The electron carries a negative elementary or unit charge while the proton carries a positive charge in an atoms, the basic unit of matter. 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. 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.

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.

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.

Two basic laws by Kirchoff, which are Kirchhoff’s current law and Kirchhoff’s voltage law are used in applied electronics for determining the current and voltage in an electrical circuit.

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.

There are three basic kinds of electrical component in applied electronics, namely the resistor, the capacitor and the inductor.

For a resistor, the electric current which will flow through them is varies with the voltage applied across 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.

It is found that silver is the best electrical conductor at room temperature. At very low temperatures, lead becomes a superconductor where its resistivity becomes 0 ohm. The resistivity of copper is close to silver and the main advantage of using copper is that it is much less expensive. Aluminum’s resistivity is not far behind and it is less expensive than copper.

A capacitor is a two-terminal element in which the current, i, is proportional to the change of voltage with respect to time, dv/dt, across the element, or where C is capacitance and is measured in farads (F). One farad is quite large, so in common capacitor values are 10 pF (10 to the power of 12 F) to 1 MF (10 to the power of 6 F). Capacitors are commonly used in a variety of electric circuits and are a main component of electronic filtering systems in electronics instruments.

An inductor is a two terminal element in which the voltage, v, is proportional to the change of current with respect to time, di/dt, across the element, or where L is the inductance. Its unit is the henry (H). Like the farad, this unit is large so more convenient units such as mH (10 to the power of 3 H) and H (10 to the power of 6 H). This is usually taken to be the defining equation for the inductance of any inductor, regardless of its shape, size, or material characteristics. One should the difference between resistance and inductance: resistance is a measure of opposition to a current whereas inductance is a measure of the opposition to any change in current. The inductance of an element often depends on its geometry.

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 common 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.

With a resistor both the current and the voltage vary as sin(wt), the current is in phase with the voltage, meaning that when the current is at a maximum, the voltage is also at a maximum. For an inductor, the current lags behind the voltage 90. For a capacitor, the current leads the voltage by 90.