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Heat transfer (Conduction, Convection and Radiation)

news-details Image Source May 18, 2021 16:49 IST · 5 min read

Heat may be defined as energy in transit from a high temperature object to a lower temperature object.

If two bodies at different temperatures are brought together, energy is transferred -i.e., heat flows-from the hotter body to the colder.

The effect of this transfer of energy usually, but not always, is an increase in the temperature of the colder body and a decrease in the temperature of the hotter body.

A substance may absorb heat without an increase in temperature by changing from one physical state (or phase) to another, as from a solid to a liquid (melting), from a solid to a vapour (sublimation), from a liquid to a vapour (boiling), or from one solid form to another (usually called a crystalline transition).

Hotter things have more heat energy than colder things, that's because the atoms or molecules move around faster in hot things than they do in cold things. This idea is called the kinetic theory.

Heat is a form of energy and temperature a measure of the amount of that energy present in a body.

Because all of the many forms of energy, including heat, can be converted into work, amounts of energy are expressed in units of work, such as joules, foot-pounds, kilowatt-hours, or calories.

The calorie (or gram-calorie) is the amount of energy required to raise the temperature of one gram of water from 14.5 to 15.5 °C.

Heat capacity or thermal capacity is a physical property of matter, defined as the amount of heat to be supplied to a given mass of a material to produce a unit change in its temperature. The SI unit of heat capacity is joule per kelvin (J/K).

Specific heat capacity is the amount of energy that must be added, in the form of heat, to one unit of mass of the substance in order to cause an increase of one unit in temperature.

The kinetic energy of an object is the energy that it possesses due to its motion. It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. The kinetic energy of an object of mass m traveling at a speed v is 1/2 mv2, unit is the joule.

Heat transfer

Solids, liquids and gases are made of tiny particles called "atoms". The temperature of the material measures how fast the atoms are moving and the heat measures the total amount of energy due to the vibration of the atoms.

Temperature flow will always occur from hottest to coldest or, higher to lower kinetic energy. Once there's thermal equilibrium between the two temperature differences, the thermal transfer stops.

There are three modes of heat transfer, which can be described as:

(1) Conduction

Conduction is how heat flows between two solid objects that are at different temperatures and touching one another (or between two parts of the same solid object if they're at different temperatures).

Hotter things have more heat energy than colder things, that's because the atoms or molecules move around faster in hot things than they do in cold things. This idea is called the kinetic theory.

An area of greater kinetic energy will transfer thermal energy to an area with lower kinetic energy, higher-speed particles will collide with slower speed particles. The slower-speed particles will increase in kinetic energy as a result.

The process of heat conduction depends on the following factors: temperature gradient (direction and rate of heat travel), cross-section of the material, length of the travel path, and physical material properties (thermal conductivity coefficient).

Conduction takes place in all phases: solid, liquid, and gas.

(2) Convection

When a fluid (air, liquid etc) is heated and then travels away from the source, it carries the thermal energy along, this type of heat transfer is called convection. The fluid above a hot surface expands, becomes less dense, and rises.

Convective heat transfer occurs when the surface temperature differs from that of surrounding fluid. At the molecular level, the molecules expand upon introduction of thermal energy.

Example: As the room heater heats the air surrounding it near the floor, the air will increase in temperature, expand, and rise to the top of the room. This forces down the cooler air so that it becomes heated, thus creating a convection current.

(3) Radiation

Thermal radiation is a process by which energy, in the form of electromagnetic radiation, is emitted by a heated surface in all directions and travels directly to its point of absorption at the speed of light.

Thermal radiation does not require an intervening medium to carry it.

Radiation occurs through a vacuum or any transparent medium (either solid or fluid). Thermal radiation is the direct result of random movements of atoms and molecules in matter.

Movement of the charged protons and electrons results in the emission of electromagnetic radiation.

Thermal radiation ranges in wavelength from the longest infrared rays through the visible-light spectrum to the shortest ultraviolet rays.

A blackened surface is an excellent emitter as well as an excellent absorber. If the same surface is silvered, it becomes a poor emitter and a poor absorber.

The heating of the Earth by the Sun is an example of transfer of energy by radiation

How vacuum flasks work ?

A thermos is a bottle with a double-walled container inside of it. The air between the two walls is sucked out during construction, creating a vacuum.

Instead of containing some kind of heating element to keep hot things hot, a thermos is designed to keep hot things hot by not allowing heat to escape and keeps cold things cold in the same way.

The vacuum prevents conduction, the tight stopper prevents air from entering or leaving the flask, so convection isn't possible either. When infrared radiation tries to leave the hot liquid, the reflective lining of the inner chamber reflects it straight back in again.

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