Heat

What is Heat?

Heat is the transfer of thermal energy between systems or objects due to temperature differences. The heat flows from the body or system with a higher temperature to one with a lower temperature until thermal equilibrium is reached. Heat is measured in joules (J) or calories (cal).

Temperature and Heat

Temperature is a measure of the average kinetic energy of the particles in a substance. Heat, on the other hand, is the energy transferred between objects due to a difference in temperature. The relationship between heat (Q) and temperature change (ΔT) is given by:

Q = mcΔT

Where:

• Q is the amount of heat energy transferred.
• m is the mass of the substance.
• c is the specific heat capacity of the substance.
• ΔT is the change in temperature.

Methods of Heat Transfer

Heat can be transferred by three main methods:

• Conduction: The transfer of heat through a material without the movement of the material itself. It occurs in solids, especially metals, where particles vibrate and transfer energy to neighboring particles.
• Convection: The transfer of heat through the movement of fluids (liquids or gases). In convection, the warmer, less dense part of the fluid rises, while the cooler, denser part sinks, creating a circulation of the fluid.
• Radiation: The transfer of heat through electromagnetic waves (such as infrared radiation). Unlike conduction and convection, radiation does not require a medium and can occur in a vacuum (e.g., heat from the sun).

Specific Heat Capacity

Specific heat capacity is the amount of heat required to raise the temperature of a unit mass of a substance by 1°C. It varies with different materials and is an important property in understanding thermal energy transfer.

Unit: J/(kg·°C)

Water has a high specific heat capacity, meaning it requires more heat energy to change its temperature compared to many other substances. This property of water helps in regulating temperatures in the environment.

Latent Heat

Latent heat is the amount of heat required to change the state of a substance without changing its temperature. This can occur during phase transitions such as melting, freezing, boiling, or condensation.

The latent heat is classified into two types:

• Latent Heat of Fusion: The heat required to convert a solid into a liquid without changing its temperature (e.g., ice melting to water).
• Latent Heat of Vaporization: The heat required to convert a liquid into a gas without changing its temperature (e.g., water boiling to steam).

The latent heat is given by:

Q = mL

Where:

• Q is the amount of heat absorbed or released.
• m is the mass of the substance.
• L is the latent heat of the substance.

Thermal Expansion

Thermal expansion refers to the change in the size (length, area, or volume) of a material as it is heated or cooled. Most materials expand when heated and contract when cooled. The linear thermal expansion of a material is given by:

ΔL = αL₀ΔT

Where:

• ΔL is the change in length.
• α is the coefficient of linear expansion.
• L₀ is the initial length.
• ΔT is the change in temperature.

This principle is used in thermometers, expansion joints in bridges, and other structures where temperature variations are expected.

Previous Year Questions

• RRB JE 2016: What is the latent heat of fusion of ice?
• The latent heat of fusion of ice is 334 kJ/kg.
• SSC JE 2017: A 100 kg mass is heated from 20°C to 60°C. Calculate the amount of heat required if the specific heat capacity of the substance is 2.5 J/(g·°C).
• Q = mcΔT = 100,000g × 2.5 J/(g·°C) × (60°C - 20°C) = 100,000 × 2.5 × 40 = 10,000,000 J (10 MJ)
• HSSC JE 2018: Which of the following is true for thermal expansion?
• Most materials expand when heated and contract when cooled.

Conclusion

Heat is a fundamental concept in physics that affects many processes in our daily lives, from weather patterns to the functioning of engines. Understanding heat transfer and its associated concepts helps in improving energy efficiency and various technological innovations.