**Electrical energy** is the energy derived from electric potential energy or kinetic energy. A typical **nuclear power plant** has an electric-generating capacity of **1000 MWe**. It produces 1 000 000 000 joules of electrical energy per second. The heat source in the nuclear power plant is a **nuclear reactor**. As is typical in all conventional thermal power stations, the heat is used to generate steam which drives a **steam turbine** connected to a generator that produces electricity. The turbines are heat engines subject to the efficiency limitations imposed by the **second law of thermodynamics**. In modern nuclear power plants, the overall thermodynamic efficiency is about **one-third **(33%), so **3000 MWth** of thermal power from the fission reaction is needed to generate **1000 MWe** of electrical power.

Since **voltage** is **electric potential energy** per unit charge, **Kirchhoff’s voltage law** can be seen to be a consequence of the **conservation of electrical energy**. **Kirchhoff’s voltage law states:**

*The algebraic sum of the voltages (drops or rises) encountered in traversing any loop of a circuit in a specified direction must be zero.*

The algebraic sum of the voltages (drops or rises) encountered in traversing any loop of a circuit in a specified direction must be zero.

The voltage changes around any closed loop must sum to zero. The sum of the **voltage rises** is equal to the sum of the** voltage drops** in a loop. No matter what path you take through an electric circuit, if you return to your starting point, you must measure the same voltage, constraining the net change around the loop to be zero. This rule is equivalent to saying that each point on a mountain has only one elevation above sea level. If you start from any point and return to it after walking around the mountain, the algebraic sum of the elevation changes that you encounter must be zero.

## Energy Units

**Energy** is generally defined as the potential to do work or produce heat. This definition causes the SI unit for energy to be the same as the unit of work – the** joule (J)**. Joule is a derived unit of energy, and it is named in honor of **James Prescott Joule** and his experiments on the mechanical equivalent of heat. In more fundamental terms, 1 joule is equal to:

**1 J = 1 kg.m ^{2}/s^{2}**

Since energy is a fundamental physical quantity used in various branches of physics and engineering, there are many units in physics and engineering. These units are summarized in following points: