DC Series Circuit Part 1

I just want clarify something before we start our new topic for today. I think you realized why I'm posting such very basic topic. This is not to insult your intelligence but the purpose of this is for you to comprehend well the basic first, because in reality some of you do not understand the basic. This is one of the common mistakes of the beginners especially taking up the course like Electrical Engineering. If you would ask me if I have a plan of posting an advanced concept. Simply the answer is YES , PERIOD. But this is after posting up all the basic concepts.

Please bear with me for a sort of introduction. Just want to clarify something.

Moving on...

Resistance in Series Circuit

Have you already read the definition of a series circuit in your Physics? Well, if you forgot it already I will define it for you again to refresh your mind. If you still remember our example of a light bulb connected across the battery source, that's already a typical example of a series circuit. But in this case, since we are dealing of resistive circuits, we will define it in this way.

A series circuit is formed when two or more resistors are connected from terminal to terminal or simply end-to-end in a circuit in such a way that there is only one path for current to flow.

Connecting the resistor in series so as to form series connection is much easier. You don't have to worry about connecting positive and negative terminal. Resistors (unlike cells) have no polarity.

I just want to clear this things up for you. If you have for example a two lamps connected together from one terminal to another leaving each other terminals of each lamps unconnected, this is a series connected but you would not have a so called series circuit. In order to have a series circuit, you would have to connect the lamps across the voltage terminals such as a battery for example using the unconnected terminals to complete a series circuit.

Remember, any number of resistance, lamps or any devices connected together in a series would only be a series circuit provided their end to end terminals are connected across to a voltage source and would offer one path of current flow. Ohhh... I just can't proceed with the discussion without showing you the typical example of a series circuit. A very good representation of a series circuit was given below.

One important thing that we need to remember here in series circuit is that their values are just added. The regular reader of this blog still remember the discussion about factors upon which the resistance of conductor depends. It was mentioned there that the resistance of conductor increases as the length of the conductor increases. In order for you to get my point here, imagine you have a 3 different length of wires in your hand with different resistances. Then, lets try to connect it together. After connecting them together, the resistance of the wire in full length would be equal to sum of resistances of wire 1 + wire 2 + wire 3.

Let's take it again into more detail... another example if I have a three lengths of wires. The first one having a resistance of 3 ohms, the second is 4 ohms and the third one is 5 ohms. If to try to connect them together, the total resistance of the end-to-end terminal would be 12 ohms. The conclusion here is that any types of resistances connected in series, their total resistance would be equal to the sum of their individual resistances.

One more thing that we need to consider when dealing with resistance connected in series is the proper identification. So, what is this all about? When dealing with a circuit, you would surely encounter different equivalent value of resistances just like what we have in our examples above. In order for you to distinguish same device with different resistances to another we have a so called subscript for them to identify.

This subscript was now written in a different manner. Today it was written like R1 ( not offset) which is similar to R1 ( which is offset) like the above diagram. R1, R2, R2 identifies same resistor with different or same resistances. The same manner was also applied to voltage E1, E2, E3 or sometimes they use V1, V2, V3 just like given in the above example diagram. For uniformity, lets use Rt with the small subscript t for the total resistance. For example on the above diagram our total resistance is Rt = R1 + R2 + R3.

Remember to use the correct subscript, because when we deal with more complicated circuits, you would not be confused. In any aspect aside from this, it is very important to distinguished one equivalent value to another.

The Flow of Current in Series Circuit

Since we already know that the current will flow in one path in a series circuit. This only means that the current will flow in every component of the circuit. Since I'm a visual person, I would give you a practical example of this. Supposed you have a circuit just like what we have on the above example, let's put an ammeter across a resistor one at a time to get the current reading. It would show that there are identical amount of current is flowing in every component in the circuit.

Take note that the current must be capable of passing in every component in the circuit without being damaged. What does this mean? Let's take gain another example. If you have a light bulb to be connected in circuit, it must be rated. If it is rated too low, the light of the bulb would be very bright and the tendency of this light bulb would be burn out because of excessive flow of current. The same thing would happen if in practical application, you mistakenly use different ratings would result sometimes to a serious trouble. The circuit would probably stop functioning or will not function properly. This is very very important.

Voltages in Series Circuit - The Kirchhoff's Second Law

 

We all know that the current are just identical in every component for a series circuit. Now, let's consider the voltage across each component in a series circuit. This is somewhat we called it a potential drop or the voltage drop. Since they have same current, the energy expended in pushing this equal amount of current through the individual component must be also the same.

