Kamis, 27 Desember 2012

rangkaian seri


Rangkaian Seri adalah rangkaian yang cara kerjannya membagi arus yang di hasilkan dari komponen lain. Apabila ada tiga buah komponen yang dirangkai secara seri maka tegangan dari satu komponen memberikan sisa tegangan yang dihasilkan sehingga proses ini dapat mengurangi pemakaian tegangan sehingga 3 komponen hanya terhitung pada 1 komponen saja. Rangkaian Serisering juga di gambarkan dalam posisi diagonal, gunanya adalah agar dapat memahami dalam pembacaan pada gambar.
Kelebihan dari pemakaian susunan seri adalah lebih banyak menghemat daya yang dikeluarkan pada baterai. Kelebihan lainnya yang di miliki susunan seri terdapat pada pengerjaan yang singkat, serta tidak memerlukan banya penghubung pada penyambungan jalur.
Selain memiliki kelebihan, rangkaian ini juga memiliki kelemahan, adapun kelemahan pada rangkaian ini adalah karena menurunkan fungsi dari komponen itu sendiri karena mendapat tegangan yang kurang. Sebab, harus berbagi dengan komponen lain.
Selama ini kita banyak mengenal dua macam susunan dasar dari rangkaian listrik, yaitu susunan atau rangkaian seri dan rangkaian paralel. Jika ada k buah tahanan (resistor) dengan nilai seragam sebesar R, cara penyusuan k buah resistor ini akan mempengaruhi nilai tahanan total yang kita peroleh.
Rangkaian SeriRumus dari rangkaian di atas adalah : R_mathrm{total} = R_1 + R_2 + cdots + R_n
Jumlah hambatan total rangkaian seri sama dengan jumlah hambatan tiap- tiap komponen (resistor). Sedangkan jika tahanan tersebut disusun secara paralel, maka nilai total di kedua ujungrangkaian akan menjadi frac{1}{R_mathrm{total}} = frac{1}{R_1} + frac{1}{R_2} + cdots + frac{1}{R_n}.
Rangkaian Seri resistor dapat di hubungkan bersama dalam rangkaian “seri” maupun parallel, atau kombinasi keduanya (seri-paralel). Namun apapun bentuk rangkaiannya semua mengikuti Hukum Ohm dan Hukum Kirchoff. Resistors dapat dikatakan dalam seri apabila mereka terangkai dalam satu garis lurus, sehingga Arus yang mengalir tidak ada jalan lain kecuali mengalir dari Resistor satu ke resistor kedua dan seterusnya.
Satu hal penting yang harus diingat tentang rangkaian resistor secara seri, resistansi total (RT) dari dua atau lebih resistor yang dihubungkan bersama-sama secara seri akan selalu lebih besar daripada nilai resistor terbesar dalam rantai dan dalam contoh di atas RT = 9kΩ adalah nilai yang lebih besar dari nilai resistor terbesar ( 6kΩ).
Demikian penjelasan tentang Rangkaian Seri, Baca juga artikel kami lainnya tentang Rangkaian ListrikRangkaian ParalelRangkaian RLCRangkaian Flip Flop dan Rangkaian LED.
Standar Kompetensi:
Memahami konsep kelistrikan dan penerapannya dalam kehidupan sehari-hari.
Kompetensi Dasar:
Menyelidiki Rangkaian Seri dan Paralel

Rangkaian lampu seperti gambar di bawah ini disebut rangkaian seri. Karena bagian-bagian dari suatu rangkaian seri disambung satu setelah yang lain, besarnya arus yang mengalir sama untuk seluruh bagian rangkaian. Apabila kamu menghubungkan tiga amperemeter ke dalam rangkaian tersebut seperti ditunjukkan pada gambar itu, ketiga amperemeter itu akan menunjukkan harga yang sama.










Apa yang terjadi jika salah satu bagian rangkaian seri terputus? Dalam rangkaian seri arus listrik hanya mempunyai satu jalan yang dapat dilewati. Karena itu apabila ada bagian yang terputus, berarti rangkaian dalam keadaan terbuka dan arus pasti tidak mengalir. Apakah hal ini sesuai dengan hasil pengamatanmu?

