Which statement captures the essence of the First Law as described?

Which statement captures the essence of the First Law as described?

• A. Energy cannot be created or destroyed; it can be transformed.
• B. Energy spontaneously reappears in different forms.
• C. Heat is the only form of energy.
• D. Work cannot change the internal energy of a system.

Answer: (A)

The main idea being tested is that energy is conserved and can only change form, not appear or disappear. In thermodynamics, the First Law ties the change in a system’s internal energy to heat added and work done: ΔU = q + w. That means adding heat to a system increases its internal energy, and doing work on the system (like compressing a gas) also raises its internal energy. Conversely, letting heat out or letting the system do work on its surroundings decreases internal energy. This is why the statement about energy not being created or destroyed, only transformed, is the best description of the First Law. It captures the idea that energy persists and can shift between forms (thermal, kinetic, potential, chemical, etc.) but overall total energy remains constant in an isolated system. The other ideas conflict with this: energy doesn’t spontaneously reappear in new forms, heat isn’t the only form of energy, and work can indeed change internal energy (compressing a gas raises its internal energy).

The main idea being tested is that energy is conserved and can only change form, not appear or disappear. In thermodynamics, the First Law ties the change in a system’s internal energy to heat added and work done: ΔU = q + w. That means adding heat to a system increases its internal energy, and doing work on the system (like compressing a gas) also raises its internal energy. Conversely, letting heat out or letting the system do work on its surroundings decreases internal energy.

This is why the statement about energy not being created or destroyed, only transformed, is the best description of the First Law. It captures the idea that energy persists and can shift between forms (thermal, kinetic, potential, chemical, etc.) but overall total energy remains constant in an isolated system.

The other ideas conflict with this: energy doesn’t spontaneously reappear in new forms, heat isn’t the only form of energy, and work can indeed change internal energy (compressing a gas raises its internal energy).