Let's consider the diagram given above. Supposed you have three resistors connected in series. The diagram shows that you have 45 V connected across the circuit. If we measure the voltage across R1, the reading would be 10 V, getting the voltage across R1 and R2 would be 30 V and lastly getting the voltage across R1, R2 and R3 would be obviously 45V. This is similar when we get the voltage across each resistor and sum it up would be also equivalent to 45V. See the diagram how it is clearly illustrated.

The fact above was expressed by the German Physicist Kirchhoff (1824- 1887) which is known as Kirchhoff s Second Law. The first law would be discuss on my next succeeding post. Please take note of what this law being state and it is very important too.

Kirchhoff s Second Law states: The sum of the voltage drop across the resistances of a closed circuit equals the total voltage applied to the circuit.


Applying Ohm's Law in Series Circuit

Simplifying the facts that we have above about series circuit. We now know that:

1. The current in the series circuit is the same everywhere. This can be mathematically expressed as It = I1= I2= I3 and so on.

2. That the total resistance in a series circuit is equivalent to the sum of the individual resistances in the circuit. This can be expressed as Rt= R1 + R2 + R3 and so on.

3. That the voltage drop across each resistances when added is equal to the voltage source connected across the circuit in series. This can be expressed as, Et = E1 + E2 + E3, and so on.

Applying ohm's law in a series circuit is very helpful especially in terms of application. For instance, you do not know the value of the resistance connected across the circuit but you have some data to resolve that problem. Ohm's Law would be a great help for this.

Some useful application of Ohm's Law in the series circuit is the simplication process. Let's take an example diagram below.

 

You will in the example above that the right hand diagram is the simplified version of the left hand diagram. With this theory, you can find some missing factor that you want to know in your circuit. You should always try to simplify the series of resistances into single resistance equivalent circuit just like what had shown above. Let's simulate a good example of this.

A sample problem...

A circuit contains two resistors connected in series across 100 Volts. The circuit current flow is 2 Amperes. One of the resistor R1 have known value of 10 ohms. You wish to know the resistance of the entire circuit, the value of the second resistor R2 and the voltage drop across each of the two resistors.

First, in Ohm's Law Rule, lets draw the diagram just what had shown above. Since we already visualize on how we are going to simplify it, let's make a list of the given and unknown values.

Data:
Et = 100 Volts
It = 2 Amperes
Rt= (unknown)

E1= (unknown)
I1 = 2 Amperes since It = I1 = I2 = 2 Amperes
R1= 10 ohms

E2= (unknown)
I2 = 2 Amperes
R2 = (unknown)

Now let's solve it!

Since we are looking for the resistance of the entire circuit, from Ohm's Law we use the magic triangle. Put your thumb on R. Now we obtained that Rt = Et/ It = 100 / 2 = 50 ohms - answerIf you will see, you cannot obtain the value of the second resistance R2 without knowing first the voltage drop across R1 which is E1. After knowing the value of E1, we can now proceed in solving the voltage drop across R2 which is E2 therefore we can solve for the unknown R2. Let's do it...

From the magic triangle, put your thumb on E, therefore voltage drop across R1 is E1 = I1 X R1 = 2 x 10 = 20 Volts -answer

From our concept above, the sum of the voltage drop across each resistances is equivalent to a voltage source connected across them. - Kirchhoff s Second Law

Et = E1 + E2 ; 100 Volts = 20 Volts + E2, solving for E2 = 100 - 20 = 80 Volts - answer (simple algebra dear)

Since we already know the value of voltage drop across the R2, we may now solve the value of R2. Putting your thumb on R of the magic triangle, we obtain that R2 = E2 / I2 = 80 / 2 = 40 ohms - answerTry to practice more solving a series circuit using ohm's law. It is important that you should grasp the use of Ohm's Law in solving series circuit.

I will continue the discussion of DC Series Circuit here in Electrical Engineering course.

See you again.

Note : I'll be back on September 3, 2009 evening (Philippines Time) for continuation of DC Series Circuit Part 2.

Cheers!

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Resistors Part 2- Color Code and How Resistance is Measured

Let's continue of what we have discussed yesterday before we go on the full discussion of a Series Circuit which will be our next topic that we will going to study here in Learn Electrical Engineering for Beginners.

Today, we will be dealing on how the resistor color coding is being used and how do we going to interpret it to obtain the reading. Then, afterward s we will touch a little bit on how the resistance is being measured. That's all will be discussed within this new post.

Let's begin now... timer start now!