Pada di atas, dalam rangkaian seri, besar tegangan sumber, Vsumber, adalah sama dengan jumlah tegangan pada lampu A dan B,

Vsumber = VA + VB

Karena arus I yang melalui lampu-lampu tersebut sama besar, maka

VA = IRA dan VB = IRB

Oleh karena itu,

Vsumber = IRA + IRB atau

Vsumber = I(RA + RB)

Arus yang mengalir melalui rangkaian tersebut dapat dihitung dengan rumus berikut ini.

Persamaan ini berlaku untuk setiap jumlah hambatan seri, tidak hanya dua seperti gambar di atas. Arus yang sama akan tetap mengalir bila satu resistor tunggal, R, mempunyai hambatan yang sama dengan jumlah hambatan dua lampu tersebut. Hambatan seperti itu disebut hambatan ekivalen rangkaian atau sirkuit tersebut. Untuk hambatan seri, hambatan ekivalen sama dengan jumlah seluruh hambatan yang dihubungkan seri.

R = RA + RB

untuk dua hambatan yang dihubungkan seri, dan

R = RA + RB + RC

untuk tiga hambatan yang dihubungkan seri, dan seterusnya.
Perhatikan bahwa hambatan ekivalen selalu lebih besar daripada setiap hambatan tunggal yang dihubungkan seri tersebut. Oleh karena itu, jika tegangan baterai tidak
berubah, penambahan lebih banyak alat secara seri selalu menurunkan arus tersebut. Untuk menghitung arus, I, yang mengalir dalam suatu rangkaian seri, pertama-tama
hitunglah hambatan ekivalen, R, dan kemudian gunakan persamaan berikut ini untuk menghitung I.



mesh


Circuit Analysis

In the previous tutorial we saw that complex circuits such as bridge or T-networks can be solved usingKirchoff's Circuit Laws. While Kirchoff´s Laws give us the basic method for analysing any complex electrical circuit, there are different ways of improving upon this method by using Mesh Current Analysis or Nodal Voltage Analysis that results in a lessening of the math's involved and when large networks are involved this reduction in maths can be a big advantage.
For example, consider the circuit from the previous section.

Mesh Analysis Circuit

Mesh Analysis Circuit
One simple method of reducing the amount of math's involved is to analyse the circuit using Kirchoff's Current Law equations to determine the currents, I1 and I2 flowing in the two resistors. Then there is no need to calculate the current I3 as its just the sum of I1 and I2. So Kirchoff's second voltage law simply becomes:
  • Equation No 1 :    10 =  50I1 + 40I2
  • Equation No 2 :    20 =  40I1 + 60I2
therefore, one line of math's calculation have been saved.

Mesh Current Analysis

A more easier method of solving the above circuit is by using Mesh Current Analysis or Loop Analysiswhich is also sometimes called Maxwell´s Circulating Currents method. Instead of labelling the branch currents we need to label each "closed loop" with a circulating current. As a general rule of thumb, only label inside loops in a clockwise direction with circulating currents as the aim is to cover all the elements of the circuit at least once. Any required branch current may be found from the appropriate loop or mesh currents as before using Kirchoff´s method.
For example: :    i1 = I1 , i2 = -I2  and  I3 = I1 - I2
We now write Kirchoff's voltage law equation in the same way as before to solve them but the advantage of this method is that it ensures that the information obtained from the circuit equations is the minimum required to solve the circuit as the information is more general and can easily be put into a matrix form.
For example, consider the circuit from the previous section.
mesh current analysis
These equations can be solved quite quickly by using a single mesh impedance matrix Z. Each element ON the principal diagonal will be "positive" and is the total impedance of each mesh. Where as, each element OFF the principal diagonal will either be "zero" or "negative" and represents the circuit element connecting all the appropriate meshes. This then gives us a matrix of:

mesh current analysis circuit
 
Where:
  • [ V ]   gives the total battery voltage for loop 1 and then loop 2.
  • [ I ]     states the names of the loop currents which we are trying to find.
  • [ R ]   is called the resistance matrix.
and this gives I1 as -0.143 Amps and I2 as -0.429 Amps
As :    I3 = I1 - I2
The current I3 is therefore given as :    -0.143 - (-0.429) = 0.286 Amps
which is the same value of  0.286 amps, we found using Kirchoff´s circuit law in the previous tutorial.

