Law of life for electronics and electricians, if you want to understand and enter into matter, you must first know the basics, and if we ignore all the tedious formulas of derivation and integration our first task would be to understand Ohm's law. (Or at least it's how I decided it. If in doubt, ask;)).
Where:
V = Voltage, which is measured in volts.
R = Resistance, which is measured in ohms.
I = Intensity, which is measured in Amperes.
W = Power, which is measured in Watts.
This image is designed for easy understanding of the formulas. I will give a simple example so that you can understand or remember. Surely when you were children, in my country is a habit, they sent you a project to make a circuit with light bulbs, a switch and batteries. A bulb is a resistance (filament), of special shape and material, very thin that when crossing a current through it, it is heated to the point of generating luminescence.
The circuit is something like this:
This circuit is similar to the one we all have in our homes to light a light bulb, in contrast to: instead of using a battery with a constant voltage, through the bulb it passes 120 Vrms (Vrms: is the equivalent of an alternating voltage to a continuous voltage, I will explain it another time) in a sine wave at a frequency of 60Hz (this may vary depending on where you live).
Suppose that the light bulb we have at home consumes a potency of: 12W, a value set by the manufacturer after being subjected to the necessary tests, and the voltage it has, is exactly 120Vrms. We have voltage and potency, then, in the resistance, it should will pass an intensity of:
I = W / V = 12 [W] / 120 [V] = 0.1 [A]
And the resistance of the wire that is in the light bulb is of:
R = V / I = 120 [V] /0.1 [A] = 1200 [Ohms]
As additional information, when a light bulb decreases its light intensity is due to the decreased voltage, example, suppose that the voltage decreased to 100Vrms.
W = V ^ 2 / R = 10000 [V ^ 2] / 1200 [Ohms] = 8.33 [W]
The potency generated (or consumated) decreased, so it will illuminate less. On the other hand suppose that there is a peak of voltage (a sudden increase of voltage, they usually happen), and increases to 140 Vrms:
W = 19600 [V ^ 2] / 1200 [Ohms] = 16.33 [W]
Too much potency for the light bulb and to prevent heat from bursting the glass, fulfilling the same function as a fuse (protection), the wire breaks to prevent fires or damage to third parties.
I hope you have liked them, comment, make your requests, share, etc., greetings;).