The calorie is a unit of heat energy, commonly used to measure the energy content of food and the heat output of appliances.

The joule is the SI unit of work and energy, representing the amount of work done when a force of one newton is applied through a distance of one meter.

Heat transfer is the movement of thermal energy between objects or systems at different temperatures, occurring through conduction, convection, or radiation.

Conduction is the transfer of heat through direct contact between objects, as in the case of a metal spoon in hot coffee.

Convection is the transfer of heat through the movement of a fluid, such as air or water, as warm fluid rises and cooler fluid sinks.

Radiation is the transfer of heat through electromagnetic waves, as in the case of the sun's rays warming the Earth.

Specific heat is the amount of heat required to raise the temperature of one gram of a substance by one degree Celsius.

Water has the highest specific heat capacity among common substances, meaning it takes more heat to raise its temperature by one degree Celsius compared to other substances.

In a closed system, the first law of thermodynamics states that the change in internal energy is equal to the heat added to the system minus the work done by the system.

A heat engine is a device that converts heat into mechanical work, as in the case of a steam engine.

The efficiency of a heat engine is calculated as the ratio of work output to heat input, multiplied by 100%.

A reversible heat engine is one that can operate in both directions, absorbing heat and performing work or vice versa, as in the case of a refrigerator.

The Carnot efficiency of a heat engine is the maximum possible efficiency that can be achieved by a heat engine operating between two reservoirs at temperatures T_H and T_C.

A thermodynamic system is a collection of matter that is being studied, such as a closed container of gas.

The system boundary is the imaginary surface that separates a thermodynamic system from its surroundings.