Resistors

Resistors make it harder for electrons to flow through a circuit, and that’s why we add resistors to the circuits because they reduce the current, it’s like having a kink in a pipe.

This will add resistance to the flow of water, which reduces how much water can flow through. And as the water is colliding with the pipe wall, it is going to lose energy. So we get a pressure drop. The same with a resistor. The material makes it difficult for electrons to flow through. The electrons are going to collide and waste energy so we get a voltage drop. This wasted energy needs to go somewhere, so it leaves as heat.

If we look at a resistor through a thermal imaging camera, we can see that heat leaving. For example, this LED  is rated for a maximum of 22 milliamps or 0.022 amps. We want to connect this to a nine volt power supply. If we use a 100 ohm resistor, then the current would be 0.09 amps, which is too much. And the LED will burn out. If we use a 450 ohm resistor than the current is 0.02 amps, which is below the limit.

So that should be OK. If we use a 900 ohm resistor, then the current is 0.01 amps, which is far too low, so the LED will not shine brightly.