This can be said when working with direct current (constant value over time and non-variable polarity), in alternating current (variable value and polarity that changes between positive and negative) several other non-negligible parameters intervene that must necessarily be taken into account, they are the frequency f (how many times a certain current passes through zero in the unit of time (1 second), the period S (the inverse of the frequency) the capacitive reactance Xc (capacitor)), the inductive reactance Xl (inductor) and the phase shift φ (fi). These quantities combined with the resistance create another equivalent quantity (i.e. born from the combination of all the parameters) called electrical impedance and indicated with the capital symbol Z, furthermore the alternating current (used in sinusoidal form) must be represented in vector form, and therefore being a mixed quantity it has a real part and an imaginary part (complex numbers), the imaginary part is separated from the imaginary part in the formulas by the letter J (iota).

There are materials that oppose the flow of current more than others, that is, they have a higher specific resistivity, they are insulators (rubber, glass, PVC, dry wood, plastic, distilled water). Unlike insulators, conductive materials have a very low specific resistivity and oppose little to the passage of electric current (copper, gold, silver, aluminum, brass, generally metallic materials). All this is due, as previously mentioned, to the specific resistivity (symbol ρ) (ro) specific to each material, definable as the bond between the electrons and the nucleus, more or less strong, and which opposes the movement of electrons from one atom to another. To know the specific electrical resistivity of a given material you can consult the table that I have prepared for you, and that you can find at this link.

There are also special materials called semiconductors (germanium and silicon) which have a neutral specific resistivity. Semiconductors treated with a process called "doping" are the basis for obtaining photovoltaic cells, transistors and integrated circuits that we will discuss later on the active components page.

But how is electrical voltage produced? The simplest generator is the electric battery (Volta's battery, later improved by Leclanchè), which produces a continuous voltage, and is made up of disks of different conductive materials superimposed and immersed in an electrolyte (galvanic effect). Rechargeable batteries are currently also available, which are correctly called "electric accumulators" which, after a recharging process, subsequently return the accumulated energy.
The amount of energy that can be accumulated is defined as "capacity" and for small capacities nickel-cadmium batteries or the improved nickel-manganese type are used, for large capacities lead-acid batteries or lithium batteries are used, each of them has positive and negative aspects, and one model rather than another is used based on the required starting current, the capacity and the space available.

To conclude, I cite the formulas to calculate at least the main parameters exposed before, that is, voltage, current and resistance. To do this, "Ohm's law" comes to our aid, which allows us to calculate one of the parameters knowing the other two. By convention, voltage is indicated with V, current with I and resistance with R. Having said this, let's move on to the formulas. Voltage V= R*I Current I= V/R Resistance R= V/I The value of the electrical power is instead given by the multiplication of voltage and current, in essence, W=V*I