The Resistor Color Code

We all know that we can find the resistance value of any resistor by using an ohmmeter. But what if we don't have an ohmmeter to use? Most of the case we can find the resistance value easier by interpreting its marking. Some resistors like wire-wound resistor have its printed value in ohms in their body. If they don't have the mark, you would require to use an ohmmeter. An example of a resistor which usually have all of the data printed directly on the resistor body with the information such as tolerance, temperature characteristics, and exact resistance value is the precision wire wound resistor. Other resistor like the carbon resistors usually do not have the data of characteristics directly marked on them, instead they have a so called color code by which they can be identified. You will wonder why it is being done this way for carbon resistors. The reason of using a color code for a carbon resistor is that they are small which is difficult to read the printed values especially when they are mounted.

Before we forgot something, there are two types of carbon resistors. The radial and an axial. They are only differ in the the way the leads are connected to the body of the resistor. Both employ the same color code but they are printed in the different manner. Radial lead resistors are not found in modern equipment. They are widely used in the past. I can't see any example of this now. Below is an example of an axial resistor.

In the picture above this axial lead resistors have its leads molded into the ends of the carbon rod of the resistor body. If you will see, the leads extends straight out in line from the body of the resistor. The carbon rod is coated with a good insulator.

Moving on...

Color coding system for resistors consists of three colors to indicate the resistance value in ohms of a certain resistor, sometimes the fourth color indicate the tolerance value of the resistor. By reading the color coded in correct order and substituting the correct value of each corresponding color coded as shown in the table below, you can immediately tell all you need to know about the resistor. The only thing that you will practice on how to use it and familiar yourselves for those value so that you can easily determine the value of the resistor color coded at a glance.

This is how you will do it.

The color of the first color band indicates the first digit of the resistance value or the first significant digit. Let's have an example below. Supposed that you have a given resistor below, the first color is yellow. If you would look at the table above it is equivalent to 4.

The second color coded of the resistor given below is violet, so this is now your second digit which is equivalent to 7 as shown in the table above.

The third color would served as your multiplier. In the case below since it is color red which is equivalent to 100 multiplier, or just simply add 2 zeros so this would look like this now:

47 ohm x 100 = 4, 700 ohms or 4.7 kilohms

Ooooppps! it seems that we are not done yet. The last color band or the fourth color band is gold which have 5% tolerance according to our table above. Therefore our final answer would be:

4.7 kilohms +/- 5% - answer

How to Measure the Resistance

We all know that voltmeter and ammeter are used for measuring the voltage and the current respectively. For the resistance, the meters that use to measure it is the ohmmeter. When using an ohmmeter, there should be no voltage present across the resistors except for the ohmmeter battery, otherwise your ohmmeter would be damaged. I can see two types of ohmmeter nowadays, the analog and the digital. Among the two ohmmeters, digital is widely used nowadays.

The above ohmmeter usually used to measure the resistance of the resistors. Ohmmeter ranges usually vary from 0-1,000 ohms to 0 -10 megohms. There are some special ohmmeters called the MEGGERS. This ohmmeter was used to measure high resistance values which are over 10 megohms. Some meggers use high voltage batteries and other use special type of hand generator to obtain the necessary voltage. These megohmeters is used to measure and test the resistance of insulation. Picture below is the example of a megger.

Ohmmeter is very easy to use by following two steps. First, the voltage must be set to the proper value. This is done with the zero adjustment by shorting outor by connecting together the two leads from the ohmmeter and setting to zero ohms on the meter with the zero adjustment control. This should be always done whenever you changed the meter range selector switch to a different scale. Now, the meter is now calibrated for the given range, you will notice that when the leads shorted out, the meter reads zero ohm, but when it opens, the meter reads infinity which indicates an open circuit. Therefore, when these leads touches the resistors subject for measurement, it will directly read the resistance in the meter multiplying it with the range selector switch. The range selector switch is serves as the multiplier or the multiplying factor whenever you are measuring the resistance using ohmmeter. The range selector switch usually marked as R, RX 10, RX 100, RX1,000, etc...
For example if the ohmmeter is switch on to R X 1,000 meaning the value of the meter will be multiplied to 1,000 to get the actual value of the resistance being measured.

That's it for today.

Tomorrow we'll continue dealing with circuits here in Electrical Engineering.

Cheers!

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Resistors Part 1 - Use and Properties

Now that you have learned the basic concept of Ohm's Law, we can now proceed in discussing the use, properties, and construction of resistors. All you can learn it here in Electrical Engineering.

Before we continue our study of circuits, we need to know more about resistance and resistors though we have touched it a little on my previous post. But its just a review. Today, it would be a bit deeper.

