Intro: Voltage, Current and Resistance
In this instructable I will attempt to teach you about the three most fundamental things in electronics, the current, the voltage and the resistance.
Before we get started please keep in mind that I've tried to exclude as much about the physics that are involved into the subject as I could. This was done in order to make the basic concept as simple as possible to understand. This is an introduction to practical electronics not a physics lesson.
So, with that being said let's get started.
Step 1: The Hydraulic Analog
Let's say we have a pipe that forms a closed loop and is completely filled with water. Currently there is no water flow inside the pipe since there is nothing forcing the water to move.
But, if we insert a water pump inside the pipe and turn it on, the pump will start absorbing water from one side and expel it from the other. This will force water to flow on a specific direction creating this way a current.
Also, if we swap our pump with a more powerful one, the total mass of water that passes from any given point inside the loop will get increased, which in other words means that we'll have a bigger current.
Step 2: But What About Electronics?
Now you may wonder, what all this has to do with electronics. The answer is very simple, an electric wire is exactly the same as a pipe filled with water. An electric wire is made of a conductive material (e.g. copper) and that material is made of atoms, but unlike non-conductive materials in conductive ones the electrons of their atoms have the potential to flow inside them, like water can flow inside a pipe.
Of course, if we just have a piece of wire not connected to anything electrons have no reason to flow inside it. But, see what happens if we insert a battery, which is a voltage source, between the two ends of the wire. By doing that we are applying a force between the two ends of the wire, which will make the electrons inside it to move on a specific direction thus creating a current. We call this force voltage and the bigger the voltage the larger will be the current.
It's important to realize that a voltage source does not generate electrons, a voltage source works the same way as a water pump, it pushes electrons that are already inside a wire making them that way to move on a specific direction, which results to a current flow.
Another important thing to remember is that current always flows from the positive pole of a voltage source to the negative.
I know, electrons go the other way around but forget about electrons for a moment, current is something more abstract, it's the effect that electron flow has on a conductor and it just happens for as current direction, to use the opposite of that of electrons.
There is no reason making things more complicated than they are, just remember that current always flows from positive to negative and forget about anything else. It's just simple as that.
Step 3: The Resistance
Let's go back to our water circuit. Now let's make the pipe a little narrower on this spot. Doing so will have as a result that less water will be able to pass from that point at any given time. And since less water will be able to pass, the water flow on the whole loop will also get decreased. In other words, the amount of current on the whole loop will get decreased.
But why on the whole loop? I'll try to explain that using another analog, imagine you are in your car and you're driving on the highway. Currently there is a fair amount of traffic and you don't have any free space before or after you, but since everyone is moving at the speed you are also moving there is no problem, you just keep going.
Now imagine that there are some workers fixing a part of the road a couple of kilometers ahead of you. This will introduce a resistance on that point of the road since fewer cars will be able to pass from that point the same time. That will force drivers before that point to slow down, which will cause those after them to slow down too and finally making you to slow down.
Now imagine that instead of a highway you are driving on a road that forms a closed loop, if the same conditions apply as before the flow of traffic won't get decreased only before the point of resistance but on the whole loop. Since now each car that passes the resistance point doesn't leave the loop but continues going in cycles, the amount of traffic at each point of the loop will be the same.
Electrical resistance behaves the same way. If we add a resistor in series with the battery we add some amount of resistance to the loop. Doing so will make the current to get decreased on the whole loop, not just before the resistor. It's important to realize that the current anywhere on a closed loop is always the same. And the larger the resistance the smaller the current will be on the loop.
Finally, another thing worth mentioning is that resistance is a property of a conductor. A resistor is just a special conductor manufactured in such a way to have a predictable resistance of a specific value. It doesn't matter how big the voltage is, the resistance will always be the same.
Step 4: The Ohm's Law
As we said earlier, the bigger the voltage we apply on a closed loop circuit the larger the current will be. And as we also said, the bigger the resistance on a closed loop circuit the smaller the current will be. This whole conclusion can be expressed using a very simple formula:
This formula is called the Ohm's law and it is the most fundamental thing in electronics. What this formula tells us is that the amount of current that flows on a closed loop circuit is directly proportional to the voltage applied on that loop and inversely proportional to the total resistance of that loop.
In other words, the bigger the voltage we apply on a closed loop circuit the larger the current will be, but the same time the larger the total resistance that our loop has the smaller the current will be. We cannot have current without a voltage, the current is the result of a given voltage applied across a given resistance. Voltage is what makes current to flow and resistance what limits it. So we could say that voltage is what determines the current on a loop with a specific resistance.
Now that we know what the Ohm's law is, let's try to calculate the amount of current that flows on our circuit.
To built our circuit we used a standard AA battery which has a voltage of 1.5V and a resistor with a resistance of 1KΩ. So by using Ohm's law we get that the current that is flowing on the loop is 1.5mA.
Step 5: The Ground
As we already explained on our previous example when the water pump is activated it starts absorbing water from one side and expel it from the other. The water pump does this by applying basically a pressure difference between the two ends of the pipe. That pressure difference is what actually forces the water to flow. Voltage is like pressure, it can only exist between two points. And since voltage can only exist between two points it can also only be measured between two points, voltage is always relative.
Many times we want to compare different voltages on different points on a circuit, so having to measure voltage between two points all the time is not always very handy. That's why the concept of the ground was invented.
We define as ground the point on a circuit where the voltage is zero volts. We use ground as the default point to measure the voltage of any other point on a circuit relatively to it. Since the voltage is always a difference between two points on a circuit and we define ground as zero volts, the difference in voltage between any point of a circuit and the ground will always be equal to the voltage on the point we want to measure.
Ground is not an electric component it is just a concept that help us with our measurements, it is just the point on a circuit were we say that voltage is zero volts. And it is zero volts because we say it is zero volts, it is just our point of reference.
Many times on a circuit there is a fair amount of points that need to be tied to the ground. So, the schematic designer in order to make the diagram easier to read, instead of joining all those points together using straight lines, he/she simply adds a ground symbol on those points.
Every time you see more than one ground symbol it simply means that all those points are tied together to the ground point. All ground symbols on a circuit diagram are actually the same point on the real circuit. The circuit diagram below describes the exact same circuit as the previous diagram.