 Particles can carry negative or positive electric charge. Let us denote this charge with the letter small q. Charge is measured in the unit coulomb and abbreviated with the letter c. Particles like the protons that make up our universe carry a very small positive charge of 10 to the power of minus 19 coulomb. That is 18 zeros with a one behind the decimal point. This is a quite tiny amount of charge for our standards. Only if we take a lot of particles they will carry a noticeable amount of charge in sum. So let's take a lot of charges. In order that different particles don't get mixed up, we put the positive particles in one box and the negative particles into another box. We add up the individual charges small q together and get a total charge that we denote with a large letter q. Our oppositely charged boxes will attract each other and move towards each other. Our boxes are just like two big charges. We say they exert an electric force on each other. Let's do a thought experiment. We fix the two boxes so that they cannot move towards each other. In this way the opposite charges will remain separate from each other. The accumulation of negative charges in the one box is called minus pole and the accumulation of positive charges in the other box is called plus pole. The charges from both poles attract each other. They want to move towards each other. But they can't do that because we have fixed them each in two places. The separation of the positive and negative poles creates a voltage between the poles. We denote the voltage with a letter u. The voltage u is smaller when less positive and negative charges are separated. In this case the total charge q in the boxes is small. The voltage u is large when many positive and negative charges are separated. In this case the total charge q in the boxes is large. The voltage arises only when positive and negative charges are separated. In our case there are two charged boxes separated from each other. The voltage always refers to two different points. So always keep the question in mind voltage between which two points. So when you read or hear something about voltage always remember that somewhere positive and negative charges are separated from each other and there is in principle the possibility to release them to generate an electric current. But more about that a little later. So far you have only learned that charge separation is the cause of voltage. Now let's try to understand what voltage means physically. With this you can understand what a number like 5 volt means. For this purpose we take a small positively charged particle with a charge small q. Let's call this particle a test charge. With this test charge we can test how big the charge separation is and therefore how big the voltage between the poles is. Let's put the test charge directly on the positive pole. The test charge is attracted to the negative pole so it experiences an electric force. This force causes the charge to accelerate. Just before it hits the negative box the test charge reaches some velocity. But if a test charge has a velocity then it also has kinetic energy. So the test charge has gained energy by moving from the positive pole to the negative pole. Let us call this gained energy w. The energy is measured in joule and the unit is abbreviated with the letter j. Let's hold on to an important insight. Charge separation results in charged particles gaining energy as they travel the distance between the positive and negative poles. If we now divide this gained energy w by the charge q of the test particle we get a quantity that gives gained energy per charge w over q. And this quantity w over q corresponds to the voltage u. This formula translated into words means voltage u is a measure of how much energy w a charged particle would gain if it travels from the positive to the negative pole. Let's make an example. A test particle with q equals one kilo travels from the positive pole to the negative pole and gains let's say 10 joule energy. So the voltage u is equal to 10 joule per kilo. Here we abbreviate the unit joule per kilo with the letter v and call it volt. In this case we say there is a voltage of 10 volt between the positive and negative pole. If we conductively connect the two poles the positive charges can move to the negative pole. We assume here that the negative charges are firmly anchored in the negative box and cannot move. Otherwise they would also move along the connection to the positive pole. That would be okay but it would make our thought experiment unnecessarily complicated. Therefore we only consider the current of positive charges. The electric current i is the amount of charge per second that travels through this connection. It is clear that the charges only travel through this connection if there is any charge separation at all. Without charge separation there is neither a voltage nor a current here. We can exploit this current to make a lamp light up. The charges move and move but with time the number of positive charges in the box decreases. The charge separation decreases, the current decreases, the light shines dimmer and dimmer. As fewer charges are separated the voltage naturally decreases as well until there are no more charges in the positive box. Then the voltage is zero, the current stops flowing, the light stops shining. The charges are no longer separated from each other but are all in one box. The opposite charges have neutralized each other. The total charge in the box is now zero. How can we make it so that the light continues to shine constantly? We must keep the charge in the box constant so that the voltage in the current remain constant as well. Then the lamp will keep the same brightness all the time. If we supply charges by whatever means to the boxes then we have thereby created what is called a voltage source with which we can then generate a non decaying current which in turn causes the light to glow steadily. There is such a voltage source even in your room, it is a socket. The two holes of the socket form the two poles so to speak and if you connect them with a plug then you have 230 volt available without interruption. You can use this voltage to power a lamp or to charge your smartphone. This constant 230 volt is provided by a power plant somewhere. You may ask is voltage dangerous? Voltage in itself is not dangerous whether it be 10 volt or 10 000 volt. As long as the charges are separated nothing special happens. The positive charges are resting in one box and the negative charges in the other box. This situation is not dangerous although the voltage between the poles is not zero. Only when we conductively connect the two poles an electric current starts to flow and that current in turn can become dangerous. The current becomes more dangerous, the greater the voltage you and the better the connection between the poles especially when the connection is your own body.