Heat capacity

Heat capacity, also known as thermal capacity, is a fundamental physical property of matter. It measures how much heat energy must be supplied to an object to raise its temperature by one unit. This property quantifies a material's ability to store thermal energy. Because it depends on the total amount of matter present, heat capacity is classified as an extensive property. This means a larger object made of the same material will have a higher heat capacity than a smaller one.

Scientists use several different terms to describe related concepts. Specific heat capacity is an intensive property, meaning it does not change based on the amount of substance. You find it by dividing the total heat capacity by the mass of the object. Molar heat capacity is found by dividing the heat capacity by the number of moles. There is also volumetric heat capacity, which measures heat capacity per unit of volume. In fields like architecture, the heat capacity of a building is often called its thermal mass.

To understand how heat capacity works, we must look at the thermodynamic processes involved. When heat is added at a constant pressure, it is called an isobaric process. In this state, the heat contributes to both the internal energy and the work done by the system. When heat is added at a constant volume, it is called an isochoric process. In an isochoric process, no expansion work is done, so the heat only changes the internal energy. Because of this difference, the isochoric heat capacity is always less than the isobaric heat capacity.

Heat capacity is not a fixed number for every substance; it changes based on environment. The value typically varies depending on the starting temperature and the pressure applied to the object. It also changes dramatically during phase transitions, such as when a solid melts into a liquid or a liquid turns into a gas. During these transitions, the heat capacity is considered infinite. This is because the heat is being used to change the state of the material rather than raising its temperature. For example, a block of iron weighing one pound has a heat capacity of about 204 J/K at 25 °C and 1 atm of pressure. This value remains reliable for temperatures between 15 °C and 35 °C.

Measuring heat capacity involves a specific procedure. A scientist starts with an object at a known, uniform temperature. They then add a known amount of heat energy to the object. They must wait until the temperature becomes uniform throughout the entire object. Finally, they measure the resulting change in temperature. This method works well for many solids. However, it is much harder to get very precise measurements for gases.

Different professional fields use different units to measure heat. The International System of Units (SI) uses joules per kelvin (J/K). In the United States, engineers often use English Engineering units. These include the British thermal unit (BTU) and the degree Fahrenheit or Rankine. Chemists frequently use calories. A small calorie (cal) is exactly 4.184 joules. A grand calorie, or kilocalorie (kcal), is 1,000 small calories, or 4,184 joules. These units allow specialists to communicate clearly within their specific disciplines.

While most systems have a positive heat capacity, some extraordinary cases show a negative heat capacity. This occurs in certain inhomogeneous systems that are not in thermodynamic equilibrium. For instance, stars and galaxies are self-gravitating bodies. According to the virial theorem, their potential energy and kinetic energy are linked. If a star loses energy by radiating it into space, its average kinetic energy actually increases. This makes the star hotter even as it loses energy. Black holes also demonstrate this behavior. As a black hole absorbs more mass and energy, it actually becomes colder.