Heat is not a thing, but a transfer of energy
In short: We often use the words heat and temperature interchangeably, but in physics, they are distinct concepts. Heat is not something a body possesses; rather, it is the energy that flows between objects when their temperatures differ, moving relentlessly from the hotter body to the colder one.
To understand thermodynamics, one must distinguish between heat and temperature. While temperature serves as a measure of the amount of energy present within a body, heat is the actual energy being transferred. This distinction was clarified during the 18th and 19th centuries, helping scientists move beyond vague notions of warmth toward precise mathematical descriptions of energy movement.
When two bodies of different temperatures interact, energy flows from the hotter object to the colder one. Such a mechanism generally causes the colder object's temperature to rise while the warmer object's temperature falls. However, this is not a universal rule. A substance can absorb heat without any change in temperature if it is undergoing a phase change, such as melting from a solid to a liquid, boiling from a liquid to a vapour, or sublimating from a solid to a vapour.
Because heat is a form of energy, it can be converted into work and is measured in units like joules, calories, or British thermal units (BTUs). For instance, a calorie is the energy needed to raise one gram of water by one degree Celsius (from 14.5 to 15.5 °C) at a constant pressure of one atmosphere. In contrast, a BTU is the energy required to raise one pound of water by one degree Fahrenheit (from 63 to 64 °F). Understanding these transfers is essential for everything from basic survival, such as why wearing a jacket keeps you warm, to advanced technological applications.
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Sources
- Summary by Intellect · source: The Physics of Heat: Crash Course Physics #22
- Also: Heat | Definition & Facts | Britannica
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