Tuesday, April 12, 2011

Ohm's Law

Why would I post an article about Ohm's Law to a blog that contains advanced materials such as electrical specifications and temporary electrical supply?Isn't that a bit out of place?

Nope. It is not out of place.

Ohm's Law is one of the most basic principles of electrical engineering. For beginners to have a sufficient understanding of electrical installations and electrical wiring, knowing and understanding Ohm's Law is a must.

For readers who have never heard of Ohm's law, please do not worry. It is just a name.

Many people who have already some practical skills of house electricity may also never heard of Ohm's law. Knowing the name is not really necessary. Know how things work is necessary.

However, if you wish to grow your knowledge of electricity, then you need to know what it is that electrical people call Ohm's law.

To grow knowledge, we need to be able to communicate with the language of that discipline. We need to understand what people are saying.

In electrical engineering, Ohm's law is like onions in cooking. Or PVC pipes in water supply.

Okey, enough on introduction. Now let's go to the substance about the Ohm's law, one of the most basic of electrical principles.

Look at the simple electrical diagram below:

Diagram 1 - The simplest electrical circuit on earth

I have tried to make the above diagram self explanatory, but maybe it is still not so explanatory.

How many components are there in the above circuit diagram? Four components: 1) The electricity source; 2) The 1.5 sq.mm PVC insulated wiring cable at the top; 3) The LOAD; 4) The 1.5 sq.mm PVC insulated wiring cable at the bottom.

There you have it. If you can get this four components, you can make a working electrical circuit and it will work.

For readers with even a minimal knowledge of electrical engineering, you may wish to visit other posts on this blog. The are quite a number of posts with advanced materials here. This post is for true beginners only and the materials here will get more and more basic. This post is like a first day in a class of electrical engineering in college.

Now allow me to elaborate deeper into the above components for readers who still feel they do not yet understand fully about the above four components.

Components number (2) and (4) are the wiring cables. So you have two lengths of wiring cables here.

Component number (1) is called "Electricity source" because it is a source of electricity. A 1.5V cell battery size AA that we normally use to power a small pocket torch light is one source of electricity. A car battery is also a source of electricity. I could as well replace the word "Electricity source" with "Car battery" and our simple electric circuit in Diagram 1 will still work.

Component number (3) is an electrical "Load". It is called a "load" because it is a burden to the electricity source (or the car battery). I could just replace the word "Load" with the word "incandescent lamp".

If I replace the "Load" with "incandescent lamp", what will happen? Simple. The lamp will light up.

The word "Load" is used to represent all types of electrical loads, whether it is an incandescent lamp or a car radio.

I haven't really got to the Ohm's Law, have I?

I will get there soon. We are actually on the way.

Now look at Diagram 2 below:

Diagram 2 -A simple electric circuit

Here I have replaced the "Electricity source" with "12 V car battery" and the "Load" with "15 Watt (12V) incandescent lamp".

I have also added the capital letter "I" after "Electric current" above the top wiring cable.

And also a capital letter "V" on the right side of the electricity source, opposite to the "12 V car battery"

Also the capital letter "P" on the left side of the electrical load, opposite of the "15 Watt (12V) incandescent lamp".

Now our electrical circuit is detailed enough. If you get these four components: a 12 Volt car battery, two lengths of 1.5 sq.mm PVC insulated wiring cable and a 15 Watt incandescent lamp, you can easily put them together and see the result.

The 10 watt incandescent lamp will light up.

The above is very basic and I believe anybody can see the common sense logic there. I can go even deeper and MUCH MORE basic, but that will take more time. We now need to get back to the direction of the Ohm's Law.

Now if the above electrical components are assembled and connected as shown, we will get a lighted incandescent lamp.

As students of electrical engineering (you are now if you have read through this post this far), you should be asking yourself what are the numbers that makes the lamp light up.

Let me help you here. These are the numbers:

A. 15 watt divided by 12 volt = 1.25 ampere. The 1.25 ampere here is what we call the electric current. It is how we give a quantity to the flow of electrons that flow through the wiring cables. This current flows from the positive side of the car battery (marked + ) travel along the top wiring cable, pass through the incandescent lamp, then continue the travel through the bottom wiring cable, return to the car battery on the negative side (marked - ), and pass through the battery to the positive side and keep circulating the loop in the same direction. The material that the lamp is made of heats up and produce light when the electric current flows through it.

B. Now we have three numbers: 12 is the strength of the car battery. People in electrical engineering called it "volt" to honor one of the scientists who discovered and developed electricity into the form that we can use today. A few hundred years ago, people still used candles to light up their whole house, not just the dinner tables. Imagine that. We can consider ourself very lucky.

