An atom is made of protons, neutrons and electrons. When the atom is in a neutral state, the number of negatively charged electrons always equals the number of positively charged protons. Neutrons have no charge. If an atom expends an electron, it becomes positively charged. The letter "e" represents an indivisible single unit of charge -- a Coulomb. Engineers can quantize or subdivide the charge of a particle, as it comes in multiples of a single unit of charge.
The term "current" means the rate of flow of charge through a conductor, with each unit of an electric current known as an ampere. To calculate the current, divide the total charge by time in seconds. In the same way that water always heads downhill to a lower level, a current always flows away from a lower potential to a higher one.
If current represents the effect, voltage serves as the cause or the push that makes the current flow, as the force of electricity. It's a measure of the amount of energy carried by a charge -- the difference between two points. Electronics applications use voltage at a point, as it conveys the difference between a reference point of zero volts and a second point. Although voltage can exist with a current, a current cannot move without voltage.
Energy is the work done when a charge transfers through an element. Energy transfer systems consist of several types of energy including kinetic energy, internal energy, potential energy, heat and work. The three types of units typically employed to measure energy are the British thermal unit (Btu), mechanical unit and the electrical unit, known as the watt-second.
The rate of doing work, known as power, equates to the rate of the transfer of energy, measured in units of energy per unit time. In electrical applications, the units are known as watts (W) or kilowatts (kW).