A circuit is a complete path around which electricity can flow. You can think of it like a pipe system with water flowing through, and each component in the system is going to use some of the water. So the amount of total water being used is power. The difference between the amount of water across a pipe is the voltage difference. At the end of the pipe system where there is no water is the ground. The speed of the water is current in a circuit, and the water is charge in a circuit. The more charge you have, the more energy and the more heat, so a higher amount of charge and current passing through will make your circuit start to get warm!
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Now that you know a little bit about circuits, lets talk about our circuit. Our LED's consume about 2-3 volts, so you can think about them as a plant that is consuming a certain amount of water. The current represents the rate at which the water is being delivered to each plant. We will be using Arduinos to power our circuits. The arduino's will be connected to laptops, so they will have a lot of power! But the problem is that each arduino always supplies 5 volts to the circuit. Since the LED can only use 2-3 volts, we need to use up 1-2 volts somewhere else in the circuit. So we add a resistor, which basically just wastes the extra voltage. So you can think of it as having another plant before our LED plant, which soaks up some of the water so that the LED gets the correct amount of water and doesn't drown.
So now let's think about when we make five led's light up. We need to make 5 different circuits. However, since we have a really large power supply from the arduino(pool of water) all the circuits can be connected to the same power source since there is enough water/charge to provide each circuit with 5 volts of power, and the ground will all be the same as well!
Here are some questions to consider
if each LED has a current of 5 mA, meaning they all receive charge at this rate, what is the total current/rate of charge that the power is supplying? Think about it like this, what is the amount of mA entering at node A given that each wire after node A gets 5mA.
