Electrical resistance is a scale of physical magnitude that measures the tendency of a body to oppose the passage of an electrical current when it is subjected to an electrical voltage. This term is also used to refer to an element of an electrical circuit that hinders the circulation of electrical charges.
The resistance R is the inverse of the electrical conductance G, defined for a cylindrical conductor as:
R = 1 / G.
Resistance for alternating current circuits and for alternating electromagnetic fields is described in terms of impedance and characteristic impedance.
The device that measures electrical resistance is the ohmmeter.
How Does an Electrical Resistance Work?
The resistance to the passage of current is due to the fact that the electrical charges (ions or electrons) that flow through an electrical conductor collide with atoms of the conductor itself. When colliding, part of its kinetic energy is converted into heat. That is, one of the effects of the passage of current in a conductor is its heating. This effect is called the Joule effect.
The resistance of a conductor depends on the material with which it is made, its size and its temperature. As the temperature increases, the resistance increases. Likewise, as the temperature drops, the resistance decreases.
What Is the Unit of Measurement of an Electrical Resistance?
In the international system, the unit of measurement for electrical resistance is the ohm (Ω) in honor of Georg Simon Ohm.
In the case of direct current and in the absence of electromotive force within the considered conductor, Ohm's law applies:
R = V / I
- V the stress to which the body is subjected;
- I is the intensity of the current through the body.
Ohm's law is valid when it is used to express the resistance of the electric conductor in macroscopic form only when it has a constant geometry.
Types of Electrical Resistance of an Electronic Circuit
The types of resistors in an electronic circuit are the following:
- Carbon film electric resistors.
- Pressed carbon resistors.
- Glazed metal heating elements.
- Wirewound resistors.
- SMD resistors.
- Metal oxide film resistors.
- Cement resistor or power resistor.
- Variable resistors or potentiometers.
- Resistors dependent on temperature, light or voltage.
- Wirewound resistors.
To determine the resistance of each of these components there are color codes. Each resistor in an electronic circuit has colored bands that are assigned to numerical values that allow the total resistance of the element to be calculated.
Examples of Use of Electrical Resistors
The electrical resistances have many utilities in different fields; here we attach three practical examples:
Electric stoves: a stove is nothing more than an electrical resistance that heats up with the passage of electricity according to Joule's law.
Refrigerators and heat pumps: all electrical appliances that require heating a working fluid or water use electrical resistances.
Electronic circuits: electronic circuits use resistors to vary the voltage and current intensity at a certain point in the circuit according to Ohm's law.
What Is a Thermistor?
A thermistor is a type of electrical resistance whose value varies depending on the temperature. All resistors increase with increasing temperature, but a thermistor does so in a more noticeable way.
There are two types of thermistors depending on the sign of the temperature coefficient:
- NTC (negative temperature coefficient), with negative coefficient. The resistance of these thermistors drops with increasing temperature.
- PTC (positive temperature coefficient), with positive coefficient. Resistance increases with increasing temperature.
Dependent resistors are resistances in which the resistance to the passage of electric current (its ohmic value) depends on different parameters such as temperature, voltage or ambient light.