Thermodynamics is the branch of physics focused on how energy, particularly heat, relates to work and the properties of matter. While the term “heat” is often used loosely in everyday language, within a scientific context, it has a precise definition. Understanding this precision is necessary to grasp the fundamental principles that govern all energy interactions. This article explores the specific thermodynamic understanding of heat, clarifying its nature as energy in motion and distinguishing it from other closely related concepts.
Defining Heat as Energy Transfer
Heat, denoted by the symbol \(Q\), is defined exclusively as the transfer of thermal energy between two systems or between a system and its surroundings due solely to a difference in temperature. This energy transfer always occurs spontaneously from the warmer object to the cooler object. It is energy in transit, existing only while the transfer is taking place.
The scientific unit for measuring heat is the Joule (J) in the International System of Units (SI), though traditional units such as the calorie (cal) and the British thermal unit (BTU) are still used in certain applications. A common misconception is that a hot object “contains” heat; thermodynamically, the object contains thermal energy, and heat is the mechanism for moving that energy across a boundary. Heat transfer into a system is conventionally assigned a positive value (\(Q > 0\)), while heat transferred out of a system is negative (\(Q < 0[/latex]).
Distinguishing Heat from Temperature and Internal Energy
Temperature ([latex]T\)) is a measure of the average kinetic energy of the particles—atoms and molecules—within a substance. A higher temperature signifies that the particles are moving or vibrating more vigorously on average. Temperature is a property of the system, meaning it describes the state of the system at any given moment.
Internal energy (\(U\)) is the total energy contained within a thermodynamic system. This sum includes the kinetic energy of the particles as well as the potential energy stored in their chemical bonds and intermolecular forces. While two objects at the same temperature have the same average particle kinetic energy, a larger object has a greater internal energy because it contains more particles.
Heat is neither temperature nor internal energy; it is a process function describing the flow of energy driven by a temperature difference. A system can possess internal energy and a specific temperature, but it cannot possess heat. Adding heat to a system generally increases its internal energy and consequently raises its temperature. The core distinction is that internal energy and temperature are properties of a substance, while heat is the means of energy exchange.
Mechanisms of Thermal Energy Movement
Thermal energy moves from one location to another through three distinct physical mechanisms: conduction, convection, and radiation. Real-world heat transfer often involves a combination of all three processes occurring simultaneously. Each mechanism is defined by the medium it uses and the way the energy is transmitted at a microscopic level.
Conduction
Conduction is the transfer of heat through direct physical contact between materials, without any bulk movement of the matter itself. This process is most effective in solids, where closely packed atoms and molecules vibrate and collide with their neighbors, passing kinetic energy along the material. Touching a metal spoon that is resting in a hot bowl of soup is a common example of conduction, as the energetic particles in the metal transfer their energy to the less energetic particles in your hand.
Convection
Convection is the transfer of heat through the movement of fluids, which include liquids and gases. When a fluid is heated, it expands, becomes less dense, and rises, while the cooler, denser fluid sinks to take its place. This circulation pattern, known as a convection current, efficiently distributes heat throughout the fluid. The process of boiling water in a pot or the circulation of air from a home furnace are prime examples of convection.
Radiation
Radiation is the transfer of heat via electromagnetic waves, such as infrared light. This mechanism is unique because it does not require a physical medium to travel, allowing heat to be transferred across a vacuum, which is how the sun’s energy reaches Earth. All objects with a temperature above absolute zero emit thermal radiation, and hotter objects emit significantly more of this energy. Feeling the warmth from a campfire or a hot stovetop element from a distance are everyday experiences of heat transfer by radiation.
Heat, Work, and Energy Conservation
The transfer of energy between a system and its surroundings can occur in only two fundamental ways: as heat (\(Q\)) or as work (\(W\)). Work in this context is mechanical energy transfer, such as a gas expanding against a piston or stirring a liquid. The relationship between heat, work, and the internal energy of a system is described by the First Law of Thermodynamics, which is a restatement of the principle of energy conservation.
This law states that the change in the internal energy (\(\Delta U\)) of a system is equal to the net heat transferred into the system minus the work done by the system. Any energy added to a system as heat must either increase the system’s internal energy or be used by the system to perform work on its surroundings. Heat and work are both forms of energy in transit, and they combine to determine the final state of the system’s stored internal energy.