Understanding Heat: Transfer, Capacity, and State Changes
Heat energy is the manifestation of the kinetic energy of particles, atoms, and molecules, which comprise the body in question. It is transmitted from hotter bodies to colder ones until a state of balance is reached, and energy transmission stops. This energy in transit is the heat.
Temperature and Heat
Temperature is a measure of the degree of heat of substances. It is measurable in °C, °K, or °F.
Heat is measured by a quantity called specific heat (Ce) and is defined as the heat needed to raise 1 gram of a substance by 1 degree Celsius. The joule is used as the unit of work and energy (1 calorie = 4.18 Joules).
Heat Capacity
Heat capacity is the amount of heat that a mass of a body can accumulate or lose. The formula is: Q = m * Ce * (Tf – Ti).
The formula to find the amount of heat (Q) required to melt a quantity of ice is: Q = m * Lf.
The formula to find the amount of heat (Q) required to evaporate a given amount of water is: Q = m * Lv.
Heat Transfer
Conduction
Conduction is typical of solids. The heat passes from one particle to the others that surround it, from these to the next, and so on.
- Good conductors: Metals such as iron, copper, etc.
- Bad conductors: Non-metals like wood, glass, etc.
Convection
Convection is typical of fluids (liquids and gases). The hotter mass, being less dense, rises, and the gaps left are filled by colder masses that descend, resulting in convection currents (vertical winds, ocean currents, etc.).
Radiation
Radiation occurs when there are no material resources that facilitate the spread. It is transmitted by waves. The absorption of this type of heat transmission depends on the color of the materials, with darker materials being more absorbent. For example, dark clothes are used in winter and light clothes in summer.
Effects of Heat
Thermal Expansion
Dilation: Any body increases in volume when heat is conveyed to it. This phenomenon is called dilation.
- Dilation of the container: It is the first that occurs and is manifested by an apparent decrease in the volume of liquid, which corresponds to the increase in the volume of the container.
- Dilation of the fluid: It is produced almost instantaneously to the container and is manifested by an increased volume of fluid inside the container (apparent dilation).
The real expansion of the liquid is the sum of the vessel dilation plus the apparent expansion of the liquid.
Changes of State
Matter exists in three states: solid, liquid, and gas.
- Solid state: Characterized by particles that are very close, and cohesive forces are very effective.
- Liquid state: Particles are farther apart, as they move faster, and the cohesive forces have less influence.
- Gaseous state: Characterized by the huge separation between particles due to their high speed. Cohesive forces produce a minimal effect.
By the action of heat, materials can change state (switch from one state to another), but while this process occurs, the body temperature does not increase despite the heat supplied.
Progressive Changes of State
Progressive changes of state occur due to an increase in heat.
- Fusion: The transition from solid to liquid. There are two types:
- Direct: Passing directly from solid to liquid without intermediate states (e.g., water).
- Pasty: Having some intermediate states before melting (e.g., butter, wax).
- Vaporization: The passage from liquid to gas. It has two forms:
- Evaporation: Occurs quietly on the free surface of the liquid (e.g., water from salt pans).
- Boiling: Occurs tumultuously in the entire volume of the liquid (e.g., boiling water).
- Progressive or direct sublimation: The direct transition from solid to gas (e.g., iodine crystals).
Regressive Changes of State
Regressive changes of state occur due to a decrease in heat.
- Solidification: The transition from liquid to solid.
- Liquefaction or condensation: The transition from gas to liquid.
- Regressive sublimation: The direct passage of gas to solid (e.g., camphor, naphthalene).