## Heat Transfer – Class 11 | Chapter – 11 | Physics Short Notes Series PDF for NEET & JEE

Heat Transfer: Heat transfer is the process by which thermal energy is exchanged between two or more systems, as a result of a temperature difference between them. The direction of heat transfer is always from the hotter system to the cooler system, and the rate of transfer depends on a variety of factors such as the thermal conductivity of the materials involved, the surface area of contact, the temperature difference, and the presence or absence of other heat transfer mechanisms such as convection and radiation.

## Mechanisms of Heat Transfer

• Conduction:Conduction is a mechanism of heat transfer that occurs when two objects at different temperatures are in contact with each other. In this process, heat is transferred from the hotter object to the cooler object through direct molecular collisions, without any net movement of matter.In a solid material, conduction occurs when vibrating molecules transfer their energy to neighboring molecules, causing them to vibrate as well. This process continues throughout the material until the heat energy reaches the cooler object or the environment. The rate of heat transfer by conduction depends on the thermal conductivity of the material, which is a measure of how well the material conducts heat.

Conduction can also occur between two fluids or between a fluid and a solid surface. In these cases, the heat transfer occurs at the boundary between the two materials, where molecules from one material collide with molecules from the other material. The rate of heat transfer by conduction in fluids is affected by factors such as the viscosity and thermal conductivity of the fluid, as well as the surface area and geometry of the objects involved.

Examples of conduction include the transfer of heat through a metal pot when placed on a hot stove, the transfer of heat through the walls of a building from the outside environment to the interior, and the transfer of heat through a metal heat sink used to cool an electronic device. Conduction is an important mechanism of heat transfer in a variety of contexts, and understanding the principles of conduction is essential in fields such as engineering, physics, and materials science.

The equation governing conduction of heat through a solid material can be expressed as:

Q = kAΔT / L

where:

• Q is the rate of heat transfer in watts (W)
• k is the thermal conductivity of the material in watts per meter-kelvin (W/mK)
• A is the surface area of the object in contact with the hotter material in square meters (m²)
• ΔT is the temperature difference between the hotter and cooler materials in kelvin (K)
• L is the distance between the hotter and cooler materials in meters (m)
• Convection: Heat transfer by convection occurs when a fluid, such as air or water, is heated and then moves, carrying heat with it. Convection can be either natural, due to density differences caused by temperature variations, or forced, where a pump or fan is used to move the fluid. The rate of heat transfer by convection depends on the properties of the fluid, the surface area of contact, and the velocity of the fluid.The equation governing convective heat transfer can be expressed as:Q = hAΔT

where:

• Q is the rate of heat transfer in watts (W)
• h is the convective heat transfer coefficient in watts per square meter-kelvin (W/m²K)
• A is the surface area of the object in contact with the fluid in square meters (m²)
• ΔT is the temperature difference between the object and the fluid in kelvin (K)
• Radiation: Heat transfer by radiation occurs when thermal energy is transferred from one object to another through electromagnetic waves. All objects emit and absorb radiation, and the rate of transfer depends on the temperature and the emissivity of the objects involved. Radiation is unique among the mechanisms of heat transfer in that it can occur even in a vacuum, and is not dependent on the presence of a material medium.The equation governing radiation heat transfer can be expressed as:Q = εσA(T_h^4 – T_c^4)

where:

• Q is the rate of heat transfer in watts (W)
• ε is the emissivity of the object (unitless, ranging from 0 to 1)
• σ is the Stefan-Boltzmann constant, equal to 5.67 × 10^-8 W/m^2K^4
• A is the surface area of the object in square meters (m²)
• T_h is the absolute temperature of the hotter object in Kelvin (K)
• T_c is the absolute temperature of the cooler object in Kelvin (K)

This equation is known as the Stefan-Boltzmann law of radiation. It states that the rate of heat transfer by radiation is proportional to the difference in the fourth power of the absolute temperatures of the hotter and cooler objects, the surface area of the object, and the emissivity of the object. The emissivity of an object is a measure of how well it emits radiation compared to a perfect black body, which has an emissivity of 1.

Understanding the principles of heat transfer is important in a wide range of fields, including engineering, physics, chemistry, and meteorology, as well as in everyday life. Many important technologies, such as refrigeration, power generation, and energy-efficient building design, rely on a thorough understanding of the principles of heat transfer.

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