C. Then we have number 1.25 for the current that flows through the lamp and the two wiring cables. This number has been called ampere, also for the same reason that the name volt was given. 1.25 is generally the volume of electrons that flow through the wiring cables and the lamp. When you think of electric current, think of the water current in a river or at a beach. The water current can be strong enough to sweep you away and drown you. If the river floods, it can even sweep away houses and cars, etc. The electric current can be just as strong as the water curent. To bring it closer to home, think of the water flow in a water hose when you wash your car.

D. Another number is 15. That is from the incandescent lamp. It is the amount of energy that is used by the lamp while producing the light. The name watt is given to just give a name to the quantity. Watt is also a given name just like volt and ampere.

Now we have three numbers with a name to label each one of them. Ampere for the volume of electric current that flows through the "loop" (or "circuit", just like circuit in a race car track). In our electrical circuit in Diagram 2, the current is 1.25 Amperes. Now please do not get confused about this quantity. When I say volume of current, it is like 1.25 liters of water per second, not 1.25 Liter in quantity. So it is more like speed of electron flow, not the quantity of electrons.

The second number is 12. As I said above, this number is a measure of the strength of the car battery. Trucks usually have a battery with the strength of 24. That is twice the strength of a car battery. Even some cars have have batteries with the strength of 24. A stronger battery can force the electrons to flow faster. Therefore, we can get a higher volume of electron per second passing through the cables and the lamp. A stronger battery is like a bigger water pump. A bigger pump can push more liter of water per second through a water hose. The car battery functions the same way. The name "volt" is just name that has been given to call the quantity. It could have been 12 Lucy instead of 12 Volt if there was a scientist by that name during the period of discovery of electricity.

The third number, 15, is a measure of the strength of an electric machine in doing work. The incandescent lamp in our electric circuit can be called a machine also if we like. The work of the lamp is to produce light. 15 is a measure of that strength. A similar lamp with the strength of 30 would produce a light twice as strong or double the brightness. You can get creative here and maybe use two lamps with the strength of 15. Then you can get the same amount of light. Again one scientist got lucky and his name was used to name this strength. The name is "Watt".

Is this tiring?

If it is, then that is good. That means your mind has been trying to adapt itself to this logic of electrical quantities. If it is a consolation, please know that if you can follow this through and understand these three quantities as I explain them, then you are already on your way to become a good electrician. Not an engineer yet, but you just showed to yourself that you have the capacity to handle one of the most powerful invisible powers on earth, ELECTRICITY.

Where is the Ohm's Law?

Please be patient. There is one more number,the fourth number, that you need to understand before I show you the Ohm's Law.

If you are feeling really impatient already, then I should tell you that learning electricity is like learning magic. I mean real magic, not the "mind trick" types that you can see on TV every day. Before a sorcerer can teach her powerful magic to her student, she must first teach the student the knowledge of defense against magic. This includes measuring how strong a magic is.

Only after mastering the knowledge and skills in the defense against magic, then only one is prepared enough to learn how to use magic. Without the knowledge in defense, the magic itself can hurt or seriously injure the student.

Enough metaphors.

Now the fourth number.

Before the invention of automobiles (or cars as we call them today), people use horses to pull carts so they could deliver goods to distant places.

They didn't have to use the horses to move the carts, they could just push or pull the cart themselves. However, that would be a lot of work. Men were smart. They wanted results the easy way. So they enslaved the wild horses and train them the skills of pulling the carts. Being less smart than men, some of the horses happily do the task while men sit on horses with the goods. Guess who got rich...

Imagine a scenario when a horse (This one man could only afford one horse and a cart. The rest of his money went to the cost of purchasing the goods.) was about to pull a cart.

Before the horse starts pulling, the cart was not moving. It was stationary. The cart was loaded with goods and the man, the owner.

The moment the horse starts pulling, the cart starts to move. Slowly in the beginning moments but it accellerate faster and faster. After a while the horse and the cart travels at a steady speed.

Now if the the man suddenly cuts the ropes that tie the horse to the cart, what would happen? The cart would slow down gradually, right? And eventually the cart would come to a stop.

In order to keep the cart moving, the horse, the source of strength need to be constantly tied to the cart.

Of course the man can also order the horse to stop thereby causing the cart to stop also. I know what you are thinking. The man need to have some means of braking the cart. Otherwise the cart would hit the back of the horse because the natural stopping of the cart would not be immediate. It would be a gradual stop, naturally.

That is the point I am trying to make.


It is the nature of things to stay still, stationary, unless there is a source of strength that forces it into motion.

Even after it has been moved into speed, the source of the strength must stay connected. Otherwise the moving object will gradually come to a stop.

