Convection, Physics tutorial

Conduction of Heat in Fluids:

Liquids and gases are generally referred to as fluids. All liquids, except mercury, which is a metal, are poor conductors of heat. Air and gases, usually are even worse conductors of heat. This can be explained by putting piece of ice wrapped inside the wire gauze and placed in the long test tube filled with water. Upper part of the tube B is set to boil after heating it for some time. It will be seen that as water boils ice remains unmelted at bottom of tube. This signifies that heat is not conducted down the tube. Heat remains at the top. Experiments have also illustrated that air is the poor conductor of heat. Steel wool, crumpled aluminum foil, wollen materials are bad conductors of heat due to large number of small air pockets inside these materials.

Convection in Fluids:

Heat is transferred in fluids through convection. Convection is the mechanical displacement of heated part of a fluid. It is the phenomenon of transfer of heat with actual movement on particles of the body. We generally put the pot of water on top of cooker to get boiled water to make food or to bath if weather is very cold. The liquid will be in contact with solid wall of the container that is generally a metal like the aluminum pot. Container is at the higher temperature than fluid. The heat is transferred to the fluid. Heated portion of fluid rises up while cold fluid on the top comes down. This procedure continues until water starts to boil. Procedure may be described in this manner. When water molecules are heated, they expand and because of this expansion, density of molecules decreases such molecules therefore experience the upthrust, according to Archimedes Principle that makes them to float. As nature doesn't let a vacuum, the colder, heavier molecules take the place of lighter ones. Therefore the current of molecules is made within liquid. Viscosity of fluid that is fluid friction logically influences convectional transfer of heat in fluids. Term convection is generally applied to transfer off heat in fluids from one point to another by actual movement of particles which make up the fluid.

Natural and Forced Convection:

When material fluid carries heat from one place to another because of differences in density because of thermal expansion, procedure of heat transfer is known as natural convection. The example of this takes place when water in the pot is made to boil. In contrast, when material of fluid is forced to carry the heat from one place to another by the blower or pump, the procedure is known as forced convection.

Convection of Heat in Liquids:

Convection in liquids may be shown by placing the crystal of potassium permanganate carefully in the round bottomed flask and then heating bottom of flask gently. There will be a rising column of colored water will take place in flask. This illustrates that convection currents have been set up. This is the natural convection. This procedure again can be described by expansion of water molecules at bottom of flask because of heat acquired. Their density decreases and as such, they float according to Archimedes' Principle. Heavier colder molecules then take places of hot ones and so the current is created within bulk of the water. This is the principle applied in domestic hot water system in cold countries. Hot water circulates by natural convection. Though in large buildings that have central heating systems, a pump is generally utilized to help in circulation of hot water.

Convection in Gases:

The particles of matter have ability to be vibrated, translate, or rotate that is due to kinetic energy of particles. Measurement of kinetic energy of the particles is done by temperature. Kinetic energy of rotation, translation or vibration increases with increasing temperature. Flow of energy occurs from the higher to lower temperature. Temperature difference between higher and lower temperature objects is reason for transfer of heat. This procedure is continued to get thermal equilibrium condition.

The simple example of heat transfer is cooling of the hot tea cup; similar warming of the cold can. Transfer of heat is from hot to cold water till both samples get same temperature.

Along coastal regions we generally see breeze flowing from sea at day time that is always reversed in night to constitute land breeze. In day, the sun heats up both land and water. But due to difference in specific capacities of land and water, land is hotter than water. Therefore warm air rises up and its place taken up by colder sea breeze. In night the land cools faster than water. Therefore warm air above water rises up and is replaced by cold air from the land, thus creating the land breeze.

Newton's Law of Cooling:

When the hot body is left in air it cools down. This can take place under natural convection or forced convection. It cools naturally when air is still. But when there is the steady draught, it can be cooled under what we call forced draught - as in ventilated cooling in the draught. Newton propounded law of cooling that is satisfactory for temperature excesses.

Newton's law of cooling defines that rate of loss of heat by the hot body is proportional to temperature difference between hot body and its surroundings. Assume body is at a temperature of ΘoC and is permitted to cool in the environment of temperature ΘooC, Newton's law stipulates that:

Q/t = k(Θ - Θo)

Or -Q/t = k(Θ - Θo)

k is constant of proportionality whose value depends on two factors (negative sign illustrates that heat is lost to surrounding):

  • Nature of exposed surface of material - surface emmisivitiy e
  • It's surface area A

Therefore -Q/t = eA(Θ - Θo)

Several scientists have really worked on Newton's law of cooling. Preston, Dulong and Petit and Lorentz are examples of such people. Summary of findings have illustrated that:

For forced convection in the strong draught Newton's law of cooling prevails, where rate of loss of heat is proportional to temperature excess

Therefore - Q/t = ∝(Θ - Θo)

For natural convection in still air, five-fourth power law predominates. That is rate of loss of heat is proportional to (temperature excess)

Therefore -Q/t = ∝(Θ - Θo)5/4

It is usual to utilize Newton's law of cooling for heat loss in Calorimetry experiments.

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