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# DC Circuit Theory

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 ).

## Electrical Voltage

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. ) commonly 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 to indicate a generated emf (electromotive force). 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 dc 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“. Note that voltage can exist across a circuit without current, but current cannot exist without voltage and as such 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, ( 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 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, being positive to negative. The diagram at the left shows the movement of the positive charge (holes) around a closed circuit flowing from the positive terminal of the battery, through the circuit and returns to the negative terminal of the battery. This flow of current from positive to negative is generally known as conventional current flow.

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 flow.

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.

### Electron Flow

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).

Electrical current is generally expressed in Amps with prefixes used to denote micro ampsμA = 10-6A ) or milliampsmA = 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 alternating 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. Note that current cannot exist without voltage so 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

Resistance, ( R ) is the capacity of a material to resist or prevent the flow of current or, more specifically, the flow of electric charge within a circuit. The circuit element which does this perfectly is called the “Resistor”.

Resistance is a circuit element measured in Ohms, Greek symbol ( Ω, Omega ) with prefixes used to denote Kilo-ohmskΩ = 103Ω ) and Mega-ohmsMΩ = 106Ω ). Note that resistance cannot be negative in value only positive.

### Resistor Symbols

The amount of resistance a resistor has is determined by the relationship of the current through it to the voltage across it which determines whether the circuit element 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. Hence the name “semi-conductor”. Semiconductors are used to make Diodes and Transistors etc.

Resistance can be linear or non-linear in nature. 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.

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.

A resistor is classed as a passive circuit element and as such cannot deliver power or store energy. Instead resistors absorbed power that appears as heat and light. Power in a resistance is always positive regardless of voltage polarity and current direction.

For very low values of resistance, for example milli-ohms, ( mΩ´s ) it is sometimes much 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).

The unit used for conductance is mho (ohm spelled backward), which is symbolized by an inverted Ohm sign . Power can also be expressed using conductance as: p = i2/G = v2G.

The relationship between Voltage, ( v ) and Current, ( i ) in a circuit of constant Resistance, ( R ) would produce a straight line i-v relationship with slope equal to the value of the resistance as shown.

## 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. In a linear circuit of fixed resistance, if we increase the voltage, the current goes up, and similarly, if we decrease the voltage, the current goes down. This means that if the voltage is high the current is high, and if the voltage is low the current is low.

Likewise, if we increase the resistance, the current goes down for a given voltage and if we decrease the resistance the current goes up. Which means that if resistance is high current is low and if resistance is low current is high.

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 DC 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 “.
• Current is Directly Proportional to Voltage ( I ∝ V )
• The effective (rms) value of an alternating current has the same average power loss equivalent to a direct 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.
• Current is Inversely Proportional to Resistance ( I 1/∝ R )
• 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 Ampere A Resistance R Ohms Ω

In the next tutorial about DC Circuits 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: V = I x R.

• i
imran mansuri

What is the relation between resistance and time…??

• Wayne Storr

Unlike a capacitor or inductor, the rate of change of either voltage or current applied to a resistor, having a resistance of x-ohm’s, is virtually instant and not time-varying, that is the jump from one constant voltage or current level to another occurs in zero time. Then there is no relationship between resistance and time in a purely resistive circuit.

• L
Lina elkassify

I have a question in ohms law what happen to a reading of ammeter and voltmeter when the resistance of reostate increases ?i am a little bit confused about it .

• R
Rubiya

Thanks for giving us such a good guidance.keep providing guidance in this way..

• a
abdelazeem Hassan Shehata

thanks a lot for your discussion, but, i want to ask the relation between V&I from ohm law V=RI is directly proportional but from P=V*I the relation between V&I inversely proportional you can help me for known the difference,please

• Wayne Storr

Ohms Law, V = IxR, or R = V/I, or I = V/R

Power, P = VxI, or I^2xR, or V^2/R

• r
rukma

why haven’t you explain power in detail

• d
darshan singh gusain

this is a good thorey

• A
Andrew

Got a question. I am an electrician and we are taught that when volts go up, amps go down. So I’m confused on why it says on here that when volts go up, current goes up? If current is amps, then this is contradictory to me. I keep thinking that I’m just missing something easy here but I ain’t getting it.

• Wayne Storr

Hello Andrew, Ohms Law states that voltage is proportional to current time resistance, V = IxR. So for a fixed value resistance, R, if we increase the voltage V, of the circuit, the current flowing through it will also increase.

