What is used to measure both internal energy and heat?

The British thermal unit (Btu) is a measurement that quantifies the amount of heat energy needed to raise the temperature of one pound of water by one degree Fahrenheit at a constant pressure. It is commonly utilized in heating and cooling systems to give an understanding of energy requirements.

While it specifically measures heat, its relation to internal energy comes from the fact that internal energy is affected by heat transfer. When heat is added to a system, the internal energy increases, and when heat is removed, the internal energy decreases. In thermodynamics, heat and internal energy are interrelated concepts, and Btu serves as a practical unit that reflects this relationship.

The other options, such as calorie and joule, are also units of energy but serve specific contexts or follow different systems of measurement (the calorie is often used in nutrition and chemistry, and the joule is a standard SI unit for energy). Kelvin, on the other hand, is a unit of temperature and does not measure energy or heat directly at all. Therefore, Btu stands out as the unit that relates both heat transfer and internal energy changes effectively in practical applications.