Mesh Current Analysis Summary.

This "look-see" method of circuit analysis is probably the best of all the circuit analysis methods with the basic procedure for solving Mesh Current Analysis equations is as follows:
  • 1. Label all the internal loops with circulating currents. (I1, I2, ...IL etc)
  •  
  • 2. Write the [ L x 1 ] column matrix [ V ] giving the sum of all voltage sources in each loop.
  •  
  • 3. Write the [ L x L ] matrix, [ R ] for all the resistances in the circuit as follows;
    •  
    •   R11 = the total resistance in the first loop.
    •  
    •   Rnn = the total resistance in the Nth loop.
    •  
    •   RJK = the resistance which directly joins loop J to Loop K.
  •  
  • 4. Write the matrix or vector equation [V]  =  [R] x [I] where [I] is the list of currents to be found.
As well as using Mesh Current Analysis, we can also use node analysis to calculate the voltages around the loops, again reducing the amount of mathematics required using just Kirchoff's laws. In the next tutorial about DC Theory we will look at Nodal Voltage Analysis to do just that.

Circuit Analysis

In the previous tutorial we saw that complex circuits such as bridge or T-networks can be solved usingKirchoff's Circuit Laws. While Kirchoff´s Laws give us the basic method for analysing any complex electrical circuit, there are different ways of improving upon this method by using Mesh Current Analysis or Nodal Voltage Analysis that results in a lessening of the math's involved and when large networks are involved this reduction in maths can be a big advantage.
For example, consider the circuit from the previous section.

Mesh Analysis Circuit

Mesh Analysis Circuit
One simple method of reducing the amount of math's involved is to analyse the circuit using Kirchoff's Current Law equations to determine the currents, I1 and I2 flowing in the two resistors. Then there is no need to calculate the current I3 as its just the sum of I1 and I2. So Kirchoff's second voltage law simply becomes:
  • Equation No 1 :    10 =  50I1 + 40I2
  • Equation No 2 :    20 =  40I1 + 60I2
therefore, one line of math's calculation have been saved.

Mesh Current Analysis

A more easier method of solving the above circuit is by using Mesh Current Analysis or Loop Analysiswhich is also sometimes called Maxwell´s Circulating Currents method. Instead of labelling the branch currents we need to label each "closed loop" with a circulating current. As a general rule of thumb, only label inside loops in a clockwise direction with circulating currents as the aim is to cover all the elements of the circuit at least once. Any required branch current may be found from the appropriate loop or mesh currents as before using Kirchoff´s method.
For example: :    i1 = I1 , i2 = -I2  and  I3 = I1 - I2
We now write Kirchoff's voltage law equation in the same way as before to solve them but the advantage of this method is that it ensures that the information obtained from the circuit equations is the minimum required to solve the circuit as the information is more general and can easily be put into a matrix form.
For example, consider the circuit from the previous section.
mesh current analysis
These equations can be solved quite quickly by using a single mesh impedance matrix Z. Each element ON the principal diagonal will be "positive" and is the total impedance of each mesh. Where as, each element OFF the principal diagonal will either be "zero" or "negative" and represents the circuit element connecting all the appropriate meshes. This then gives us a matrix of:

mesh current analysis circuit
 
Where:
  • [ V ]   gives the total battery voltage for loop 1 and then loop 2.
  • [ I ]     states the names of the loop currents which we are trying to find.
  • [ R ]   is called the resistance matrix.
and this gives I1 as -0.143 Amps and I2 as -0.429 Amps
As :    I3 = I1 - I2
The current I3 is therefore given as :    -0.143 - (-0.429) = 0.286 Amps
which is the same value of  0.286 amps, we found using Kirchoff´s circuit law in the previous tutorial.

