefore, temperature is a function of both position and time.

$Q = 200$
$A = 50 \times 10^{-4}\ m^2$
${\theta}^{} _{1} = 80^\circ C    ;   \ \ {\theta}^{} _{2} = 0$
$t = 10\ s ; \     l  = 0.40\ m$
$Q = \dfrac{KA\Delta T t}{L}$
$200 = \dfrac{K  \times 50\times 10^{-4}\times    80\times    10}{0.4}$ 
$K = 20\ W/mK$ 

From the formula:

Thermal conductivity (K) is an intrinsic property of a material, describing its ability to conduct heat. It does not depend on the geometry (length, area) or mass of the object.

Coefficient of thermal conductivity of a rod is an intrinsic property. It depends on the material of the rod and not on the dimensions of the rod.

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.

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

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

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    $4.8\times 3.36\times 10^{5} = \dfrac{K(0.35) (3600)(100)}{0.1}$
$\implies \ K = 1.24 \ Js^{-1} m^{-2} k^{-1}$

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.

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

As heat flows from higher temperature to lower temperature, so a temperature gradient is required.