We know that there is a certain amount of resistance in all electrical equipment which we use. Sometimes, this resistance is not enough to control the flow of current to the extent desired. If you did not get my point here, let's have a few example of this. I will going to give an example by illustration as what had shown below. The circuit shown below, a switch and a current limiting resistor are used to control the flow of current through the motor. When starting a motor, the switch is kept open and the resistance is thereby added into the circuit to control the flow of current. After the motor has started, the switch is then closed in order to bypass the current limiting resistor.

There are wide variety of resistors, some of them have fixed values and some are variables.

What resistor is made up of?

Resistors are made up of special resistance wire, graphite (carbon) composition, or of metal film. Wire wound resistors are usually used to control large currents, while carbon resistors controls current which are relatively small.

Vitreous enameled wire-would resistors are constructed by winding resistance wire on a porcelain base, attaching the wire ends to metal terminals, and coating the wire and base with powdered glass and bake enamel to protect the wire and conduct heat away from it.

Fixed wire wound resistors with a coating other than vitreous enamel are also used. The example below is the example of this one.

Wire-wound resistors may have fixed taps, which can be also used to change the resistance value in steps, or sliders, which can be adjusted to change the resistance of any fraction of the total resistance. The picture below is the example of this one.

Precision wound resistors of manganin wire (a special wire that does not change resistance very much with high temperature) are used where the resistance value must be very accurate, such as in test instruments. The picture below is the example of this one.

Carbon resistors are used for low current applications. They are made from a rod of compensated graphite (carbon) that is mixed with clay and binders. By varying the amount of each component, it is possible to vary the resistance values obtained over a very wide range. Two lead wires are called pigtails are attached to the end of the resistance rod, and the rod is embedded in a ceramic or plastic covering, leaving the pigtails protruding from the ends. Take a look for a sample below.

You will find other type of resistor called a deposited film resistor used for special applications. These resistors are made by depositing a thin film of resistance metal or carbon on a ceramic core and then coating the resistor with either a ceramic or enamel protective coating. In many cases you will find that these resistors have radial leads meaning the leads come off at right angles to the body of the resistor. In some cases the deposited film is laid down on the core as spiral, similar to winding a wire around the tube, in order to increase the length of the resistance element without making the resistor too long. The example of this one was shown below.

Resistor Tolerance and Values

Let's consider this topic before we go on the color code for resistors. You need to find out the something about resistor tolerances and something about the preferred values of resistance that you will find in the circuits. Special resistors may have tolerances of as little as 1% , 0.1% or even 0.01% but most resistors that you will see have much greater tolerances. Large wire-wound resistors usually have tolerances of 10% or 5%. Carbon resistors are available in 20%, 10% and 5% tolerances.

So what are those tolerance mentioned above means? Let's take an example...

If you had a 10 kilohm resistor with a tolerance of 20%, the actual value of the resistor could anywhere from 10 kilohm - 10 kilohm (0.20) = 8 kilohm to 10 kilohm + 10(0.20) = 12 kilohm. That is how you will going to use the tolerance given for a specific resistor.

You will wonder how many different resistance values you can get for a resistor. It depends on the tolerance. Considerations such as this have led to the establishment of a set of preferred values of resistance in each tolerance where the highest tolerance of one value is about equal to the lowest tolerance of the next highest value. The table of preferred resistance was shown below. Later, you will find that resistors are available in different power ratings as well.

The numbers on the chart above show only the first two digits. Thus, it means for example, 33 means that 3.3, 330, 3.3 kilohm, 330 kilohm and 3.3 megohm resistors are available.

On my next post I will show you how the resistor color code is being used for carbon resistors and how the resistance is being measured.

I will tee off shortly. Stay more here in Learn Electrical Engineering for Beginners.

Cheers!

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The Basic Concept of Ohm's Law

I just can't wait for another morning without my new post here in Learn Electrical Engineering for Beginners. It's going interesting here because I've noticed that there are new another batch of subscriber who want to learn about Electrical Engineering. Since you've found this site, you're halfway to learning the Electrical Engineering. For those who are new in this site, you can still catch up with our previous 2 last topics here.

• What is Electric Circuit - Part 1
• What is Electric Circuit - Part 2

Ok, let's begin with our new basic topic.

The Relationship of Voltage, Current, and Resistance

This next topic, you will learn about Ohm's Law, this is one of the most basic and important that you will use throughout your career here in Electrical Engineering.

The concept is just simple: given a constant resistance in a circuit, the current flow increases as the voltage applied to the circuit increases. Given a constant voltage (emf) applied to the circuit, current flow decreases as the resistance of the circuit increases. You can finalize these ideas or concepts as follows: Current flow in the circuit increases as the voltage is increased, and decreases as the resistance is increased.

