The ohm (symbol: Ω) is the SI unit of electrical resistance, named after Georg Simon Ohm. An ohm is equivalent to a volt per ampere. Since resistors are specified and manufactured over a very large range of values, the derived units of milliohm (1 mΩ = 10−3 Ω), kilohm (1 kΩ = 103 Ω), and megohm (1 MΩ = 106 Ω) are also in common usage.
The reciprocal of resistance R is called conductance G = 1/R and is measured in Siemens (SI unit), sometimes referred to as a mho. Thus a Siemens is the reciprocal of an ohm:S = Ω − 1. Although the concept of conductance is often used in circuit analysis, practical resistors are always specified in terms of their resistance (ohms) rather than conductance.
The behavior of an ideal resistor is dictated by the relationship specified in Ohm's law:
Ohm's law states that the voltage (V) across a resistor is proportional to the current (I) passing through it, where the constant of proportionality is the resistance (R).
Equivalently, Ohm's law can be stated:
This formulation of Ohm's law states that, when a voltage (V) is present across a resistance (R), a current (I) will flow through the resistance. This is directly used in practical computations. For example, if a 300 ohm resistor is attached across the terminals of a 12 volt battery, then a current of 12 / 300 = 0.04 amperes (or 40 milliamperes) will flow through that resistor.
Main article: Series and parallel circuits
In a series configuration, the current through all of the resistors is the same, but the voltage across each resistor will be in proportion to its resistance. The potential difference (voltage) seen across the network is the sum of those voltages, thus the total resistance can be found as the sum of those resistances:
As a special case, the resistance of N resistors connected in series, each of the same resistance R, is given by NR. Resistors in a parallel configuration are each subject to the same potential difference (voltage), however the currents through them add. The conductances of the resistors then add to determine the conductance of the network. Thus the equivalent resistance (Req) of the network can be computed:
The parallel equivalent resistance can be represented in equations by two vertical lines "||" (as in geometry) as a simplified notation. For the case of two resistors in parallel, this can be calculated using:
As a special case, the resistance of N resistors connected in parallel, each of the same resistance R, is given by R/N.
A resistor network that is a combination of parallel and series connections can be broken up into smaller parts that are either one or the other. For instance,
The power P dissipated by a resistor (or the equivalent resistance of a resistor network) is calculated as:
The first form is a restatement of Joule's first law. Using Ohm's law, the two other forms can be derived.
The total amount of heat energy released over a period of time can be determined from the integral of the power over that period of time:
IDENTIFICACIÓ PER CODI DE COLORS
Exemple: Si un resistor té les bandes colors següents: VERMELL GROC VERMELL OR
- El resistor té un valor NOMINAL de 2400,000 Ohms +/- 5 %
- El valor màxim d'aquest resistor és de: 25200,000 Ω
- El valor mínim d'aquest resistor és de: 22800,000 Ω
- El valor REAL del resistor es troba els valors màxim i mínim.
Els colors dels resistors no indiquen potencia que pot dissipar, per la mida que té dóna una idea de la dissipació màxima que pot tenir.
Els resistors comercials dissipen 1/4 watt, 1/2 watt, 1 watt, 2 watts, etc..
Com més gran el resistorm, més disipació de potencia (calor).
Ley de Joule – Potència en una resistència
Abans de conèixer la Lley de Joule (potència en una resistència o conductor), primer cal entendre el concepte d'energia.
Energia: És la capacitat o l'energia necessària per a realitzar un treball. Per ejemple, si es connecta una bateria o una pila a un focus s'observa que l'energia entregada es converteix en llum i es dissipa calor calor.
La unitat d'energía és el Joule (J), i la rapidesa amb la que es consumeix aquesta energia es mesura en Joules/segon. A aquesta magnitud se l'anomena POTÈNCIA. També es pot dir que dóna idea de la velocitat amb la que es realitza un treball.
Potència: Aleshores, la potencia és la velocitat amb la que es consumeix energia, i la fórmula és: P = W / T (potència = energia x unitat de temps). Si es consumeix un Joule en un segon es diu que s'ha consumit un Watt (Vat) de potència.
Podem calcular la potència a partir de la fórmula P=V.I
En el cas de les resistències, també es poden fer servir aquestes fórmules aïllant de Llei d'Ohm:
P = V2/R: si no coneixem el corrent
P = I2xR: si no coneixem la tensió