Voltage, Current and Resistance
Relationship Between Voltage, Current and Resistance
The fundamental relationship between voltage, current and resistance in an electrical circuit is
called Ohm's Law. 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.
But if we separate them from each other they want to reform and start to exert a potential of attraction called a potential difference.
Now if we create a closed circuit these loose electrons will start to move and drift back to the
protons due to their attraction creating a flow of electrons. This flow of electrons is called an electrical current.
The electrons do not flow freely through the circuit as the material they move through creates a restriction to the electron
flow. This restriction is called resistance.
Then all basic electrical or electronic circuits consist of three separate but very much related
electrical quantities called: Voltage, ( v ), Current, ( i )
and Resistance, ( Ω ).
Voltage, ( V ) 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.
Then the difference in voltage between any two points, connections or junctions (called nodes) in a
circuit is known as the Potential Difference, ( p.d. ) sometimes called the 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.
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, ( I ) 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
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, positive to negative. 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. To continue with this line of thought, in all circuit diagrams and schematics,
the arrows shown on symbols for components such as diodes and transistors point in the direction of conventional current
Then Conventional Current Flow gives the flow of electrical current from positive to
negative and which is the opposite in direction to the actual flow of electrons.
The flow of electrons around the circuit is opposite to the direction of the conventional current flow
being negative to positive.
The actual current flowing in an electrical circuit is composed of electrons that flow from the negative pole of the battery
(the cathode) and return back to the positive pole (the anode) of the battery.
This is because the charge on an electron is negative by definition and so is attracted to the positive
terminal. This flow of electrons is called Electron Current Flow. Therefore, electrons actually flow around
a circuit 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 ). Note that electrical current can be either positive in
value or negative in value depending upon its direction of flow.
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 Irms produces the same average power loss equivalent to a DC
current Iaverage . 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
The Resistance, ( R ) of a circuit is its ability
to resist or prevent the flow of current (electron flow) through itself making it necessary to apply a greater voltage to
the electrical 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 ). Note that Resistance cannot be
negative in value 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 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. Remember that resistance is always positive, and never negative.
Resistance can also be classed as an attenuator as it has the ability to change the characteristics of
a circuit by the effect of loading the circuit or by temperature which changes its resistivity.
For very low values of resistance, for example milli-ohms, ( mΩīs )
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 represents the ability of a conductor or device to conduct electricity. In other words the
ease by which current flows. High values of conductance implies 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, 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.
The relationship between Voltage, ( v )
and Current, ( i ) in a circuit of constant
Resistance, ( R ).
Voltage, Current and Resistance Summary
Hopefully by now you should have some idea of how electrical Voltage,
Current and Resistance are closely related together. 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. Then we can see that current flow around a circuit is directly proportional ( ∝ )
to voltage, ( V↑ causes I↑ )
but inversely proportional ( 1/∝ ) to resistance as,
( R↑ causes I↓ ).
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 |
|Voltage||V or E||Volt||V|
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.