• R
Rod

Hi Andrew, you are probably thinking of power transfer and in particular transformers. So for example if you had a plug in power pack and it had an output of 12V at 1A the input would be 240V at 0.05A (ideal with no losses). What you are maintaining is the power P=V x I so 12V x 1A = 240V x 0.05A = 1W. On a larger scale the national grid operates at 450000V to transmit power around the country then uses transformers to step the voltage down to 11000V and then (450V 3 phase) 230V so when you take say 10A at 230V to boil your kettle the 450000V power lines will only see an increase of 0.005A. (Simplified) 🙂

• B
BasiX

no when the voltage goes the the current goes up becuase the more current there is the faster its used due to (light bulb shortage) for an example the higher the voltage the higher the risk of shoting the bulb right….? so with a voltage ov 1.5 the amps will be 1.5 in a 1 light bulb circuit………… and amps will go down but the current moved faster, in short the website is right……

• g
glennboie

hi andrew, are you into a topic about transformer? if it is then it could be the reason why the one who taught you said if voltage go up the current go down. Transformer operates in that manner with response to a specific voltage and specific power output in its secondary. so for example you have a 500 watts transformer the output in terms of voltage can be increased but the current is somehow sacrifice during its transfer… the reason for it is you cannot transfer some power on the secondary which does not exist in its primary..,… check about this topic…

• R
Richard Comerford

For power to be constant, if volts go up, current must go down. So for 60 W (P=VI) the voltage can be 30 V and the current 2 A, or 60 V and 1 A. But since V=RI, what must change is resistance ; in the first case the resistance is 15 ohms, in the second, 60 ohms.

However, if the resistance is fixed at 15 ohms, and the voltage is increased to 60 V, the current will also increase , to 4 A.

• R
Rwagasana Gerard

In the equation U = RI, if I increases (R being constant) U increases also and vice versa

• f
faisal osman dafala

iam interistig in electric field thanks

• G
Greg

Can anyone tell me what the likely methods used are (how is it done) in a tennis racket-looking mosquito zapper? It is chargeable (220 volts AC), and once fully charged, the output is 1500 volts DC (need to press a button on the handle to turn it “on” … then ready to swat at mosquitoes).

Thanks for replies.

• Wayne Storr

These bug zappers, like stun guns, use transformers or capacitor-diode multiplier circuits to generate the pulsed high voltage, which can be as high as 3 to 4000 volts, to kill any bug that comes in contact with the mesh grill. Rechargeable batteries allow it to be used unconnected and a simple oscillator circuit provides the high frequency pulsed drive for the output voltage circuit. Remember though, bug zappers are not a toy, do not do anything stupid with them to other people.

• G
Greg

Zapping people for fun. Great idea … thanks for bringing this possibility to my attention 🙂

• A

I amused , it’s a great website demonstrates each electrical word clearly and gives smart examples.please keep it up!
Next to that i’ve two questions that isn’t clear to me on this lesson.
1.you said that ‘current cann’t exist with out voltage ‘,does it not contradict to current source ?
2.what is the main distinction point between voltage and voltage source?
Thanks….

• Wayne Storr

Current flows around a closed circuit as a result of an electric voltage being applied across it. If no voltage is present no current can flow.
Current sources are devices capable of producing and maintaining a constant flow of current regardless of changes in voltage at its terminals.
Voltage is defined as the potential difference between two point in an electrical circuit.
A voltage source supplies a constant electrical voltage in the form of a direct voltage (DC) or an alternating voltage (AC) regardless of the current flowing through it.

• Uncle Milty

Great tutorial, Wayne. I don’t mean to be nit-picky, but I think it is important to clarify to the general reader that Constant Current & Voltage Sources are theoretical only and do not exist in the real world. However, they are used in modeling real world sources for purposes of circuit analysis. For example, in DC circuits a battery would be modeled as a Constant Voltage Source connected in series with a resistor representing the internal resistance of the battery. The voltage drop may be small or negligible at low currents, but becomes significant at higher currents. A good example of this would be the starter circuit of a typical auto. The nominal 12 volts available at the battery might drop to 10.5 volts or lower during engine startup because the high starter current required has to flow first through the battery’s internal resistance and then through the wiring to the starter. This internal resistance can be calculated using Ohm’s Law, R=E/I. Let’s say the starting current is 100 amps. The internal resistance would then be 1.5v/100a=0.015 ohms (or 15 milliohms). 🙂

• Wayne Storr

An ideal voltage source is one that is capable of supplying the same voltage potential to a circuit at every instant of time regardless of the changes in current. As the current is soley dependent on the circuit elements this makes the voltage source an independant source. Independant voltage sources are used in circuit analysis to model electrical sources such as batteries, alternators, dynamos, etc. Depending on the actual direction of the current, the voltage source can either provide power or absorb power, and it could theoretically deliver an infinite amount of power.

A constant voltage source such as a car battery, will have a 12V terminal voltage that remains essentially constant as long as the current through it does not exceed a few amperes, any higher and the battery may become a dependent source as the voltage value then depends on the value of a voltage (or current) elsewhere in the circuit. Normally we can not specify the value of a dependent source unless we know the value of the voltage on which it depends.

Similarly, an independent current source will maintain a constant current through it while the voltage across it will be set by the rest of the circuit.

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