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

Why is it convection and not conduction?

Conduction is the transfer of the heat through a solid object, liquid or gas. In order for the heat to be conducted, there should be physical contact between particles and some temperature difference. When one part of an object is heated, the molecules within it vibrate against one another, begin to move faster and more vigorously, when these molecules hit other molecules within the object they cause heat to be transferred through the entire object. Substances that conduct heat readily are called conductors, while substances that don't conduct heat readily are called insulators. Metal (e.g. copper, platinum, gold iron, etc) is a good conductor of thermal energy, aluminum is a good insulator. Wood falls somewhere in between. Conduction is greater in solids, where molecules are in constant contact.

As density decreases so does conduction. Therefore, fluids (and especially gases) are less conductive. This is due to the large distance between atoms in a gas, fewer collision between atoms means less conduction. Conductivity of gases increases with temperature. Conductivity increases with increasing pressure from vacuum up to a critical point that the density of the gas is such that molecules of the gas may be expected to collide with each other before they transfer heat from one surface to another. After this point density, conductivity increases only slightly with increasing pressure and density. In liquids (except liquid metals) and gases, the molecules are further apart, giving lower chance of molecules colliding and passing on the thermal energy.

Convection is the right answer because the heat transfer in liquid & gasses due to convection and the collision accident also occurs in convection because the molecular force of attraction is min. In liquid & gasses so when the liquid & gasses gain some heat then their bonds breaks and electron start moves freely & collies to each other.

While in the conduction the molecular force of attraction is max. In solid and bonding between the molecules are strong so the molecules are only vibrating with respect to each other. So The transfer of heat in conduction due to lattice vibration.
The transfer of heat due to;
* lattice vibration =30%.
* free-electron =70%.

In conduction The molecules collide with surrounding molecules, making them vibrate too, resulting in the transportation of thermal energy to neighboring part of the object.

In convection heat transfer, by real movement of matter, that occurs only in fluids. Fluid alludes to any substance, whose molecules move freely from one place to another, such as liquid and gases. It happens naturally or even forcefully.

In radiation, The heat transfer mechanism in which no medium is required is called radiation.

So, I think the given answer is right.

It must be conduction. Because in conduction molecules are closely packed. When one molecule is heated it vibrates higher than lower energised molecules or lower temperature molecules and collide with and finally transfer some energy to them. In this way heat transfer in solid by conduction.

Conduction in solid is due to sum of molecular vibration and flow of free electron. But conduction in metals is predominantly due to flow of free electron. In case of crystalline material, conduction is due to molecular vibration.

Convection in fluids is due to molecular collision.

Conduction is the process by which heat energy is transmitted through collisions between neighboring atoms or molecules.

Conduction occurs more readily in solids and liquids, where the particles are closer together than in gases, where particles are farther.

Convection is right answer. As they said molecular collision in the question so that it's mean that the movement of particle so that actual migration of particles are only seen in convection. Therefore the answer is convection.

No, it is the conduction mode of heat transfer. By collision molecules transfer energy from one to another. In convection mode, molecules themselves carry the heat from one place to another.

Answer [A] is correct. Conduction heat transfer takes place by Molecular collision whereas Convection heat transfer is due to the Bulk motion of the fluid.

It is 100% Convection, molecules collide in either gases or liquids, how can a solid collide with itself?

Please anyone explain.