Which equation expresses the First Law balance as Energy In = Energy Out + Energy Stored?

Which equation expresses the First Law balance as Energy In = Energy Out + Energy Stored?

• A. Energy In = Energy Out + Energy Stored
• B. Energy In = Energy Out - Energy Stored
• C. Energy In + Energy Out = Energy Stored
• D. Energy In = Energy Stored - Energy Out

Answer: (A)

Energy conservation for a system means the energy that comes in must either go out or be stored—the stored portion is the accumulation of energy in the system. Expressed mathematically, the net energy entering minus the energy leaving equals the rate of energy storage: Ein − Eout = Energy Stored. Rearranging gives Ein = Eout + Energy Stored. This form directly shows that all incoming energy is accounted for by either energy leaving the system or being stored. The other forms mis-sign the storage or misplace how energy flows are accounted for. They would imply storage-energy behaves differently in the balance or that input and output energies combine to equal storage, which contradicts the strict accounting of energy in minus energy out equaling stored energy.

Energy conservation for a system means the energy that comes in must either go out or be stored—the stored portion is the accumulation of energy in the system. Expressed mathematically, the net energy entering minus the energy leaving equals the rate of energy storage: Ein − Eout = Energy Stored. Rearranging gives Ein = Eout + Energy Stored. This form directly shows that all incoming energy is accounted for by either energy leaving the system or being stored.

The other forms mis-sign the storage or misplace how energy flows are accounted for. They would imply storage-energy behaves differently in the balance or that input and output energies combine to equal storage, which contradicts the strict accounting of energy in minus energy out equaling stored energy.