In physics, charge, also known as electric charge or electrostatic charge and symbolized q, is a characteristic of a unit of matter that expresses the extent to which it has more or fewer electrons than protons. In atoms, the electron carries a negative elementary or unit charge; the proton carries a positive charge. The two types of charge are equal and opposite. The unit of electrical charge in the International System of Units is the coulomb (symbolized C), where 1 C is equal to approximately 6.24 x 10 to the power of 18 elementary charges.

Current is a flow of electrical charge carriers, usually electrons or electron-deficient atoms. The common symbol for current is the uppercase letter I. The standard unit is the ampere, symbolized by A. One ampere of current represents one coulomb of electrical charge moving past a specific point in one second.

An electric field is the effect produced by the existence of an electric charge, such as an electron, ion, or proton, in the volume of space or medium that surrounds it. The electrical potential energy at a given point is defined to be the negative of the work an external force would have to do to move a charge from the location chosen as the zero reference level to the point. The electrical field and potential is generated by an electrical charge or a varying magnetic field. The vector electric field E(r) determines the electromotive force F = qE(r) and also can be represented by a scalar potential field E(r) = -Grad[(r)]

Voltage is a quantitative expression of the potential difference between two points in an electrical field. The greater the voltage, the greater the flow of electrical current, that is, the quantity of charge carriers that pass a fixed point per unit of time through a conducting or semi-conducting medium for a given resistance to the flow. Voltage is symbolized by an uppercase italic letter V or E. The standard unit is the volt, symbolized by a non-italic uppercase letter V.

It is the voltage or potential difference across that is responsible for an electrical current to flow.

For many conductors of electricity, the electric current which will flow through them is directly proportional to the voltage applied to them. When a microscopic view of Ohm’s law is taken, it is found to depend upon the fact that the drift velocity of charges through the material is proportional to the electric field in the conductor. The ratio of voltage to current is called the resistance, and if the ratio is constant over a wide range of voltages, the material is said to be an “ohmic” material. If the material can be characterized by such a resistance, then the current can be predicted from the relationship.

Kirchhoff’s current law states that the total currents flowing into or out of any node is zero. This rule is a consequence of the fact that the flow of charge is conserved in steady-state flow. There would be a build up of charge at any circuit junction if less charge flowed out of the junction than flowed into the junction.

Kirchhoff’s voltage law states that the sum of voltage drops around a loop is zero. The superposition theorem mentions that the total current in any part of a linear circuit equals the algebraic sum of the currents produced by each source separately.

The current and voltage in any circuit can be either constant or time-dependent. The former is known as direct current (DC), which is the continuous flow of electricity through a conductor such as a wire from high to low potential. In direct current, the electric charges flow always in the same direction, which distinguishes it from alternating current (AC). A form of the latter is alternating current (AC), which basically is an electric current which repeatedly changes polarity from negative to positive and back again. The most commonly used form of alternating current does so in a sine wave pattern. The mean value over time is zero. For a 230V AC, the peak voltage or A is approximately 325V.