Mesh Current Analysis Summary.

This "look-see" method of circuit analysis is probably the best of all the circuit analysis methods with the basic procedure for solving Mesh Current Analysis equations is as follows:
  • 1. Label all the internal loops with circulating currents. (I1, I2, ...IL etc)
  •  
  • 2. Write the [ L x 1 ] column matrix [ V ] giving the sum of all voltage sources in each loop.
  •  
  • 3. Write the [ L x L ] matrix, [ R ] for all the resistances in the circuit as follows;
    •  
    •   R11 = the total resistance in the first loop.
    •  
    •   Rnn = the total resistance in the Nth loop.
    •  
    •   RJK = the resistance which directly joins loop J to Loop K.
  •  
  • 4. Write the matrix or vector equation [V]  =  [R] x [I] where [I] is the list of currents to be found.
As well as using Mesh Current Analysis, we can also use node analysis to calculate the voltages around the loops, again reducing the amount of mathematics required using just Kirchoff's laws. In the next tutorial about DC Theory we will look at Nodal Voltage Analysis to do just that.

Kirchoffs Circuit Law

We saw in the Resistors tutorial that a single equivalent resistance, ( RT ) can be found when two or more resistors are connected together in either series, parallel or combinations of both, and that these circuits obey Ohm's Law. However, sometimes in complex circuits such as bridge or T networks, we can not simply use Ohm's Law alone to find the voltages or currents circulating within the circuit. For these types of calculations we need certain rules which allow us to obtain the circuit equations and for this we can use Kirchoffs Circuit Law.
In 1845, a German physicist, Gustav Kirchoff developed a pair or set of rules or laws which deal with the conservation of current and energy within electrical circuits. These two rules are commonly known as: Kirchoffs Circuit Laws with one of Kirchoffs laws dealing with the current flowing around a closed circuit, Kirchoffs Current Law, (KCL) while the other law deals with the voltage sources present in a closed circuit, Kirchoffs Voltage Law, (KVL).

Kirchoffs First Law - The Current Law, (KCL)

Kirchoffs Current Law or KCL, states that the "total current or charge entering a junction or node is exactly equal to the charge leaving the node as it has no other place to go except to leave, as no charge is lost within the node". In other words the algebraic sum of ALL the currents entering and leaving a node must be equal to zero, I(exiting) + I(entering) = 0. This idea by Kirchoff is commonly known as theConservation of Charge.

Kirchoffs Current Law

Kirchoffs Current Law
Here, the 3 currents entering the node, I1, I2, I3 are all positive in value and the 2 currents leaving the node, I4 and I5 are negative in value. Then this means we can also rewrite the equation as;
I1 + I2 + I3 - I4  - I5 = 0
The term Node in an electrical circuit generally refers to a connection or junction of two or more current carrying paths or elements such as cables and components. Also for current to flow either in or out of a node a closed circuit path must exist. We can use Kirchoff's current law when analysing parallel circuits.

Kirchoffs Second Law - The Voltage Law, (KVL)

Kirchoffs Voltage Law or KVL, states that "in any closed loop network, the total voltage around the loop is equal to the sum of all the voltage drops within the same loop" which is also equal to zero. In other words the algebraic sum of all voltages within the loop must be equal to zero. This idea by Kirchoff is known as the Conservation of Energy.

