Voltage, Current and Resistance

Relationship

Any basic electrical or electronic circuit consists of three separate but very much related quantities, Voltage, ( V ), Current, ( I ) and Resistance, ( Ω ).

Voltage.

Voltage is the potential energy of an electrical supply stored in the form of an electrical charge, and the greater the voltage the greater is its ability to produce an electrical current flowing through a given circuit. As energy has the ability to do work this potential energy can be described as the work required in joules to move the electrical current around a circuit from one point or node to another. The difference in voltage between any two nodes in a circuit is known as the Potential Difference, p.d. or sometimes called Electromotive Force, (EMF) and is measured in Volts with the circuit symbol V, or lowercase "v", although Energy, E lowercase "e" is sometimes used.

A constant voltage source is called a DC Voltage with a voltage that varies periodically with time is called an AC voltage. Voltage is measured in volts, and one volt can be defined as the electrical pressure required to force an electrical current of one ampere 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 = 10³ V ). Voltage can be either positive or negative.

Batteries or power supplies are mostly used to produce a steady D.C. (direct current) voltage source such as 5v, 12v, 24v etc in electronic circuits and systems. While A.C. (alternating current) voltage sources are available for domestic house and industrial power and lighting as well as power transmission. The mains voltage supply in the United Kingdom is currently 230 volts a.c. and 110 volts a.c. in the USA with general electronic circuits operating on a voltage supply of between 1.5V and 24V d.c. The circuit symbol for a constant voltage source usually given as a battery symbol with a positive, + and negative, - sign indicating the direction of the polarity. The circuit symbol for an alternating voltage source is a circle with a sine wave inside.

A simple relationship can be made between a tank of water and a voltage supply. The higher the water tank above the outlet the greater the pressure of the water as more energy is released, the higher the voltage the greater the potential energy as more electrons are released. Voltage is always measured as the difference between any two points in a circuit and the voltage between these two points is generally referred to as the "Voltage drop". Any voltage source whether DC or AC likes an open or semi-open circuit condition but hates any short circuit condition as this can destroy it.

Electric Current.

Electrical Current is the movement of electrical charge and is measured in Amperes, symbol I, for Intensity). It is the continuous and uniform flow of electrons (negative particles of an atom) around a circuit that are being "pushed" by the voltage source. In reality, electrons flow from the negative (-ve) terminal to the positive (+ve) terminal of the supply and for ease of circuit understanding conventional current flow assumes that the current flows from the positive to the negative terminal. Generally in circuit diagrams the flow of current through the circuit usually has an arrow associated with the symbol, I, or lowercase I to indicate the actual direction of the current flow. However, this arrow usually indicates the direction of conventional current flow and not necessarily the direction of the actual flow.

In electronic circuits, a current source is a circuit element that provides a specified amount of current for example, 1A, 5A 10Amps etc, with the circuit symbol for a constant current source given as a circle with an arrow inside indicating its direction. Current is measured in Amps and an amp or ampere is defined as the number of electrons or charge (Q in Coulombs) passing a certain point in the circuit in one second, (t in Seconds). Current is generally expressed in Amps with prefixes used to denote micro amps (μA = 10^-6A) or milli amps (mA = 10^-3A). Electrical current can be either positive or negative.

Current that flows in a single direction is called Direct Current, or D.C. and current that alternates back and forth through the circuit is known as Alternating Current, or A.C.. Whether AC or DC current only flows through a circuit when a voltage source is connected to it with its "flow" being limited to both the resistance of the circuit and the voltage source pushing it. Also, as AC currents (and voltages) are periodic and vary with time the "effective" or "RMS", (Root Mean Squared) value given as I_rms produces the same average power loss equivalent to a DC current I_average . Current sources are the opposite to voltage sources in that they like short or closed circuit conditions but hate open circuit conditions as no current will flow.

Using the tank of water relationship, current is the equivalent of the flow of water through the pipe with the flow being the same throughout the pipe. The faster the flow of water the greater the current. Any current source whether DC or AC likes a short or semi-short circuit condition but hates any open circuit condition as this prevents it from flowing.

Resistance.

The Resistance of a circuit is its ability to resist or prevent the flow of current (electron flow) through it making it necessary to apply a bigger voltage to the circuit to cause the current to flow again. Resistance is measured in Ohms, Greek symbol (Ω, Omega) with prefixes used to denote Kilo-ohms (kΩ = 10³Ω) and Mega-ohms (MΩ = 10⁶Ω). Resistance cannot be negative only positive.

The amount of resistance determines whether the circuit is a "good conductor" - low resistance, or a "bad conductor" - high resistance. Low resistance, for example 1Ω or less implies that the circuit is a good conductor made from materials such as copper, aluminium or carbon while a high resistance, 1MΩ or more implies the circuit is a bad conductor made from insulating materials such as glass, porcelain or plastic. A "semiconductor" on the other hand is a material whose resistance is half way between that of a good conductor and a good insulator such as silicon and germanium and is used to make Diodes and Transistors etc.

Resistance in a circuit prevents short circuits (unless its very low) by limiting and controlling the amount of current flowing in a circuit by the voltage supply connected to it and therefore the transfer of power from source to load. Resistance is not affected by frequency and the AC impedance of a pure resistance is equal to its DC resistance. Resistance also has the ability to change the characteristics of a circuit by the effect of load resistance or by temperature which changes its resistivity.

For very low values of resistance, for example milli-ohms, (mΩ´s) it is sometimes more easier to use the reciprocal of resistance (1/R) rather than resistance (R) itself. The reciprocal of resistance is called Conductance, symbol (G) and it is the ability of a conductor or device to conduct electricity with high values of conductance implying a good conductor and low values of conductance implying a bad conductor. The unit of conductance is the Siemen, symbol (S).

Again, using the water relationship, resistance is the diameter or the length of the pipe the water flows through. The smaller the diameter of the pipe the larger the resistance to the flow of water, and therefore the larger the resistance.

Relationship between Voltage and Current in a circuit of constant resistance.

Summary

This relationship between Voltage, Current and Resistance forms the basis of Ohms Law which is discussed in the next tutorial and a basic summary of the three units is given below.

• Voltage or potential difference is the measure of potential energy between two points in a circuit and is commonly referred to as its "volt drop".
• When a voltage source is connected to a closed loop circuit the voltage will produce a current flowing around the circuit.
• In D.C. voltage sources the symbols +ve (positive) and -ve (negative) are used to denote the polarity of the voltage supply.
• Voltage is measured in "Volts" and has the symbol "V" for voltage or "E" for energy.
• Current flow is a combination of electron flow and hole flow through a circuit.
• Current is the continuous and uniform flow of charge around the circuit and is measured in "Amperes" or "Amps" and has the symbol "I".
• The effective (rms) value of an AC current has the same average power loss equivalent to a DC current flowing through a resistive element.
• Resistance is the opposition to current flowing around a circuit.
• Low values of resistance implies a conductor and high values of resistance implies an insulator.
• Resistance is measured in "Ohms" and has the Greek symbol "Ω" or the letter "R".