This tendency to stay still, and to gradually come to a stop if the driving force is cut loose, is the way things are in this world.

This RESISTANCE to motion is the way of nature.

If we need to light up the lamp in Diagram 2, we need to overcome the RESISTANCE against the flow of electrons that is in the nature of the materials that make up the electric circuit.

Have I made this point clear?

The speed of the cart is dependent on a number things. The strength of the horse and the combined weight of the cart, the goods and the man, the owner.

However, the type of ground surface also plays an important role.

Also the quality of contact surfaces between the moving "tires" of the cart and other mechanical parts that don not move.

All this factors determine the TOTAL or overall RESISTANCE of the LOAD being pulled by the horse, the SOURCE OF STRENGTH.

This overall resistance dictates how fast the horse can drive the load.

In our simple electric circuit above, the overall RESISTANCE dictates how much electrons per second can flow through the lamp and the wiring cables.


Hey! Where is the Ohm's Law?

Let's not waste everybody's time. The Ohm's Law is here.

With the STRENGTH of the car battery, the STRENGTH of the light producing lamp, the SPEED of the electron flow inside the wiring cables, and the total RESISTANCE of all materials along the path of the electron flow, you now know HOW THE OHM'S LAW WORKS.

The Ohm's Law

Please... do not get intimidated by engineering names such as this Ohm's Law here. It is just a name.

However, nobody can learn engineering without some use of mathematics.

Here you do not need much. There will more as the subject gets more advanced.

V = I x R

R = V / I

I = V / R

P = V x I; P = I x I x R; P = V x V / R

There you have it. The mighty Ohm's Law.

What does it mean? How does it relate to the cart, the horse, the car battery and the circuit in Diagram 2?

The above formulas are a representation of the natural behaviours of the things and quantities that I explained above, when they are electrically connected as shown in Diagram 2.

Let me explain..

(a) Mathematical symbols V, I and P are already shown in Diagram 2.

(b) The symbol R resresents RESISTANCE. This is the resistance against the flow of electrons that is what we normally call electric current.

(c) If you refer to other websites about Ohm's Law, most websites (not all) usually only show the first three formulas as the immediate components of Ohm's law. However, if you recall how I explained the logic about the current, the voltage and the wattage of the incandescent lamp, you will notice that the RESISTANCE only emerged at the end, which was at the end of the horse's story. This is my own way of introducing the concept to you. I start with what is real and visible. Then use them to explain those that are invisible.

(d) The resistance is invisible. You cannot see the resistance, the "R".

(e) The volt is visible as the voltage of the battery. Take or buy any battery and you will see the voltage clearly written on it. A cell battery size AA will have 1.5V written, plus the positive (+) and negative (-) labels clearly indicated on each end of the terminals. A car battery would have 12V or 24V label plus the positive and negative terminal labels. These are all VISIBLE and REAL. That volt is the "V" in the formulas.

(f) The watt of the incandescent lamp would be clearly embossed or printed on the crystal glass, or the metal base of the bulb. A 15 watt incandescent bulb in the above circuit diagram would have 15W and 12V DC written on it. The label 12V DC indicates that the bulb has been manufactured to be used on a 12V direct current battery. The label DC is an additional information so that consumers do not connect it to some other types of voltage supply such as a 12V alternative current in a laboratory or in an electrical workshop. The bulb is real and the these information on the bulb is visible. The watt is represented by symbol "P" in the formulas.

(g) What about the electric current? The magnitude of the current is represented by the symbol "I" in the formulas. But is the current real? Is it visible? Yes, the current is real but it not visible anywhere. I had to do a calculation to know the magnitude of the current in the above circuit.

(h) And the resistance R?

Now do you feel that I am going in a circle?

That has been done on purpose. Electricity is invisible. That is the reason some people just cannot understand enough to be comfortable about it.

That the very reason a beginner need to appreciate and "feel" the logic between the four quantities of electricity: voltage (V), wattage or power (P), current (I) and resistance (R).

I ran you you around in circles to train your mind to the logic and relationships between these four quantitities.

I used analogies such as horses and carts to give you a picture that you can put to the logic and the relationships.

If you have difficulties in following on the train of the logic, keep reading this post. You will feel tired mentally. However, if you persist, sooner or later you you will be able to "see" the electricity: the invisible current, the invisible resistance, the invisible power and the invisible voltage.

It is the relationships between these electrical quantities that is called the Ohm's Law. The above formulas only help to summarize it.

I will end this post here. It is not yet finished. We need to play with numbers in order to have more "feel" for the formulas in the Ohm's Law.

See you in the next post.

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