Lesson 1: The Concept of Heat

Video Lesson

Lesson Competencies

At the end of the lesson, you will be able to

• Calculate heat transfer using the formula Q=mcΔT.
• Analyze the role of heat in thermodynamics (e.g., laws of thermodynamics).
• Evaluate energy efficiency in heating systems (e.g., home insulation, engines).

Brainstorming Question

What is temperature?

Key Terms and Concepts

Heat

• heat and temperature are completely different quantities.
• Heat is a measure of change, never a property possessed by an object or system.
• Therefore, it is classified as a process variable. However,
• temperature is a fundamental physical quantity which is to the average kinetic energy of particles of the substance.
• It should also be noted that work and heat are closely related.
• Both can change the temperature of a substance, and
• heat can be turned into work and work can be turned into heat.
• This equivalence is the basis of how heat engines power modern society
• 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,
• 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 general, if two objects at different temperatures are brought in thermal contact, energy flows spontaneously from the hotter object to the cooler one.
• The spontaneous transfer of energy from hotter region to cooler region is called heat. . Since heat

Thermal Equilibrium

• 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.

Thermal Equilibrium

• It is obtained when touching objects within a system reach the same temperature.

Figure: Thermal equilibrium.

Heat, internal energy and thermodynamic work

• Another aspect of the definition of heat is that a body never contains heat.
• That is, we cannot say “stored heat energy” as we cannot say “stored electric current”.
• Heat is not something you get it sitting down. Rather, heat can be identified only as it crosses a boundary
• When an object is heated, its temperature rises, since the average kinetic energy of its particles increases but during a phase change, the temperature remains constant.
• In other words, the energy stored in the substance is increased.
• This energy stored in the object is what we call internal energy.
• When the tea is cooling down, it is releasing its internal energy in the form of heat.
• Internal energy is equal to the sum of internal kinetic energy due to molecular motion (translational, rotational, and energy of vibration) and internal potential energy due to molecular attractive forces.
• Heat and work are two different ways of transferring energy from one system to another.
• Heat is the transfer of thermal energy between systems, while
• work is the transfer of mechanical energy between two systems.
• If heat is supplied to an enclosed gas, the temperature of the gas increases by then increasing its internal energy.
• If the gas is closed by a movable piston as shown in Figure 5.2, work is done by the expanding gas in raising the piston up.
• Thus, the heat supplied is converted partly into internal energy and partly into work done.
• We can also increase the internal energy of the gas by pushing the piston down and compressing the gas.