Thermal physics

Thermal physics is an important branch of physics that deals with the study of heat, temperature, and the conversion of energy into different forms. It explores how thermal energy is transferred between different systems and how the state of matter changes as a result. This field of study plays a vital role in understanding how everything from small particles to large systems behave under the influence of temperature.

Heat and temperature: understanding the basics

First, it's important to distinguish between heat and temperature, which are often used interchangeably but have different meanings in physics. Heat is a form of energy transfer between systems or objects with different temperatures. It's important to remember that heat is not a substance; it's a process of energy transfer.

Temperature , on the other hand, is a measure of the average kinetic energy of the particles in a substance. This determines the direction of heat transfer. For example, when you touch a hot cup, heat flows from the cup to your hand because the cup has a higher temperature. Remember that temperature is measured in units such as Celsius (°C), Fahrenheit (°F) and Kelvin (K).

Thermodynamic systems and environments

In thermal physics, the system refers to the part of the universe that is being studied, while the surroundings are everything outside the system. Systems can be of three types:

• Open system: can exchange energy and mass with its surrounding environment.
• Closed system: can exchange energy, but not mass, with its surroundings.
• Isolated system: It cannot exchange energy or mass with its surroundings; it is completely isolated.

Understanding how systems interact with their surroundings helps us understand how energy is transferred and transformed in various processes.

Laws of thermodynamics

Thermal physics is strongly guided by the laws of thermodynamics, which are fundamental principles that describe the behavior of energy. Let's explore these laws:

First law of thermodynamics

The first law of thermodynamics is essentially the law of conservation of energy. It states that energy cannot be created or destroyed; it can only change form. The mathematical expression of this law is:

ΔU = Q - W

Where:

• ΔU is the change in the internal energy of the system.
• Q is the heat added to the system.
• W is the work done by the system.

Second law of thermodynamics

The second law of thermodynamics introduces the concept of entropy, which is a measure of the disorder or randomness of a system. It states that in any energy exchange, if no energy enters or leaves the system, the potential energy of the situation will always be less than that of the initial state. In simple terms, energy systems go from a state of order to a state of disorder.

This is the law that makes perpetual motion machines impossible, since they violate this principle by claiming to generate energy from nowhere or to operate forever without energy input.

Third law of thermodynamics

The third law of thermodynamics states that as the temperature of a system approaches absolute zero (0 Kelvin), the entropy of a perfect crystal approaches zero. Absolute zero is the lowest possible temperature where the motion of particles is minimal.

This law helps us understand that reaching absolute zero is practically impossible, as it would mean removing all the energy from the system, which contradicts other laws of thermodynamics.

Specific heat capacity and calorimetry

The specific heat capacity of a substance is the amount of heat needed to raise the temperature of one kilogram of that substance by one degree Celsius (or one Kelvin). It tells us how much energy is needed to change the temperature of a given substance. The equation used to find the specific heat is:

Q = mcΔT

Where:

• Q is heat energy (in joules).
• m is the mass of the object (in kilograms).
• c is the specific heat capacity (in joules/kg°C).
• ΔT is the change in temperature (in °C or K).

Calorimetry is the study of measuring heat changes resulting from chemical reactions and physical transformations. A commonly used instrument in this field is the calorimeter, which isolates a system to measure the heat involved in reactions.

Heat transfer system

Heat transfer can occur in three different ways: conduction, convection, and radiation. Understanding these mechanisms is important to the study of thermal physics.

Conductivity

Conduction is the process of heat transfer through a substance without any movement of the substance. It occurs mainly in solids where the particles are close together. When a particle is heated, it vibrates and transfers energy to the neighbouring particles.

An everyday example of conduction is when a metal spoon heats up from the handle to the tip when placed in a hot liquid.

Convection

Convection is the transfer of heat through a fluid (liquid or gas) caused by the movement of the fluid. When one part of a fluid is heated, it becomes less dense and rises, while the cooler, denser fluid sinks. This creates a convection current that facilitates heat transfer.

Examples of convection include boiling water, where the hot water at the bottom rises, and cooler water comes down to take its place, creating a circular motion.

Radiation

Radiation is the transfer of heat via electromagnetic waves. Unlike conduction and convection, radiation does not require a medium and can occur in a vacuum. The Sun's heat reaching the Earth is a prime example of radiation.

Objects continuously emit radiation, and the ability of an object to emit or absorb radiation depends on the texture and color of its surface. Darker and rougher surfaces absorb more radiation than lighter and smoother surfaces.

Phase changes of matter

As substances absorb or release heat, they can undergo phase changes, transitioning between solid, liquid, and gaseous states. These changes occur without a change in temperature until the entire substance is transformed.

Melting and freezing

Melting occurs when a substance changes from a solid to a liquid, as heat is absorbed. In contrast, freezing is the change from a liquid to a solid, as heat is released.

Boiling and condensation

Boiling is the change from liquid to gas, which occurs at a specific temperature called the boiling point, while condensation is the change from gas to liquid, which releases heat.

Sublimation and deposition

Sublimation is the direct change from solid to gas without passing through the liquid state, while deposition is the opposite process, in which a gas is converted into a solid without going through the liquid state.

Kinetic theory of gases

The kinetic theory of gases is a model used to describe the behavior of gases and provides information about the gas laws. It assumes that gas particles are in constant, random motion, and the temperature of the gas is related to the average kinetic energy of the particles.

Some of the key points are as follows:

• Gas particles are in constant motion and their collision with the walls of a vessel produces pressure.
• The volume occupied by the gas particles is negligible compared to the volume of the container.
• There are no intermolecular forces between gas particles.

Practical applications of thermal physics

The applications of thermal physics are evident in everyday life and technology, from refrigerators and air conditioners to power plants and engines. Understanding thermodynamics and heat transfer principles helps us use energy efficiently and develop sustainable technology.

By considering these concepts, students gain an appreciation for the importance of thermal physics in scientific progress and practical problem solving.

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