Voltage, current, resistance, Ohm’s law.

You have never studied electronics it’s probably a good idea to learn about basic quantities voltage, current and resistace before you continue with electronics. If you know about it it’s still good to read this in order to confirm you don’t have bad knowledge about something. It’ll probably be boring because it’s mostly theory, but I’ll try to spice it a bit.

Before I start

I assume you’ve all studied physics and you know about electrons, protons … and stuff. If you don’t it will help to read about it, but it’s not 100% required. For those that don’t know (or don’t remember) what electrons and protons are, and don’t want to search and read about it I’ll say with just few words.  You probably know that the atom is the “smallest” particle … until someone found out that the atom consists of even smaller particles – a core that consists of neutrons and protons that stay in the center and electrons that fly arount the core.  The electrons are really small (light) particles compared to the neutrons and protons. They have charge and it’s negative (probably the most important part). The protons have positive charge and they are 10 000 times heavier than electrons. Neutrons do not have charge so they are not interesting for this article. If you want to know more – read the wiki page.

Also I want to state that I simplified some of the terms and didn’t mention others in order to make it easier to understand. So keep that in mind while you read so you don’t get surprised later. As a quick example – the rechargeable batteries. I didn’t say what is the voltage difference when you connect them. I didn’t mention it because I had to explain what internal resistance is and why one of the batteries can have 1.4 volts, the other 0 volts and yet between the two positive terminals is zero when you connect them.


Why do I start with current and not voltage? Well current is (the simple way to say it) a flow of electrons.  Because I’ll speak about positive and negative charges moving I want to clarify that only electrons can move from one atom to another. Protons do not move!!! When when we say that positive charges are moving around actually it means that the lack of electrons is moving. Yes, it is supposed to be confusing. As you know the electron can fly away if given enough energy. But what’s left then – an atom with one electron less than it’s supposed to have. That’s positively charged atom. When we say that positive charges move we don’t mean that the positive atom is moving. It means that if an electron from another atom comes to the positively charged atom then it looks like the positive charge moved to the other atom.

Well I’m trying to confuse you with movement of positive charges because in electronics we say that the direction of current is from (+) to (-). Just take it as it is – current flows from (+) to (-). Even though in metals usually electrons are moving (from – to +), it’s a lot simpler to take one direction for current and it’s from plus to minus.  Remember that. It’s important.

Current direction

Current direction (source wikipedia)

So the current can exist everywhere. Even in dielectrics which usually are not conductive, but in real world they conduct very little amount of current.

Still not get it? Don’t worry try, try again until you understand. Keep reading and then get back to this paragraph.

Almost forgot. Current is measured in amperes.


As stated in wikipediaVoltageelectrical potential differenceelectric tension or electric pressure“. It means that voltage can’t exist in one point (because it’s a difference of two things).  It’s hard to explain that. Let’s take air pressure as compassion. Let’s say we have two balloons – one full of air and one empty.  There is a pressure in one of the balloons of some value (not 0). There is no pressure in the other balloon or we say that the preasure is 0. There is preasure difference so if air pressure was voltage we can say that there is voltage between the two balloons. However there is no air flowing from one balloon to the other so we can say that the current is zero. If you connect the two balloons with a pipe then air will flow from one of the balloons will flow to the other. Then we have both voltage and current. But after a while the preasure in the two balloons will be the same – half of the value. So if the air preasure was electric current we could say that there is no voltage between the two balloons – because the preasure is the same. 

Voltage is measured in volts.