Kirchoffs Voltage Law

Kirchoffs Voltage Law
Starting at any point in the loop continue in the same direction noting the direction of all the voltage drops, either positive or negative, and returning back to the same starting point. It is important to maintain the same direction either clockwise or anti-clockwise or the final voltage sum will not be equal to zero. We can use Kirchoff's voltage law when analysing series circuits.
When analysing either DC circuits or AC circuits using Kirchoffs Circuit Laws a number of definitions and terminologies are used to describe the parts of the circuit being analysed such as: node, paths, branches, loops and meshes. These terms are used frequently in circuit analysis so it is important to understand them.
  • Circuit - a circuit is a closed loop conducting path in which an electrical current flows.
  • Path - a line of connecting elements or sources with no elements or sources included more than once.
  • Node - a node is a junction, connection or terminal within a circuit were two or more circuit elements are connected or joined together giving a connection point between two or more branches. A node is indicated by a dot.
  • Branch - a branch is a single or group of components such as resistors or a source which are connected between two nodes.
  • Loop - a loop is a simple closed path in a circuit in which no circuit element or node is encountered more than once.
  • Mesh - a mesh is a single open loop that does not have a closed path. No components are inside a mesh.
  • Components are connected in series if they carry the same current.
  • Components are connected in parallel if the same voltage is across them.
Kirchoffs Circuit Law

Example No1

Find the current flowing in the 40Ω Resistor, R3
Kirchoffs Law Example
The circuit has 3 branches, 2 nodes (A and B) and 2 independent loops.
Using Kirchoffs Current LawKCL the equations are given as;
At node A :    I1 + I2 = I3
At node B :    I3 = I1 + I2
Using Kirchoffs Voltage LawKVL the equations are given as;
Loop 1 is given as :    10 = R1 x I1 + R3 x I3 = 10I1 + 40I3
Loop 2 is given as :    20 = R2 x I2 + R3 x I3 = 20I2 + 40I3
Loop 3 is given as :    10 - 20 = 10I1 - 20I2
As I3 is the sum of I1 + I2 we can rewrite the equations as;
Eq. No 1 :    10 = 10I1 + 40(I1 + I2)  =  50I1 + 40I2
Eq. No 2 :    20 = 20I2 + 40(I1 + I2)  =  40I1 + 60I2
We now have two "Simultaneous Equations" that can be reduced to give us the value of both I1 and I2 
Substitution of I1 in terms of I2 gives us the value of I1 as -0.143 Amps
Substitution of I2 in terms of I1 gives us the value of I2 as +0.429 Amps
As :    I3 = I1 + I2
The current flowing in resistor R3 is given as :     -0.143 + 0.429 = 0.286 Amps
and the voltage across the resistor R3 is given as :     0.286 x 40 = 11.44 volts
The negative sign for I1 means that the direction of current flow initially chosen was wrong, but never the less still valid. In fact, the 20v battery is charging the 10v battery.

Application of Kirchoffs Circuit Laws

These two laws enable the Currents and Voltages in a circuit to be found, ie, the circuit is said to be "Analysed", and the basic procedure for using Kirchoff's Circuit Laws is as follows:
  • 1. Assume all voltages and resistances are given. ( If not label them V1, V2,... R1, R2, etc. )
  •  
  • 2. Label each branch with a branch current. ( I1, I2, I3 etc. )
  •  
  • 3. Find Kirchoff's first law equations for each node.
  •  
  • 4. Find Kirchoff's second law equations for each of the independent loops of the circuit.
  •  
  • 5. Use Linear simultaneous equations as required to find the unknown currents.
As well as using Kirchoffs Circuit Law to calculate the various voltages and currents circulating around a linear circuit, we can also use loop analysis to calculate the currents in each independent loop which helps to reduce the amount of mathematics required by using just Kirchoff's laws. In the next tutorial about DC Theory we will look at Mesh Current Analysis to do just that.

hUBUNGAN ANTARA TEGANGAN, ARUS DAN HAMBATAN

 

Voltage, Current and Resistance

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Relationship Between Voltage, Current Resistance

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

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.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 EnergyE 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

voltage sources

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, ( 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. The diagram at the left shows Conventional Current Flowthe 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. Electron FlowThe 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 ). 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 from flowing.

Resistance

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.

Resistor Symbols

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 obeyOhm'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, ( 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 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 relationship


Voltage, Current and Resistance Summary

Hopefully by now you should have some idea of how electrical VoltageCurrent and Resistance are closely related together. The relationship between VoltageCurrent 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 ".

QuantitySymbolUnit of
Measure
Abbreviation
Voltageor EVoltV
CurrentIAmpA
ResistanceROhmsΩ
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.