Voltage, Current and Resistance definations

Relationship

All materials are made up from atoms, and all atoms consist of protons, neutrons and electrons. Protons, have a positive electrical charge. Neutrons have no electrical charge while Electrons, have a negative electrical charge. Atoms are bound together by powerful forces of attraction existing between the atoms nucleus and the electrons in its outer shell. When these protons, neutrons and electrons are together within the atom they are happy and stable. However, if we separate them they exert a potential of attraction called a potential difference. If we create a circuit or conductor for the electrons to drift back to the protons the flow of electrons is called a current. The electrons do not flow freely through the circuit, the restriction to this flow is called resistance. Then all 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. Voltage can be thought of as the force that pushes electrons through a conductor and the greater the voltage the greater is its ability to "push" the electrons 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 electrons in the form of an 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. sometimes called Voltage Drop.

The Potential difference between two points is measured in Volts with the circuit symbol V, or lowercase "v", although Energy, E lowercase "e" is sometimes used. Then the greater the voltage, the greater is the pressure (or pushing force) and the greater is the capacity to do work.

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, with one volt being 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. General electronic circuits operate on low voltage DC battery supplies 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.

Voltage Symbols

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.

Electrical Current

Electrical Current is the movement or flow of electrical charge and is measured in Amperes, symbol i, for intensity). It is 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. 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.

Conventional Current Flow

Conventionally this is the flow of positive charge around a circuit. The diagram at the left shows the movement of the positive charge (holes) which flows from the positive terminal of the battery, through the circuit and returns to the negative terminal of the battery. This was the convention chosen during the discovery of electricity in which the direction of electric current was thought to flow in a circuit. In circuit diagrams, the arrows shown on symbols for components such as diodes and transistors point in the direction of conventional current flow. Conventional Current Flow is the opposite in direction to the flow of electrons.

Electron Flow

The flow of electrons around the circuit is opposite to the direction of the conventional current flow. The current flowing in a circuit is composed of electrons that flow from the negative pole of the battery (the cathode) and return to the positive pole (the anode). This is because the charge on an electron is negative by definition and so is attracted to the positive terminal. The flow of electrons is called Electron Current Flow. Therefore, electrons flow from the negative terminal to the positive.

Both conventional current flow and electron flow are used by many textbooks. In fact, it makes no difference which way the current is flowing around the circuit as long as the direction is used consistently. The direction of current flow does not affect what the current does within the circuit. Generally it is much easier to understand the conventional current flow - positive to negative.

In electronic circuits, a current source is a circuit element that provides a specified amount of current for example, 1A, 5A 10 Amps 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.

Resistor Symbols

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 such as silicon or germanium, is a material whose resistance is half way between that of a good conductor and a good insulator. Semiconductors are used to make Diodes and Transistors etc.

Resistance can be linear in nature or non-linear in nature. Linear resistance obeys Ohm's Law and controls or limits the amount of current flowing within a circuit in proportion to the voltage supply connected to it and therefore the transfer of power to the load. 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. Resistance is pure and is not affected by frequency with the AC impedance of a resistance being equal to its DC resistance and as a result can not be negative. resistance is always positive. Also, resistance is an attenuator which 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

Hopefully by know you have an idea of how voltage, current and resistance are related. The relationship between Voltage, Current and Resistance forms the basis of Ohm's law which in a linear circuit states that if we increase the voltage, the current goes up and if we increase the resistance, the current goes down. 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".

Quantity	Symbol	Unit of Measure	Abbreviation
Voltage	V or E	Volt	V
Current	I	Amp	A
Resistance	R	Ohms	Ω

In the next tutorial about DC Theory we will look at Ohms Law which is a mathematical equation explaining the relationship between Voltage, Current, and Resistance within electrical circuits and is the foundation of electronics and electrical engineering. Ohm's Law is defined as: E = I x R

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