Heat: Definition, Transfer Mechanism and Important Terms

Heat is a form of energy that produces the sensation of hotness and coldness in us. This energy plays an essential role in nature. One of the best examples of heat energy is the sun itself which is a must for survival on Earth.

• Heat energy is used for several activities like cooking, transportation, etc.
• When a body is heated various changes take place.
• The temperature of the body may rise with an expansion or contraction of the body or the body may liquify or vaporize with no change in temperature.
• Heat energy is also defined as the total kinetic energy of the molecules of the substance.
• That means the heat energy of a substance increases with the increase in kinetic energy of its molecules and vice-versa.

Key Terms: Heat energy, Kinetic energy, Laws of thermodynamics, Latent heat, Specific Heat, Temperature, Thermal expansion, Thermal equilibrium.

Read More: Intermolecular Forces

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What is Heat?

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The term "Heat" refers to energy crossing a boundary of a thermodynamic system as a result of a difference in temperature. Heat also refers to the thermal energy contained in a system as internal energy and that is reflected in the temperature of the system.

• Heat energy flows from a hot body to a cold body.
• It is the total kinetic energy of the molecules of the substance.
• According to the first law of thermodynamics, the heat involved in a process is the difference in the final state and initial state of a system, and subtracting the work done in the process.
• The measurement of the transfer of heat energy is called calorimetry.
• The vessel which is largely used in the measurement of heat is called a calorimeter.
• SI unit of heat energy is joule (J).
• In CGS, the unit of heat energy is calorie (cal).
• One calorie is equivalent to 4.186 joules.
• British thermal unit (BTU) is also used to measure heat energy.
• One Btu is equivalent to 1055.06 joule

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What is Temperature?

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The temperature of a substance is a physical quantity that measures the degree of hotness and coldness of the substance. It is the average kinetic energy of a body.

• An object at a higher temperature is said to be hotter than that one at a lower temperature.
• The temperature of a substance determines whether the heat energy flows from one body to another body, or not.
• Due to the temperature difference between the two bodies, heat energy is transferred from one body to another body.
• Heat energy flows from a body at a higher temperature to a body at a low temperature.
• SI unit of temperature is kelvin (K), whereas degree Celsius (^oC) is a commonly used unit of temperature.

Measurement of Temperature

A device used to measure the temperature of a body is called a thermometer. The branch of science which deals with the measurement of the temperature of a substance is known as thermometry and the property of a substance that changes with temperature is called thermometric property.

The thermometric property of mercury is used to establish a temperature scale, which helps the temperature of other bodies. The four temperature scales used for measuring the temperature of other bodies are:

1. Celcius scale
2. Fahrenheit scale
3. Reaumur scale
4. Kelvin scale

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Difference Between Heat and Temperature

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The main difference between heat and temperature is given below

Read More: Mean Free Path

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Heat Transfer Mechanisms

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The heat energy transfer from a body at a higher temperature to a body at a lower temperature. The heat transfer from one body to another body by any of the following processes:

1. Conduction
2. Convection
3. Radiation

Conduction

The process by which heat is transferred from the hot part of a body to the cold part through the transfer of energy from one particle to another particle of the body without the actual movement of the particles from their mean positions is called Conduction.

Read More: Valance Electrons

Convection

The process of heat transfer from one part of a fluid to another part by the actual movement of the particles of the fluid is called Convection. Liquid and gases are heated by the process of convection. There are two types of convection:

1. Forced convection: If the fluid is circulated by an external agent like a blower or pump, the convection process is known as forced convection.
2. Natural convection or Free convection: If the flow is caused by the difference in density due to thermal expansion, such as hot air rising. The process is called Natural convection.

Radiation

The process of transfer of heat from one place to another place without heating the intervening medium is called Radiation. It is the transfer of heat by electromagnetic waves such as visible light, infrared and ultraviolet rays.

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Thermal Equilibrium

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Thermal means "caused by heat". Heat moves from a hot substance to a cold substance till the temperature of the materials becomes the same, which is a state of thermal equilibrium.

• In mechanics, the term equilibrium means that the net force and torque on a system are zero.
• But in thermodynamics, if the temperatures of two systems are equal then we can say that both systems are in thermal equilibrium.

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Thermal Expansion

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The process due by which solid material experiences change in volume, length, or area of heating is called Thermal Expansion.

• If we provide some amount of heat to any material, there will be an increase in its size or shape.
• Thermal expansion is due to the increase in the amplitudes of the particles.

Types of Thermal Expansion

There are three kinds of Thermal Expansion.

1. Linear Expansion
2. Superficial Expansion
3. Volumetric Expansion

1. Linear Expansion

When the length of any rod will increase due to heat applied to it, such an increase is known as linear expansion.

2. Superficial Expansion

When the area of any square or rectangle increases due to heat applied to it, in such a case this increase is called superficial expansion.

3. Volumetric Expansion

When the volume of a cube increases due to applied heat, such an increase is known as volumetric expansion.

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Thermodynamics

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The study of heat and its transformation to mechanical energy is called Thermodynamics. It comes from a Greek word meaning “Movement of heat”.

A collection of a large number of molecules or atoms within a certain boundary such that it has certain values of pressure, volume, and temperature is called a thermodynamic system. There are three laws of thermodynamics.

1. Zeroth Law of Thermodynamics
2. First Law of Thermodynamics
3. Second Law of Thermodynamics

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Zeroth Law of Thermodynamics

Zeroth law states that “if two systems each are in thermal equilibrium with a third system separately, then they are in thermal equilibrium with each other.

Examples: 

If the temperature of A = temperature of C

and temperature of B = temperature of C

Then, the temperature of A = temperature of B

First Law Of Thermodynamics

According to Claussius's statement of the first law of thermodynamics, the heat (ΔQ) given to a system is equal to the sum of the internal energy (ΔU) of the system and the work done (ΔW) by the system against external pressure.

ΔQ = ΔU + ΔW

Second Law Of Thermodynamics

Kelvin-Planck's statement: No process is possible whose sole result is the absorption of heat from a reservoir and the complete conversion of heat into work.

Clausis statement: No process is possible whose sole result is the transfer of heat from a cooler object to a hotter object.

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Important Terms Related to Heat

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Some important terms related to heat are:

• Latent heat: It is the amount of heat required to change the state of a unit mass of a substance without change in its temperature, and pressure.
• Specific heat: Specific heat of a material is the amount of heat required to raise the temperature of a unit mass of a substance by one-degree Celsius.
• Thermal Heat: It is the amount of heat required to raise the temperature of a body by one-degree Celsius.
• Heat Engines: Any device that transforms heat, partly into work or mechanical energy is called a heat engine. 

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Things to Remember

• Heat means something that seems warm or hot.
• The total Kinetic energy of a material is known as HEAT.
• It is the average kinetic energy of a body or we can also say the numerical measurement of the degree of hotness is Temperature.
• Heat transfer mechanisms are ways by which hot/thermal energy can be carried between objects or things.
• The branch of physics that deals with heat energy change into mechanical energy or work or change of work into heat energy is known as Thermodynamics.
• The process due by which solid material experiences change in volume, length, or area of heating is called Thermal Expansion.