Heat Transfer and Thermodynamics: A Comprehensive Look
Understanding Heat and Temperature
A body is a limited portion of matter, and matter is formed by molecules. The motion of these molecules varies according to the state of aggregation of matter.
- In solids, the molecules move, vibrating around a fixed point.
- In liquids, the molecules move more freely.
- In gases, the molecules move randomly and at high speeds.
Molecular Theory: This theory explains the movement of molecules. When molecules move faster, they possess greater energy. Therefore, an increase in molecular motion results in a higher temperature. The energy we are referring to within a body is called its internal energy. Temperature indicates the internal energy of a body.
Heat is a type of energy transfer that occurs from a body at a higher temperature to one at a lower temperature. Energy is the ability to do work. It is a capability that manifests in various forms, such as solar energy, electrical energy, and chemical energy.
Principles of Thermodynamics
- First Principle of Thermodynamics: Energy is neither lost nor created; it is transformed.
- Second Principle of Thermodynamics: Heat is transferred from a body at a higher temperature to a body at a lower temperature.
Thermal Equilibrium
The heat transfer process ends when both temperatures are equal, ceasing heat transfer. (Kelvin = Celsius + 273).
Thermometers
A thermometer consists of a thin glass tube with mercury inside a bulb. When heated, the liquid inside expands according to the temperature of the object it contacts. Conversely, when cooled, the liquid contracts. There are three temperature scales: Celsius, Fahrenheit, and Kelvin (absolute).
Absolute Zero
Absolute zero is the lowest possible temperature, where objects have no internal energy and their molecules cease to move. The Kelvin scale is considered absolute because its zero point is defined as absolute zero.
Forms of Heat Transmission
- Conduction: In solids, heat spreads through conduction. When heating a solid, such as an iron bar, its molecules vibrate rapidly and collide with neighboring molecules. These collisions transfer energy to molecules moving more slowly. This process varies among materials; some solids, like metals, conduct heat very well, while others, called insulators (e.g., wood and plastics), do not.
- Convection: In liquids and gases, heat is transferred by convection. When heating water in a container, the water closest to the heat source heats up first. The molecules expand, becoming lighter, and rise, creating circular currents that displace energy through the movement of matter.
- Radiation: Between the sun and the earth, there is no fluid to transmit heat by convection, nor a solid to transmit heat by conduction. The sun radiates heat as electromagnetic waves, which can propagate even in a vacuum.
Expansion
Heating a body increases its temperature and simultaneously increases its volume, resulting in expansion. The extent of expansion depends on the state of matter and the material composition of the body.
- Solid Linear Expansion: When heating a solid body where length dominates, the expansion is linear and depends on the substance’s nature.
- Surface Expansion: In laminar bodies where length and width predominate, the area increases with temperature.
- Cubic Expansion: In solids where no dimension predominates, heating increases the volume.
Fluids and Gases
The volume of a liquid increases when heated. If a gas is heated in a closed container, and the volume cannot increase, the pressure increases.
The formation of cracks in ceilings is caused by the expansion and contraction of materials due to temperature changes. Iron sheets can become twisted.
Thermostats are devices that maintain a constant temperature in a medium. They utilize the principle of thermal expansion, consisting of two metal sheets with different expansion coefficients. When heated, the metals expand unevenly, interrupting an electrical circuit. When cooled, the bimetal returns to its straight form, closing the circuit.