The relationship of voltage, current, and resistance was studied by German Physicist, George Simon Ohm. His statement of this relationship, called Ohm's Law. Obviously, the title of the law was derived itself on his name. This is one of the fundamental laws of Physics.

What does this vital laws state? This is simple as:

With Constant Resistance

• lower voltage gives small current.
• higher voltage gives large current.

With Constant EMF

• lower resistance passes large current.
• higher resistance passes small current.

In a more technical expression, you can state it as:

Ohm's Law states that the current flowing in a circuit is directly proportional to the voltage applied (emf) and inversely proportional to the resistance.

It is also possible to express Ohm's Law as a mathematical equation (relationship) as further indicated below:

CURRENT = EMF / RESISTANCE

In electrical terms (notation), current is always represented by the letter "I", resistance by the letter "R" and the voltage by the letter "E" or you can used letter "V". You can therefore rewrite the mathematical given above in another way like what as shown below:

I = E / R

Using our simple algebra, you can also derived it in these ways:

E = I x R or as R = E / I

Which of the three ways ( formulas) of expressing Ohm's Law you might choose to employ depends on two things: 1. what facts you know and 2. what facts you need to know about the circuit you are considering.

Ohhh... I think you've got a headache on how to remember those formulas above. Let's have a little trick on how you can remember it better. Let's draw a triangle with a horizontal line across it half-way up from its base. Write letter E in the small triangle, which has been formed above the line, and the letters I and R below the line, it will look like as what as shown below. This is what you called the magic triangle!

Ok! now consider a circuit in which you know the values of any two of the three factors - voltage, current, and resistance- and want to find out the third. The rule for working the magic triangle to give the correct formula is as follows:

Put your thumb over the letter in the triangle whose value you want to know- and the formula for calculating that value is given by the two remaining letters.

Here is how to do it:

1. If you know the values of current and resistance in the circuit. But the value of voltage is unknown and you cannot measure it because of some reason ( no available voltmeter to use). Draw the magic triangle, put your thumb you want to calculate, which in this case the E- and you are left with the formula you need. ---- I X R.

2. You know the values of current and voltage, but in this case you have no ohmmeter to measure the resistance. Put your thumb over the letter R and you are left with the formula E / I. Substitute the known values for E and I, and your answer is R.

3. The voltage and the resistance of a circuit are known to you; but in this case the ammeter you need to measure the current is lost or broken. Put your thumb over the symbol I. and read off the formula you need: E/R.

Ohm's law cannot work properly if you will not know on how to expressed its values in the correct units of measurement. The next topic will show you the Ohm's Law Rules.

Ohm's Law Rules

Ohm's Law will work for you and give you the correct answer in any problem situations which you may try to solve. In the Ohm's Law equation, the first rule is that:

CURRENT is ALWAYS expressed in AMPERES.

VOLTAGE is ALWAYS expressed in VOLTS.

RESISTANCE is ALWAYS expressed in OHMS.

Let's have an example of the above rule.

Take you have a circuit in which you have to measure the resistance to be 100 ohms, and the current to be 300 milliamperes (mA). Obviously, if you use the Ohm's Law without knowing the correct rule mentioned above, you will arrived like this: E = I X R = 100 X 300 = 30,000 your answer will be definitely wrong by a factor of 1,000.

You have to use the conversion tables as what you have learned in Physics, and you must rewrite all factors in the simple expression above in amperes, volts and ohms. When you did this, you will obtain:

E = I X R = 100 X 0.3 = 30 volts - answer

Thus, giving you the correct answer.

There is a second rule in which you must apply whenever you are attempting to solve an Ohm's Law problem involving quantities and values in an electric circuit. The rule is: Always sketch a rough diagram of the circuit you are considering, before you start making calculations based on the values in the circuit which are already known to you. This rule is very useful especially when we already dealing with the more complex circuits.

Let's have an example of the above second rule:

Supposed you have an unknown resistor connected across the battery, and you find by measurement that the voltage across it is 24 volts. You measure the current flowing as 6 amperes. You want to know the resistance of the resistor; but you have no ammeter.

This is how you will solve it.

First, draw the circuit diagram, and fill it in with the information you already have. Sketch out the magic triangle ( illustration given below) . The magic triangle tells you that R = E/ I.

Into this equation, you may substitute the known values and get R = 24 / 6 = 4 ohms - answer

Since you know now already the basic concept of Ohm's Law, you are now ready to face the challenges here in Electrical Engineering. On my next post let's discuss some applications about it.

