Which statement best summarizes the First Law of Thermodynamics as described?

Which statement best summarizes the First Law of Thermodynamics as described?

• A. Pressure-volume work is always zero.
• B. Energy is created when work is done.
• C. Energy cannot be created or destroyed; it can be transformed.
• D. Heat spontaneously appears from nothing.

Answer: (C)

The First Law is about energy conservation: energy cannot be created or destroyed, only transformed from one form to another. In thermodynamics, the change in a system’s internal energy equals the heat added to the system minus the work the system does on its surroundings. This keeps total energy constant: any energy you put in as work is transferred into the system (as internal energy or heat), and any energy leaving as heat or work must come from somewhere. For example, if a gas expands and does work on a piston, the system’s internal energy drops unless heat flows in to compensate. Conversely, compressing the gas increases the energy of the system because you’re doing work on it. The other statements misstate energy accounting: energy isn’t created by performing work, PV work isn’t always zero, and heat cannot spontaneously appear from nothing.

The First Law is about energy conservation: energy cannot be created or destroyed, only transformed from one form to another. In thermodynamics, the change in a system’s internal energy equals the heat added to the system minus the work the system does on its surroundings. This keeps total energy constant: any energy you put in as work is transferred into the system (as internal energy or heat), and any energy leaving as heat or work must come from somewhere. For example, if a gas expands and does work on a piston, the system’s internal energy drops unless heat flows in to compensate. Conversely, compressing the gas increases the energy of the system because you’re doing work on it. The other statements misstate energy accounting: energy isn’t created by performing work, PV work isn’t always zero, and heat cannot spontaneously appear from nothing.