In the world of electronics and power systems, electrical resistance is a fundamental concept. It is a physical quantity that characterizes the ability of a conductor or a section of a circuit to counteract the passage of electric current. It is thanks to this property that we can control the flow of electrons, convert electrical energy into heat or light, and protect complex circuits from overload. Understanding the nature of this phenomenon is necessary for every engineer, electrician and enthusiast working with electrical equipment.
Historically, the unit of measurement of this value in the International System of Units (SI) is called Om. In honor of the German physicist Georg Simon Ohm. If a voltage of one volt occurs at the ends of the conductor, and the current strength is one ampere, then the resistance of such a conductor is equal to one ohm. In real practice, values can range from fractions of ohms in powerful power lines to millions of ohms in dielectrics that isolate conductors from each other.
The physical essence of the process lies at the micro level: free electrons moving under the influence of an electric field constantly collide with atoms of the crystal lattice of the material. These collisions cause the chaotization of motion and the transfer of some of the energy to atoms, which manifests as heating of the conductor. Ohm's Law For a section of a circuit, it mathematically describes this dependence by linking three key parameters: voltage, current strength and desired resistance.
Physical nature and dependence on conductor parameters
The resistance of any uniform conductor is directly proportional to its length and inversely proportional to the cross-sectional area. This means that the longer the wire, the more obstacles the electric current encounters in its path. Conversely, the thicker the conductor, the easier it is for electrons to move inside it, as the available space for their movement increases. This relationship is described by the classical formula that connects the geometric dimensions and properties of the material.
The key parameter here is the resistivity, which is a tabular value for each chemical element and alloy. For example, silver and copper have very low values of this parameter, making them ideal materials for power transmission. At the same time, nichrome or tungsten have a high resistivity, which allows them to be used in heating elements and incandescent lamps, where an effective conversion of electricity into heat is required.
⚠️ Note: As the temperature of most metals rises, their electrical resistance increases. This is due to the increased amplitude of the oscillations of atoms in the crystal lattice, which makes it difficult for electrons to move.
Temperature resistance is another important characteristic that cannot be ignored when designing circuits. For pure metals, it is positive, and for semiconductors and electrolytes it can be negative. This means that when semiconductor devices are heated, their conductivity can increase dramatically, which sometimes leads to thermal breakdown and failure of the device.
Ohm's Law and Basic Calculation Formulas
The main tool for calculations in DC circuits is Ohm's law. It states that the current strength on a portion of the circuit is directly proportional to the voltage applied and inversely proportional to the resistance of that portion. This formulation makes it easy to calculate any of the three parameters if the other two are known. Engineers use these calculations daily to select components and analyze equipment operating modes.
For convenience of calculations, the Ohm’s law triangle is often used, covering the desired value with a finger. If it is necessary to find a voltage, multiply the current strength by resistance. If you want to know the current, divide the voltage by resistance. And to find resistance, divide the voltage by the current strength. This simplicity makes Omaom's law of all electrical engineering.
In AC circuits, the situation is somewhat complicated by the appearance of reactive resistance, which depends on the frequency of the signal. However, for active elements such as resistors, Ohm's law holds true at any given time. It is important to consider that in complex schemes, the total resistance (impedance) consists of the active and reactive components.
R = U / IWhere R - resistance (Om), U - voltage (Volt), I - current (Ampere). Using this formula, you can quickly determine the parameters of the chain section. For example, if 12 volts are applied to the resistor and the current is 0.5 ampere, then the resistance is 24 ohms.
When calculating, always translate all values into the basic units of the SI: Volts, Ampere and Ohms to avoid errors with the orders of numbers (millies, micro, kilo).
Classification of resistors and their application
In electronics, devices with a given electrical resistance are called resistors. They are the most common elements on PCBs and serve to limit current, divide voltage and create temporary delays. Resistors are constant, the value of which does not change during operation, and variables that allow you to adjust the parameters of the circuit manually.
Permanent resistors are made of various materials: carbon, metal film, wire. Each type has its advantages. Metal film elements have high accuracy and low noise level, which is critical for audio equipment. Wire models are able to dissipate more power and are used in power electronics and loading devices.
Variable resistors, or potentiometers, are widely used in household appliances to adjust the volume, brightness of light or speed of rotation of engines. Their design involves moving the contact slider along the conductive layer, which changes the length of the active section and, therefore, the amount of resistance. There are also adjustable resistors that are only adjustable when the equipment is set up and rarely change during operation.
- 🔹 Linear resistors The change in resistance occurs proportionally to the angle of rotation of the handle.
- 🔹 Logarithmic resistors Most often used in audio technology to match human hearing.
- 🔹 Multi-turn potentiometers - allow you to perform very accurate configuration of the parameters of the scheme.
