Mechanical Engineering - Heat Transfer, Refrigeration and Air Conditioning - Discussion

3. 

Which of the following statement is wrong?

[A]. The heat transfer in liquid and gases takes place according to convection.
[B]. The amount of heat flow through a body is dependent upon the material of the body.
[C]. The thermal conductivity of solid metals increases with rise in temperature,
[D]. Logarithmic mean temperature difference is not equal to the arithmetic mean temperature difference.

Answer: Option C

Explanation:

No answer description available for this question.

Jaya Krishna said: (Mar 20, 2013)  
For solids Thermal Conductivity is dependent on lattice vibrations and movement of free electrons, as temp increases movement of free electrons is hampered by lattice vibrations.

Ajit Kumar said: (Jul 30, 2013)  
Thermal conductivity is a property of material and it is independent of temperature.

Narendra said: (Aug 14, 2013)  
Thermal conductivity is proportional to square root on I/M.

Swapnil said: (Aug 15, 2013)  
Thermal conductivity is inversely proportional to temperature(According Fourier's law of heat conduction).

Rs Rajput said: (Sep 29, 2013)  
Heat transfer in metal can be,

1. By lattice vibration.

2. By free electron transfer(free electron provide the heat flux in the direction of decreasing temperature).

(Factor two is more dominating to factor one but both act simultaneously).

According to second point. If we increase the temperature of metal these decrease the tendency free moving electron.

These may be reason for decreasing conductivity of metal.

Satish said: (Jan 2, 2014)  
K(Thermal conductivity)inversely proportional to temperature(T)(Fourier's law of heat conduction).

Saikumar said: (Jan 22, 2014)  
In solids the molecules are closely packed. The heat transfer more because this arrangement. Hence the conductivity increases with increases with increase in temperature.

Jyosna said: (Apr 26, 2014)  
Thermal conductivity is inversely proportional to the temperature, according to fourier's law.

Mohnesh said: (Aug 9, 2014)  
Thermal conductivity is property. But it is not change with respect to temperature.

Manish said: (Oct 29, 2014)  
Thermal conductivity is the property of any material which show the speed of vibration and electron transfer from one point to another.

Bir Bahadur Singh(Prakash Kumar) said: (Feb 19, 2015)  
Thermal conductivity is inversely proportional to temperature gradient.

Ravi Kumar said: (Aug 5, 2015)  
For pure metals temperature increases k value decreases for alloys it will increases.

Rohan said: (Sep 6, 2015)  
If temperature of solid is increased it will expand thus its molecules will get separated thus reducing the thermal conductivity?

Friend said: (Oct 31, 2015)  
Just remember that perfect conduction can be achieved at sub zero temperature state called superconducting that means as the temperature decreases conductivity increases. Hence it would be easy to remember that as temperature increases conductivity decreases.

Friend said: (Oct 31, 2015)  
Just remember that perfect conduction can be achieved at sub zero temperature state called superconducting that means as the temperature decreases conductivity increases. Hence it would be easy to remember that as temperature increases conductivity decreases.

Rahul Raja said: (Apr 27, 2016)  
As the resistance is more in the path of a solid body there will be more collision due to which thermal conductivity of solid decreases.

Amrit said: (May 13, 2016)  
Actually, thermal conductivity varies differently in the different physical state of matter. In solid if temperature increases then thermal conductivity decreases (for). In gas if temperature increases thermal conductivity increases. And for liquid, it depends on upon the temperature.

Gnagadharac Rao said: (May 17, 2016)  
Thermal conductivity depends on the metal property, and not temperature.

Atif said: (Jul 12, 2016)  
For metals, the thermal conductivity is mainly a function of the motion of free electrons. As the temperature increases, the molecular vibrations increase (in turn decreasing the mean free path of molecules). So, they obstruct the flow of free electrons, thus reducing the conductivity.

Pradeep said: (Sep 7, 2016)  
Not only convection. Heat flow in still water also occurs due to conduction. Hence option A is partially correct.

Netrapal said: (Jan 19, 2017)  
Thermal conductivity is directly proportional to temperature gradient.

Prabha said: (Mar 9, 2017)  
In solid metals the coducvity decrease as temp rises.

Updesh Yadav said: (May 31, 2017)  
If temperature increases thermal conductivity increases according to made easy.

Vilas said: (Sep 8, 2017)  
What is option D? Please explain it.

Sujit said: (Oct 3, 2017)  
For solids, Thermal Conductivity is dependent on lattice vibrations and movement of free electrons, as temp increases movement of free electrons is hampered by lattice vibrations. So that free electrons provide more resistance to heat flow.
according to Fourier's law of conduction

Q= -k*A*(dT/dx).

k=thermal conductivity of material
A=c/s area
dT= temp. difference
dx=thickness

So, k inversely proportional to dT.

Ks37 said: (Apr 5, 2018)  
For metals, the thermal conductivity is mainly a function of the motion of free electrons. As the temperature increases, the molecular vibrations increase (in turn decreasing the mean free path of molecules). So, they obstruct the flow of free electrons, thus reducing the conductivity.

Ilyas said: (Jul 9, 2018)  
K=Ko(1+bT) here b is negative for solid so thermal conductivity decreases while increase in temperature.

Sidd said: (Aug 27, 2018)  
Explain the correct statement in clear.

Sunny said: (Oct 29, 2018)  
Why option A is true?

In liquids in some extent conduction also takes place.

Amiq Syed said: (Jan 15, 2019)  
The thermal conductivity of solid metal's decrease with the increase of temperature.

Dwaipayan said: (May 15, 2019)  
LMTD=MTD is valid only when both the fluids in the heat exchanger are subjected to phase change only.

So option D is not correct.

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