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Stefan Boltzmann Law Stefan Boltzmann Law The Stefan–Boltzmann law, also known as Stefan's law, states that the total energy radiated per unit surface area of a black body per unit time (also known as the black-body irradiance or emissive power), , is directly proportional to the fourth power of the black body's thermodynamic temperature T. Heat Transfer Definition :- Heat is the form of energy. We also know that there is a transfer of heat only if there is a temperature difference. Heat Transfer is defined as:- The transmission of energy from one region to another region as a result of the temperature difference between them. OR The Heat transfer taking place between the bodies of different temperature. Types of Heat transfer :-

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There are three modes of Heat transfer :Conduction :- Conduction is a process where the heat transfer takes place between the two solid bodies in contact, two regions of the same solid body. This will happen because of the hot, vibrating, and rapidly moving molecules transfer the heat to their neighboring atoms. Convection :- The convection is a type of heat transfer where the heat transfer takes place through a medium and the medium may be liquid or the gas. The heat transfer takes place by the movement of fluid from one place to another. The heat transfer here is due to the bulk motion of the fluid. Convection is described by the Newton’s law of cooling; the law states that the rate of heat loosed by a body is proportional to the difference in temperatures between the body and its surroundings. Radiation :The Third mode of energy transfer is Radiation Heat Transfer. The transfer of heat from hot body to a cold body with any material medium for propagation. Every object in the universe is made up of atoms and molecules. These atoms and molecules vibrate due to thermal energy present in them. Every object emits electromagnetic radiations because of the thermal vibrations of these atoms and molecules. In case of energy transfer, the radiation conversion of radiated electromagnetic energy to thermal energy takes place. Radiation Heat Transfer can also be termed as transfer of energy through waves.

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Heat Transfer Examples Examples of Heat Transfer through Conduction: -Take one long piece of metal. Put first end of this metal in the flame. Gradually, you will find that the temperature of the other end of the metal in your hand starts increasing. Energy gets transferred from the first end under flame to the second end of the metal in your hand by conduction. Examples of Heat Transfer through Natural Convection :- Natural convection is the convection which occurs naturally due to the bouncy effect. When water is heated in a pot then the particles, atoms or molecules of the water which are in contact with the pot gain energy. Kinetic energy of these particles gets increased. As a result, density of these particles decreases and they start moving upward towards the open surface of the port. Cool water which is in contact with the air in the port starts sinking downward and convection current gets established. Cool water in the upper surface is denser than the hot water in contact with the surface of the pot. Convection Currents flow in circular fashion. Therefore, heat is transferred by the movement of water (fluids). Forced convection: The convection that takes place by the force that is the force created from the fans, stirrers, pumps and etc. Examples of Heat Transfer through Radiation :- Radiations coming out from the burner of the electric stove or toaster coils. Transfer of heat energy from Sun to Earth or to us takes place due to the electromagnetic rays emitted by the sun. Another example is the heating of food inside the microwave.

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Stefan Boltzmann Law