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Thermal Physics

Thermal Physics

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Thermal Physics: Understanding Temperature

Temperature is a concept we all know, but how do we define it using physics? Describing the internal thermal energy of a substance using temperature is crucial in physics and thermodynamics. So, what is it exactly?

The thermal energy of a substance is linked to the average kinetic energy of its molecules or atoms. In a system with two objects, the hotter object has a higher temperature and higher average kinetic energy. Thermal energy flows from the hotter object to the colder one until both objects reach the same temperature. This is what the zeroth law of thermodynamics is all about.

The zeroth law of thermodynamics is so important to thermal physics that it is now the first law. It states that if two objects are in thermal equilibrium with a third object, they are in thermal equilibrium with each other. This law helps us understand how the temperature of a substance is measured and how it relates to its internal energy.

Understanding thermal physics and the laws of thermodynamics can help us grasp the fundamental principles that govern energy transfer and temperature.

The zeroth law is a fundamental concept of thermal physics that provides a basis for the definition of temperature and mathematical laws about the effects of temperature
The zeroth law is a fundamental concept of thermal physics that provides a basis for the definition of temperature and mathematical laws about the effects of temperature

Thermal Physics: Understanding Temperature and Heat Transfer

Temperature is a fundamental concept in physics and thermodynamics. It is defined as the internal thermal energy of a substance, which is proportional to the average kinetic energy of its constituent molecules or atoms. The hotter an object is, the higher its temperature and average kinetic energy. Thermal energy flows from a hotter object to a colder one until they reach thermal equilibrium.

The zeroth law of thermodynamics defines temperature as a property that describes the direction of any thermal energy transfer between objects. This law is crucial in understanding how the temperature of a substance is measured and how it relates to its internal energy.

To measure temperature, we need a temperature scale. The Celsius scale is used by most of the world and is based on the freezing point (0°C) and the boiling point of water (100°C). The Fahrenheit scale, primarily used in the United States, is based on the same fixed points but with different increments. The Kelvin scale, also known as the absolute temperature scale, uses the triple point of water (273.16°K) and absolute zero (0°K) as fixed points. Temperatures on the Kelvin scale are always positive.

Heat transfer is the process by which thermal energy is transferred between objects in a system. Thermodynamics deals with the relationships between heat, work, temperature, and energy in systems. The laws of thermodynamics provide a framework for understanding these relationships.

The first law of thermodynamics states that a change in internal energy of a system (ΔU) to the thermal energy or of energy via work done to or by the system (W). This law shows that energy cannot be created or destroyed.

The second law of thermodynamics introduces the property of entropy, which is the total disorder or randomness of a system. When two previously isolated systems are allowed to interact, they will eventually reach a state of thermal equilibrium. The total entropy of the combined system will be greater than the sum of the two isolated systems.

The third law of thermodynamics states that as a system’s temperature approaches absolute zero and all thermal energy is removed, the system reaches a constant ground state. The value of entropy at this point is known as the system residual entropy.

All substances have internal energy, which is the sum of the kinetic energy and potential energy of their constituent molecules. The kinetic energy of molecules in a substance is directly related to its temperature, while their potential energy depends on the phase of the substance.

Understanding thermal physics and the laws of thermodynamics can help us grasp the fundamental principles that govern energy transfer and temperature.

The diagram shows how the kinetic and electrostatic potential components of a substance’s internal energy increase through phase changes as energy is transferred into the substance at a steady rate
The diagram shows how the kinetic and electrostatic potential components of a substance’s internal energy increase through phase changes as energy is transferred into the substance at a steady rate

Thermal Physics: Energy Transfer, Specific Heat, and Ideal Gases

Energy is transferred into a substance at a steady rate, and during phase changes, the electrostatic potential energy increases while the kinetic energy of its molecules (temperature) remains constant. The total internal energy is a sum of these two components and increases at the same rate as energy is transferred into the substance. The specific latent heat of a substance is the energy required to change its phase per unit mass while at a constant temperature, while the specific heat capacity is the amount of energy required to change a substance's temperature by a certain amount per unit mass. Different substances have different specific heat capacities, with water having a high specific heat capacity and iron having a low one.

Ideal gases are theoretical substances whose molecules occupy negligible space and are not affected by electrostatic force interactions. The number of atoms or molecules in a volume of gas is described using the mole, which is the SI unit for the amount of substance. One mole of a substance contains as many atoms or molecules as there are atoms in 0.012kg of carbon-12, which is approximately 6.02⋅1023.

Understanding thermal physics is crucial in grasping the fundamental principles that govern energy transfer and temperature. The zeroth law of thermodynamics defines temperature as a property that describes the direction of any thermal energy transfer between objects. Temperature scales are defined by two fixed points at specific temperatures, with a number of increments between them. The internal energy of a substance is comprised of molecular kinetic energy and electrostatic potential energy. Moles are the SI unit for the amount of substance that indicates the number of atoms or molecules in a given sample of a given substance. Ideal gases are an approximation of real gases whose behaviour can be modelled using the ideal gas equation.

Thermal Physics

What is Thermal Physics?

Thermal physics is a subject covering the fields of thermodynamics and the kinetic theory of gases. Statistical mechanics is also used to study these by approximating the behavior of large numbers of particles.

How to calculate thermal energy?

The amount of energy in a substance is its internal energy (U), comprised of thermal energy and electrostatic potential energy. To calculate the amount of thermal energy required for temperature changes the specific heat capacity of materials can be used: Change in energy (E) = mass (m) · specific heat capacity (c) · change in temperature (ΔT).

What is Q in thermal physics?

The term Q represents a change in the thermal energy of a substance. The first law of thermodynamics states: a change in internal energy (ΔU) of a system is comprised of the thermal energy added to the system (Q) plus any net gain or loss of energy via work done to or by the system (W).

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