In our study into direct current (DC) circuit theory, it is necessary for us to understand some fundamental properties of materials that make up every circuit. The basic unit of every material is called and atom. An atom consists of protons, neutrons and electrons.

Protons are the positive charge components of the atom, so they have a positive electrical charge. Neutrons are neutral with no electrical charge, while Electrons are the negative charges components of the atom, so they have a negative electrical charge.

All components of the atom are bound together by a very powerful force of attraction existing between the positive charges nucleus of the atom and the negative charges of the electrons in its outer shell.

The proton, neutron and electron co-exist together at a stable state when the atom not excited (at its neutral state). When excited or separated, a potential difference (PD) is formed. This excitement occurs when we have a closed circuit thereby forcing the loose to move and drift back to the protons due to their attraction creating a flow of electrons. The consequent flow of electrons is referred to as electric current (I).

As the current move through the material, referred to as the conduction, the force in the conductor trying to restrict or stop the flow of the electrons is referred to as the resistance (R).

As a result of this, all basic electrical or electronic circuits consist of three separate but very much related electrical quantities called: Voltage (V), Current (I) and Resistance (R).

**Electrical Voltage**

Electric Voltage (V) is referred to the potential energy of an electrical supply stored in the form of an electrical charge. The electric voltage is the force that pushes electrons through a conductor in a closed circuit. The more value of the voltage the more the greater the force exerted in pushing the electrons round the circuit. As energy has the ability to do work this potential energy can be described as the work required in joules to move electrons in the form of an electrical current around a circuit from one point or node to another.

The voltage drop (the difference in voltage between any two points of a circuit) is referred to as the potential difference. The potential difference between any two points of a circuit is measured in volts (V). It is also referred to as emf (E) if the potential difference is as a result of an electromotive force. If the voltage source is constant, we have a direct current (DC) voltage while in a case when the voltage source varies with time (periodically) it is referred to as an alternating current (AC) voltage.

Voltage is measured in volts. Volts is defined as the electrical pressure needed to move one ampere of electric current through a resistance of one Ohm. Voltages are generally expressed in Volts with prefixes used to denote sub-multiples of the voltage such as microvolts ( μV = 10^{-6} V ), millivolts ( mV = 10^{-3} V ) or kilovolts ( kV = 10^{3} V ). Voltage can be either positive or negative depending on the direction of flow of electric current.

Batteries or power supplies are mostly used to produce a direct current (DC) voltage source in electronic circuits and systems. While alternating current (AC) voltage sources are available for domestic house and industrial power and lighting as well as power transmission.

**Voltage Symbols**

Voltage in a circuit is always referred to as voltage drop because it is usually measured as the difference between any two points in a circuit. It is important to note that voltage can exist across a circuit without current, but current on the other hand cannot exist without a voltage source and as such any voltage source.

**Electrical Current**

The current flowing in a given circuit often referred to as electric current electrical current (I) is the movement or flow of electrical charge which is measured in Amperes (A). Electric current occurs when a voltage source is connected to a circuit (closed circuit) which leads to the continuous and uniform flow (called a drift) of electrons (the negative particles of an atom) around a circuit that are being “pushed” by the voltage source. Electrical current is generally expressed in Amps with prefixes used to denote micro amps ( μA = 10^{-6}A ) or milliamps ( mA = 10^{-3}A ). It is important to note that electrical current can be either positive in value or negative in value depending upon its direction of flow around the circuit.

**Resistance**

Resistance (R), just as the name suggests is a force attempting to stop the flow of current or more specifically the flow of electric charge within a circuit. The circuit element which does this is referred to as the “Resistor”. Resistance is a circuit element measured in Ohms, Greek symbol (Ω, Omega) with prefixes used to denote Kilo-ohms (kΩ = 10^{3}Ω ) and Mega-ohms ( MΩ = 10^{6}Ω ). It is important to note that resistance in a circuit cannot be negative in value.

**Resistor Symbols**

Resistance can be linear or non-linear in nature, but never negative. Linear resistance obeys Ohm’s Law as the voltage across the resistor is linearly proportional to the current through it. Non-linear resistance does not obey Ohm’s Law but has a voltage drop across it that is proportional to some power of the current.

The reciprocal of resistance is called Conductance, symbol (G) and represents the ability of a conductor or device to conduct electricity. In other words the ease by which current can flow through a conductor. High values of conductance imply a good conductor such as copper while low values of conductance implies a bad conductor such as wood. The standard unit of measurement given for conductance is the Siemen (S) of mho (℧).