Let’s exchange the two balloons for rechargeable batteries. One (Battery 1) is empty 1.4v and the other (battery 2) full (0v).  If we connect the negative terminals together. then the positive of one is 1.4V the positive of the other is 0. So between positive and negative terminals of battery 1 there is 1.4 volts. Between the positive and negative of battery 2 there is 0 volts. Current is not flowing, but we still have voltage. Also between the two positive (we always need two points when we speak about voltage) terminals there is 1.4 volts. If we connect the two positive terminals battery 1 will start discharging and battery 2 will start charging. If we stop before the voltage equalized we’ll have more voltage,let’s say 1.2V – completely random – in battery 1, and less voltage, let’s say 1V in battery 2. Then the voltage between the two positive terminals is 0.2V


We’ll not talk about current flowing when the switch is closed because it’s a bit complicated and we still haven’t mentioned resistance.

Ok. Let’s say that you have two 1000 volts batteries. If you connect the negative terminals and touch the positive ones will you die? No, because the difference is 0.

And of course you can’t understand completely any of the tree quantities before you read about all three of them. So after you read you rest you can get back to voltage again.


Resistance is a feature of materials. It shows how much materials DO NOT conduct current. Yes, it’s the opposite of (electrical) conductance. If you didn’t know that conductance is a thing, now you know. Conductance is measured in siemens (not the company). So all we say about resistance is true for conductance, but reversed.

Resistance is measured in Ohms – named to the famous scientist Georg Simon Ohm. I can’t thank him enough because ohms law is the simplest formula that solves most of the problems in electronics and I like when it’s simple.

So we say that a material – just any material – has a resistance 1 ohm when we put a potential difference of 1 volt and the current flowing is 1 ampere.  There is nothing more to it. If you know what current and voltage is, then resistance is just a formula of the other two:

Ohms law

U = R*I.

It’s that simple. You won’t believe how many problems are solved by just this little formula … as long as you know what current, voltage and resistance is. If you have a 12V (rated) light bulb and a 12V battery which for a change will give exactly 12V and not 11 or 10 or 13, and you connect the bulb to the battery and measure that it’s drawing 0.1A of current then you know that the resistance of the bulb is 12/0.1 = 120 Ohms. Simple! Now if you have a resistor of 10 ohms and you connect it to the same 12 volt battery then you know that the current flawing is 1.2A.


Power is the energy that is transferred in an electric circuit. The formula for power is P = U*I. Let’s take the 12V light bulb from above and see what’s it’s electric power: 12*0.1 = 1.2Watt.

Common misunderstandings about electric quantities.

A lot of people have bad idea of what voltage, current and power is. Especially when it comes to power supplies and circuits that draw power (electric load). For example if you have the same 12V/0.1A light bulb and you connect it to a power adapter which is rated 12V and 0.1A then the bulb will work just fine. What will happen if you connect it to power adapter that’s rated 12V/1A? Will the bulb blow? What about if you connect it to 400Watt computer power supply? Well the 12V/1A label on the power adapter means that it’s output is 12V and it can supply maximum current of 1A. That also means that it can supply less current than 1A. But if you check the ohms law: U=R*I or if you replace the known values U=12V, R of the bulb (we calculated) is 120 Ohms. So if the voltage is 12 volts the bulb is the only thing that affects how much current will flaw and the current will be exactly 0.1A regardless of the maximum current stated on the label of the power supply. It’s the same with the 400 Watt PC power supply. If you connect the bulb on the 12V output, then it’ll draw exactly 0.1A current and the power dissipated will be 1.2W. Yes, the 400W power supply won’t blow the bulb because the power and the current depend on voltage and the resistance of the load (except in special power supplies that supply constant current … but let’s leave them for later).

There you have it. What? You still don’t understand? Don’t worry, it’ll become more clear with practice. Every failed or successful circuit you make will help you understand the above better.

To make it easier for you – use the contact page to contact page to write me if you don’t understand something and I’ll explain it to you and make corrections to the article to make it more accessible. Or if you think something can be explained better – all criticism is welcome.

Good luck!


I’ve found easy to understand tutorials on the net. Well currently only one youtube channel, but I like how he explain things. Check these out and if you still do not understand something – ask me and I’ll try to change this article in order to make it more clear and easy to understand.

Posted in Beginners