Hope you learn something new basic technique here in Learn Electrical Engineering for Beginners.

Cheers to all and Good night!

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What is Electric Circuit? - Part 2

Before I proceed with our new topic for today, I just want to give thanks for those who are currently subscribed here in Learn Electrical Engineering for Beginners. I hope this will be a helpful site for you which I will always give a full and detail information for every subject matter.

Well, it's enough for my short introduction because I'm already running out of time for the updates. Yesterday I had mentioned on my previous post about our topic on full definition of Electric Circuits that I will going to differentiate between DC and AC Circuits. Please keep in mind that after discussing the difference between the two, I will going to discuss first to you all DC Circuits related topics so that we would not be confused. The principles that we will be discussing here in DC Circuits will also be used again when we touch AC Circuits. Please keep that in mind and this is very important. I just want to keep my ideas and discussions organize here in Electrical Engineering site. I was also trying to catch up the attention of those Electrical Engineering who study online as well as those non- electrical engineers.

Moving on...

Still remember on our review on Physics on my previous post in Voltage, Current, Power and Energy. I already give the definition of DC and AC Current. But for those recent readers, here is the short definition and which is almost the same when dealing with circuits.

In electricity, we deal both on direct current (DC) and alternating current (AC). In DC Circuits, the current always flows in the same or one direction. In AC Circuits, the direction of current flow reverses periodically- this means at one instant the current flow in one direction and in the next instant, in the opposite direction. Remember in our review? this flow reversal in AC current is usually done regularly. What does it mean? If we talk about 60Hz AC power, we mean that the direction of flow reverses 60 times ( or cycles) per second. Graphically, here are the difference for you to comprehend it well. ( You may click the image to enlarge)


Ohhh... before I forgot there are other types of current such as Exponential Current and Dumped Sinusoidal Current. But these are somewhat on the deeper concepts that we might be able to touch on our future discussion. But to give you an idea here are the graphical difference between the two. ( You may click the image to enlarge)



As what I had mentioned earlier, on the first part of the circuitry discussion we will deal with the function of direct current in circuits containing resistance (resistive circuits) and we will use Ohms Law and Kirchhoff's Laws as the tools for analysis and understanding the relationship between current, voltage and resistance. This will be your foundation for your future understanding of ac circuitry. Therefore, it is very important that you completely understand every concept I presented here in dc circuitry.

Continuing our understanding of Electric Circuit.

It may help you to grasp the concept of an electric current flowing through a closed circuit. Imagine that the electrons which make up the current form a moving stream which revolves through the complete circuit.

This moving streams of electrons maintains a constant density throughout its entire length. The number of electrons entering the positive terminal of a battery from a wire is always exactly balanced by the number of electrons which the battery forces to move its own negative terminal and out into the wire.

Therefore, there is no way either the conductor wire or the battery possess either more or less electrons in a complete circuit. If the circuit loop is broken, the electron orbiting stream instantly stops revolving through the circuit; but both wire and battery will still hold exactly same number of electrons as they did when the circuit was made. The only difference is that the wire is now holding some of the electrons which was previously in the battery. Likewise, the battery had taken an equal number from the wire.

The number of electrons in the electron stream is depends on the strength of the voltage applied forcing the electrons to move. The lower the voltage, the weaker.

I will just tell you this in advance that when a resistance of any kind is inserted into the circuit loop, it also restrict the number of electrons flowing therefore reduces the current. You will notice that in some of our applications when we touch different circuitry laws. The flow of current is restricted by this resistance.

Also keep in mind a closed loop of wire is not always an electric circuit. Remember that in our definition of Electric Circuits, I had mentioned there the 2 conditions that makes up an electric circuit. Current, voltage and resistance are present in any electric circuit where electrons move around the close loop. The pathway for current flow is actually the circuit, and its resistance controls the amount of current flow around the circuit.

DC circuits consists of a source of DC voltage, such as batteries plus the combined resistance of the electrical load connected across this voltage. While working with DC circuits, you will find out how the total loads can be changed with various combinations of resistances, and how these combination of resistances control the circuit current and affect the voltage. This concept will also be applied in AC circuitry.

There will be two types of circuits that we will be dealing with: these are series circuits and parallel circuits. No matter how complex the circuit is, still it can be simplified down into series connection to or a parallel circuit connection.

One last thing.

The Load

Previously on our last topic we had mentioned about the load in the electric circuit. So what the heck is the load? How does it works in the electric circuit?

