Heat

Understanding Heat and Thermodynamics

Imagine you’re holding a warm cup of coffee in your hands. The warmth you feel is actually the transfer of heat from the hot liquid to your skin. But what exactly is this mysterious force that makes your coffee cozy? In thermodynamics, heat is defined as energy transferred between a system and its surroundings through various modes such as thermal conduction, radiation, and friction.

The First Law of Thermodynamics: A Key Concept

The first law of thermodynamics states that for a closed system, the heat involved in a process is the difference in internal energy between the final and initial states. This means that if you pour hot water into an ice cube tray, the heat from the water will be transferred to the ice, causing it to melt. The change in internal energy can be measured using calorimetry, which measures the quantity of energy transferred as heat by its effect on interacting bodies.

Historical Perspectives: From Phlogiston to Kinetic Theory

Historically, the concept of heat was speculative and involved theories such as phlogiston theory and caloric theory. These ideas suggested that heat was a separate form of matter. However, in the late 18th century, the ‘mechanical’ theory of heat began to take shape. This theory proposed that changes in internal energy were due to friction and other factors, aligning with the idea that heat is related to particle motion.

Notation and Units: Measuring Heat

The symbol Q for heat was introduced by Rudolf Clausius and Macquorn Rankine. Negative values indicate heat released into surroundings, while positive values indicate absorbed heat. The unit of heat in the International System of Units (SI) is the joule (J). Other units used in engineering include watt (W), which measures heating rate.

Calorimetry: Measuring Heat Capacity

A calorimeter is a device used to measure heat capacity and the heat absorbed or released in chemical reactions. For instance, in 1780, Antoine Lavoisier used a calorimeter to investigate heat release by respiration. This tool helps scientists understand how much energy is transferred as heat during various processes.

The Modern Understanding of Heat

By the late 19th century, the modern understanding of heat began with Thompson’s mechanical theory of heat (1798) and was later developed by Nicolas Clément and Sadi Carnot. The concept that heat consists of motion was further popularized by James Prescott Joule and Scottish physicist Joseph Black. John Tyndall’s work in 1863 helped spread this idea to the English-speaking public.

Carathéodory’s Definition: A Rigorous Approach

In 1909, Carathéodory introduced a rigorous mathematical definition of heat based on the concept of energy transferred as work. This approach does not use calorimetry or temperature in its primary definition but regards calorimetry only as a secondary way of measuring quantity of energy transferred as heat.

Heat Transfer and Engines

A heat engine is a model consisting of four bodies: the working body, hot reservoir, cold reservoir, and work reservoir. A cyclic process leaves the working body unchanged, with reversible work transfers and irreversible heat transfer. Energy is taken from the hot reservoir and sent to the two other reservoirs, with the second law requiring that no cycle can occur without energy received by the cold reservoir.

Heat Pumps: Transferring Heat

A single cycle starts with the working body colder than the cold reservoir. Energy is taken in as heat from the cold reservoir and work is done on the working body, making it hotter than the hot reservoir. This process transfers energy from a colder to a hotter reservoir, which is not regarded as by an inanimate agency but rather as harnessed work.

Entropy: A Measure of Disorder

Entropy is a measure of disorder or randomness in a system. It can also be thought of as the amount of thermal energy unavailable to do work in a system. The second law of thermodynamics states that if heat is supplied to a system in a reversible process, the increment of heat δQ and the temperature T form the exact differential dS = δQ / T.

Heat Transfer Models: Conduction, Radiation, and Convection

Heat transfer occurs between bodies through conduction, radiation, and convection. Convection is regarded as transport of internal energy, while conduction and radiation allow for heat transfer between bodies. Latent heat is the energy released or absorbed during a phase transition without a change in temperature.

Conclusion

The concept of heat is essential in understanding thermodynamics. From its historical roots to modern definitions, heat plays a crucial role in our daily lives and scientific endeavors. Whether it’s warming your hands on a cold day or powering the engines that drive our world, heat remains an indispensable force. Understanding how heat works helps us harness its power more effectively and efficiently.