Serial and parallel connection
In real electrical circuits, resistors are rarely used alone. Most often, they are combined into groups to obtain the necessary denominations or power distribution. There are two main types of connection: serial and parallel. Each of these methods affects the overall resistance of the chain in different ways.
In a series connection, the end of one element connects to the beginning of the next. Current in such a circuit flows through all elements without branching, so the current strength is the same everywhere. The total resistance in this case is equal to the simple sum of the resistances of all the resistors included in the chain. This is the easiest way to increase the overall resistance of the site.
Parallel connection implies that all the beginnings of resistors are connected at one point, and all the ends at another. In this case, the voltage on all elements is the same, and the current is divided between the branches. The total resistance of such a chain is always less than the resistance of the smallest resistor included in it. The formula for calculating two elements looks like the product of their resistances divided by the sum.
⚠️ Note: When two identical resistors are connected in parallel, the total resistance is reduced exactly twice. This is a useful rule for quick calculation of the parameters of the scheme.
Formula for three or more parallel resistors
To calculate the total resistance R generals in parallel connection of three or more resistors, the formula is used: 1/R general = 1/R1 + 1/R2 + 1/R3. After summing the inverse values, the result must be inverted again.
Table of dependence of parameters of conductors
For a visual comparison of the influence of various factors on electrical resistance, it is convenient to use tabular data. Below are the resistivity values for common materials at 20 degrees Celsius. These data are reference and may vary slightly depending on the purity of the material.
| Materials | Specific resistance (Ohm·mm2/m) | Temperature coefe. (1/°C) | Application |
|---|---|---|---|
| Silver | 0.016 | 0.0040 | High-precision electronics |
| Copper. | 0.0175 | 0.0043 | Cable products, windings |
| aluminum | 0.028 | 0.0042 | Power lines |
| Iron | 0.098 | 0.0060 | Cores, constructive. |
| Nichrome | 1.1 | 0.0001 | Heating elements |
From the table, it can be seen that nichrome has a resistance almost 60 times greater than copper. That is why nichrome wire of the same length and thickness will be heated much more when passing through it current. Aluminum, although inferior to copper in conductivity, is much easier and cheaper, which determines its mass use in the energy sector.
Measurement methods and tools
Various measuring instruments are used to determine the resistance in laboratory and field conditions. The most common tool is a digital multimeter. Before measuring, you must make sure that the circuit is de-energized, as measuring resistance under voltage can cause damage to the device.
The multimeter measurement process involves the selection of a range of measurements. If the denomination is unknown, it is recommended to start at the maximum limit and gradually reduce it to improve the accuracy of the readings. The chips of the device should be tightly pressed to the contacts of the measured element, avoiding touching metal parts with your hands, so as not to make an error from body resistance.
☑️ Checking before measuring resistance
For measuring very small resistances, such as relay contacts or powerful motor windings, conventional multimeters can give a large margin of error. In such cases, microohmmeters or the four-wire connection method are used, which allows to exclude the effect of the resistance of the measuring wires on the result.
The accuracy of resistance measurement depends on the quality of contact of the probes and the absence of parasitic currents in the measured circuit.
Practical importance and safety
Understanding the nature of electrical resistance is critical to safety when working with electrical installations. Insulating materials have a tremendous resistance, which prevents current leakage and damage to humans by electricity. Violation of the integrity of the insulation or its moistening sharply reduces resistance, creating a risk of short circuit.
In everyday conditions, knowledge of the basics of resistance helps to correctly select the cross-section of wires for connecting powerful consumers. If the wire is too thin, its resistance will be high, which will lead to excessive heating and possible fire. Therefore, when installing wiring, it is always necessary to perform calculations of current load.
In addition, the resistance of the human body is also variable and depends on many factors: skin moisture, the presence of damage, the path of current passage. In dry condition, skin resistance can be hundreds of thousands of ohms, but when moisturized drops to 1000 ohms or less, making even low voltages deadly.
How does resistance affect energy loss in power lines?
When transmitting electricity over long distances, a significant portion of the power can be lost as heat due to the resistance of the wires. To minimize these losses, high voltage (which reduces current strength at the same power) and large cross-section wires of low resistivity materials such as steel core aluminum are used.
Why is the phone charger warming up?
Heating occurs due to the internal resistance of the charger components and voltage converters. Some of the electrical energy is inevitably converted into heat according to the Joule-Lentz law. High-quality devices have heat removal systems and more efficient circuits that minimize losses.
Can resistance be negative?
In passive elements, resistance is always positive. However, in active electronics, there is the concept of "negative resistance", which is a differential characteristic of some devices (for example, tunnel diodes) at certain sections of the volt-ampere characteristic, where the increase in voltage leads to a drop in current.
What happens to resistance in superconductivity?
When certain materials are cooled below the critical temperature, their electrical resistance drops to absolute zero. A current in a closed superconducting circuit can circulate for years without losing energy. This phenomenon is widely studied for use in MRI magnets and particle accelerators.