In a closed system, what happens to total energy as energy changes form?

• A. It remains constant; energy is conserved
• B. It increases
• C. It decreases
• D. It disappears

Answer: (A)

In a closed system, total energy stays the same even as it changes form. Energy can move between types—kinetic, potential, thermal, chemical, electrical, light—but the overall amount doesn’t change because energy is conserved. For example, a swinging pendulum converts gravitational potential energy to kinetic energy as it falls, and back to potential energy as it rises; with negligible air resistance, the sum of kinetic and potential energy remains constant. If some energy becomes heat due to friction, that heat is still part of the system’s energy, so the total energy is unchanged. Similarly, a battery can transform chemical energy into light and thermal energy, but the total energy across all forms in the system stays constant.