Chapter 19: Thermal Energy

19.1 Introduction to Thermal Energy

In this chapter, we will explore yet another form of energy known as thermal energy. Thermal energy is the energy that arises due to the motion of particles within a substance. As the kinetic energy of the particles within a substance increases, the temperature of that substance also increases, thus resulting in a higher thermal energy. We will discuss heat, temperature, specific heat capacity, and the modes of heat transfer.

19.2 Temperature and Heat

Temperature is a measure of the average kinetic energy of particles in a substance. It is essential to differentiate between temperature and heat. Heat is the flow of energy between substances due to a difference in temperature. When two substances at different temperatures come into contact, heat flows from the substance with the higher temperature to the substance with the lower temperature until both substances reach the same temperature, otherwise known as thermal equilibrium. Temperature is a state variable. It is something that describes the state an object is in. Heat is not a state variable. It is a flow of energy.

19.3 Specific Heat Capacity

Specific heat capacity is a property of a substance that describes how much heat is required to change the temperature of a certain mass of the substance by a certain amount. That may sound like a lot of information to take in. However, it’s not too complicated when you consider the units involved. Specific heat capacity is given in units of Joules per kilogram per degree Celsius (J/kg°C). The heat transfer (Q) can be calculated using the formula:

where is the mass of the substance, is the specific heat capacity, and is the change in temperature.

Water has a specific heat capacity of 4186 J/kg°C. Thus, we require 4186 Joules of energy to raise the temperature of 1 kg of water by 1 degree Celsius.

19.4 Heat Transfer Methods

There are three primary methods of heat transfer: conduction, convection, and radiation.

1. Conduction: Conduction is the transfer of heat through a solid material by direct contact between neighboring particles. In this process, heat energy is transferred from the particles with higher kinetic energy to those with lower kinetic energy. The rate of heat transfer through conduction depends on the material’s thermal conductivity.
2. Convection: Convection is the transfer of heat through fluids (liquids and gases) due to the motion of particles. When a fluid is heated, its particles gain kinetic energy, causing the fluid to expand and become less dense. As a result, the warmer fluid rises, and the cooler fluid sinks, creating a convection current that transfers heat throughout the fluid.
3. Radiation: Radiation is the transfer of heat through electromagnetic waves, such as infrared radiation. Unlike conduction and convection, radiation does not require a medium for heat transfer, allowing it to occur through a vacuum, like the heat transfer from the Sun to Earth.

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Continue to Chapter 20: Introduction to Fluid Mechanics

Back to Chapter 18: Elastic Potential Energy

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