In basic electric circuit the device that transforms the electrical energy from the source of power (emf) into some useful function -such as heat, light, mechanical power, etc.- is called the load. The load aside from transforming and electrical energy into some useful purposes, can be utilized to changed or control the amount of energy being delivered from the source.

A load could be a motor, a telephone, a lamp, a heater or some other appliances -( name it ). the term load means the electric power delivered by the source. If you don't get it, I will give you an example. When it is stated that the load is increased or decreased, it means that the source is delivering less or more power. Remember a load can be: a device which utilizes the power from the source and the power that is taken from the source.

One more last thing...

The Switches

I included this because this is one of the common part of electric circuitry either on DC or AC Circuitry. We have been using switches everyday and all our life. We could see it in our lamps, a radios, flahlights etc. It is a controlling device which open and close the circuit. There are many types of switches you will encounter in your study of Electrical Engineering. But this will be discussing separately when we touch practical applications in our course outline.

This is where it ends our topic discussing what an electric circuit is. On my succeeding post, we will now begin to study in detail the relationship between voltage, current and resistance.

Hope you appreciate this post today presenting it in my own little way. Learning is fun here in Electrical Engineering for Beginners.

Cheers!

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What is Electric Circuit?

Last time we have a little review of basic Physics. The reason why I included those topics here in Electrical Engineering as a beginning topic because those topics are mostly given in the board exam during our time. Please bear with me for three times posting for I have to do a little SEO for this blog. The reason why I'm doing this is to drive more traffic to this site. I was currently standing number 6 for a keyword Electrical Engineering for Beginners for which this site is really intended to.

Anyway, let's begin with our new topic for today. This is already the start of our major discussion where every detail should be understand by everyone.

The Electric Circuits

Since we will be dealing with electric circuit starting today up to the rest of the topic, it is recommended that you should have an accurate picture of what electric circuit is and how the electric current behaves on it.

Just recall for a moment what we had reviewed on our previous topic about the current flow on Voltage, Current, Power and Energy. You have learned that if you connect a length of wire or a conductor across the positive and negative terminals of a source of electromotive force ( emf) let say, a battery, the potential difference (voltage) makes the current flow and also that electrical energy is needed to keep the current flowing.

Any combination of a conductor and a source of electricity connected together to permit the electrons to travel around a continuos stream is called electric circuits. The figure shown below is a simple electric circuit that we are talking about ( you may click the image to enlarge)


Many millions of free electrons that have already been separated from the outer orbits of their respective atoms by the heat of room temperature, and which have been wandering aimlessly in all directions through the wire, now come under a common controlling force. They repelled by the more negative ( or less positive) charge which have been set-up at one end of the wire, and strongly attracted by the less negative (or more positive) charge which have been connected at the other end. This movement are converted into a disciplined current flow from more negative to more positive, and the electric current flows.

Take note that electrons are negative charges of electricity and have practically no weight at all. It only means that when a potential difference is applied to the wire, they respond to it immediately. Likewise, when the potential difference is removed, the electrons stop their disciplined flow in a single direction at once and resume their wanderings through the conductor material.

Now...

What are the conditions required in order to maintain the flow of an electric current in a circuit? Please take note of these conditions. They are simple but very important when it comes to actual application.

1. There must be a source of potential difference or voltage to provide the energy which forces electrons to move in a disciplined way in a specific direction.

2. There must be continous (complete) external path for the electrons to flow from negative terminal to the positive terminal of the source of voltage.

We have mentioned about external path. This is usually made up of two parts: the conductors or the wires, and the load to which the electric power is to be delivered for some useful effects. In the above illustration given, the lamp or the small bulb is load in the given sample.

An electric circuit is thus completed its electrical pathway, consisting of not only a conductor in which the current will flow (negative to positive), but also of a path through a source of potential difference from positive back to negative.

A small bulb connected across a dry cell is an example of electric circuit. Current flows from the negative (-) terminal of the cell, through the small bulb (the load), to the positive terminal. The action of the cell is that it provides a regenerative path for the flow of electrons to be maintained.

As long as this electrical pathway remains unbroken at any point, it is a closed circuit and the current flows. But if the pathway is broken, it becomes an open circuit and no current flows.

An Open Circuit

A Closed Circuit

This is how the Electric Circuit is defined. On my next post, I will going to differentiate about DC and AC Circuits. We should be able to comprehend well the difference between the two. We will be discussing it separately on our course outline for they have completely separate ideas. I would like to organize my ideas and lessons presented in this Electrical Engineering site.

I hope you enjoy with this simple discussion today. I'll be back shortly.

Cheers!

