Heat

Definition and Units of Heat

• Heat is the transfer of thermal energy between systems due to a temperature difference.
• It refers to the kinetic energy of vibrating and colliding atoms in a substance.
• Heat is not the same as internal energy.
• Thermal energy is stored in the microscopic degrees of freedom of the modes of vibration.
• The unit of measurement for heat is the joule (J) in the International System of Units (SI).
• Other traditional units used in engineering include the British thermal unit (BTU) and the calorie.
• The rate of heating is measured in watts (W).
• The symbol 'Q' is used to represent heat in equations.
• Heat flux is defined as the rate of heat transfer per unit cross-sectional area.

Heat and Thermodynamics Laws

• Heat and entropy are closely related in thermodynamics.
• The second law of thermodynamics states that heat transfer increases the entropy of a system.
• In a transfer of energy as heat, there are changes of entropy in both the surroundings and the system.
• The total change of entropy in the system and surroundings is equal to the uncompensated change of entropy.
• For a closed system, the change in internal energy is equal to the amount of heat supplied.
• Enthalpy is a thermodynamic quantity that includes both the internal energy and the pressure-volume work done by a system.
• The change in enthalpy of a system is equal to the heat transferred at constant pressure.
• The first law of thermodynamics relates the change in internal energy, heat, and work done by a system.
• The second law of thermodynamics states that heat cannot spontaneously flow from a colder object to a hotter object.
• Reversible processes are idealized processes that can be analyzed mathematically.
• Irreversible processes involve entropy production and are more common in real-world situations.
• The second law of thermodynamics sets limits on the efficiency of heat engines and refrigerators.

Work and Thermodynamic State Variables

• Work is the amount of energy transferred to or from a system.
• The sign convention for work is that work done by the system on its surroundings is negative, while work done on the system by its surroundings is positive.
• The thermodynamic work done by the system is defined by its state variables, such as volume.
• It does not necessarily involve mechanisms in the surroundings, such as shaft work.
• Isochoric work is an example of work that involves variables in the surroundings.
• The internal energy of a system is a state function.
• In cyclical processes, state functions return to their initial values upon completion of a cycle.
• The differential increment for internal energy in an infinitesimal process is an exact differential.
• In contrast, infinitesimal increments of heat and work are inexact differentials.
• The integral of any inexact differential in a process where the system returns to the same state does not necessarily equal zero.

Heat Transfer and Laws of Thermodynamics

• Heat transfer can occur through conduction, convection, and radiation.
• The second law of thermodynamics states that heat transfer increases the entropy of a system.
• The second law of thermodynamics states that heat cannot spontaneously flow from a colder object to a hotter object.
• Reversible processes are idealized processes that can be analyzed mathematically.
• Irreversible processes involve entropy production and are more common in real-world situations.
• The second law of thermodynamics sets limits on the efficiency of heat engines and refrigerators.

Perspectives and Terminology of Heat

• James Clerk Maxwell outlined four stipulations for the definition of heat.
• Heat can be transferred from one body to another according to the second law of thermodynamics.
• Heat is a measurable quantity and can be treated mathematically.
• Heat cannot be treated as a material substance as it can be transformed into non-material substances, such as mechanical work.
• Heat is one form of energy.
• The use of 'heat' as an abbreviation for the quantity of energy transferred led to confusion.
• In classical thermodynamics, heat refers to thermal energy.
• Enthalpy is recommended for referring to heat content at constant volume.
• Different perspectives on heat include the jiggling motion of atoms and molecules and the logical structure of thermodynamics.
• Calorimetry is used to measure the quantity of heat.

Heat Mentions

Heat Data Sources