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Solutions To Brain Teasers Number 2

As I promised last time, I will going to show you the solutions to teasers on our previous topic about the Factors Affecting Resistance here in Electrical Engineering. By the way, this topic is still covered on our review of basic Physics for this is very important one when we reached our major topics in this Electrical Engineering course.

Last time, I leaved you four worded problems in order for you to analyze and understand the principles fully. But if you still running out of time to solve it, I will show to you it now.

The first problem given was:

Problem 1 : The resistance of a copper wire 2, 500 cm long and a 0.090 cm in diameter is 0.67 ohm at 20 degree celcius. What is the resistivity of copper at this temperature?

The solution here is quiet simple. We just have to substitute the given values from the formula since we have uniform units:

p = RA/ l = 0.67 ohm x [ TT ( 0.090 cm) ^2/ 4 ] / 2, 500 cm
    = 1.7 x 10 ^-6 ohm.cm - answer

The unit of resistivity in the British engineering system of units differs from that just given in that different units of length and area are employed. The unit of area is the circular mil., the area of the circle 1 mil (0.001 in) in diameter, and the unit of length is the foot. Since the areas of two circles are proportional to the squares of their diameters, the area of a circle in circular mils is equal to the square of its diameter in mils. In this system of units the resistivity of a substance is numerically equal to the resistance of a sample of that substance 1 ft long and 1 circular mil in area, and is expressed in ohm-circular mils per foot.

The abbreviation CM is often used for circular mils. This should not be confused with the abbreviation used for centimeters (cm). We will use the more standard cmil.

Let's solve another problem using the principles above.

Problem 2 : Find the resistance of 100 ft of copper wire whose diameter is 0.024 in and whose resistivity is 10.3 ohm.cmils/ft.

Convert first the given diameter in mils. Since, 1 mil = 0.001 in as mentioned above.

Therefore, d= 0.024 in = 24 mils.

Then getting the area we have, A = d^2 = 24 ^2 cmils. Substituting the values:

R = pl / A = (10.3 ohm.cmils/ft)(100 ft) / 24 ^ 2 cmils.
R = 1.8 ohm - answer

Problem 3 : A silver wire has a resistance of 1.25 ohm at 0 degree celcius and the temperature coefficient of resistance of 0.00375 per degree celcius. To what temperature must the wire be raised to double the resistance?

Since we are asking for the temperature, just derive the formula of EQ . 1 in our previous post here in Electrical Engineering topics. It will be:

 

t = Rt - R0 / Ro oo = ( 2.50 -1.25 ) ohm / 1.25 ohm x 0.00375 /C
t = 266 degree celcius - answer

It should be clearly understood that R0 in the above equation ordinarily refers to the resistance at 0 degree celcius and not to the resistance of any other temperature. A value of oo based upon the resistance at room temperature, for example, is appreciably different from the value based upon 0 degree celcius. This may be made clearer by the graphic analysis of the variation of resistance with temperature. ( Click the image to enlarge)

In the above illustration, the resistance Rt of a conductor at any temperature t is plotted. For a pure metal, this curves gives a linear relation ( approximately). Note the fact that the curve does not pass through the origin; i.e. at 0 degree celcius the resistance is not zero. Hence, we cannot say that R oo t. The slope of the curve delta R/delta t is constant . Since,

oo = delta R / delta t /Ro = slope / Ro

it is clear that the value of oo depends upon the base temperature chosen for Ro. In computations involving temperature variation of resistance, the value of Ro must be obtained by using the equation below:

Rt = Ro + Ro oo t = Ro( 1 + oo t)

Understanding the principles of effect of temperature in resistance.

Problem 4 : A tungsten filament has a resistance of 133 ohm at 150 degree celcius. If oo = 0.0045/C., what is the resistance of the filament at 500 degree celcius?

From the EQ 1: ( of our previous post)

Ro = Rt / 1 + oo t = 133 ohm / 1 + (0.00450/C ) x 150 degree celcius

Ro = 79.4 ohms

Getting the resistance at 500 degree celcius,

R500 = R0( 1 + oo t500)
R500 = 79.4 ohms [ 1 + (0.00450/C) x 500 degree celcius]

R500 = 258 ohms - answer

Since it is the resistivity factor that changes with temperature. The equations of EQ 1 and EQ 2 from our previous lecture may be written with p in place of R.

pt = p0( 1 + oot) ------------------------------------- EQ. 3

I will not give the table for resistivities and temperature coefficients of resistivity of materials for it is always given in the problem. You don't have to memorize it.

I think this is now over for the basic review....

On my next post, we will now begin to discuss the real scope of Electrical Engineering.

Cheers!

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