Electrical resistance is a measure of how much a material opposes the flow of electric current through it. It’s a fundamental concept in electrical engineering and physics, governing how easily or difficultly electrons can move through a conductor.
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Electrical Resistance Formula and Unit
According to Ohm’s law, The resistance R of an object is defined as the ratio of voltage V to current I through it.
The electrical resistance formula is (R = V / I), The unit of electrical resistance is the ohm (Ω).
where:
- V is the voltage (in volts),
- I is the current (in amperes),
- is the resistance (in ohms).
What Does Electrical Resistance Depend on?
The resistance of an object depends on the material it is made of, while some other factors can affect the value of the resistance. These factors are:
- Temperature.
- Conductor length.
- Conductor cross-sectional area.
Let’s dive into the details.
Effect of Temperature On Electrical Resistance
In metallic material, any temperature rising offers more resistance to the movement of electrons causing the current, so with increasing temperature the resistance of metallic conductor increases.
This is the reason of using cables derating factors tables when sizing cables for electrical loads.
These factors correct the current carrying capacity of the cable according to the surrounding temperature.
The higher the surrounding temperature, the higher the cable temperature and the lower the cable current carrying capacity because the cable conductor’s resistance increases.
I highly recommend using our CABLES TABLES android app which has cables derating factors tables. It’s 100% free, No credit card is needed.
Conductor Length
The resistance of a conductor is directly proportional to its length. A longer conductor has higher resistance, leading to a greater voltage drop across it.
When selecting a cable for a motor, first calculate the voltage drop. If it exceeds the acceptable limit, choose a cable with a larger cross-sectional area to reduce resistance and minimize the voltage drop.
Conductor Cross-Sectional Area and Resistance
The resistance of a conductor is inversely proportional to its cross-sectional area. A larger cross-sectional area results in lower resistance, helping to overcome voltage drop issues effectively.
What are Resistors?
A resistor is a piece of conducting material of a particular electrical resistance meant for use in electrical circuits.
Electrical resistors are made of a variety of materials depending on several factors such as the desired resistance, and amount of energy that it needs to dissipate.
How to Measure Resistance?
An ohmmeter measures resistance, but it can’t do so in a live circuit. To measure resistance accurately, the component must be isolated from the rest of the circuit.
To measure resistance with a multimeter, set it to resistance mode and place the probes on both ends of the resistor.
If the resistor is part of a circuit and can’t be isolated, use Ohm’s law to calculate the resistance by measuring the voltage and current.
By measuring the voltage (V in Volt) across the resistor (R in Ohm) and the current (I in Amp)passing through it.
Then, R = V / I
Electrical Resistance Calculations
Ohm’s law is a way of calculating resistance.
Ohm’s law is, R = V/I
Example of applying Ohm’s law
Assume we have an electrical circuit with a battery of 12 V, and a current of 2A, applying Ohm’s law;
R = V/I = 12 / 2 = 6 Ohms.
What is Resistor color code?
Resistor color code is a simple method to quickly identify the resistor value without using any measuring tools.
This color-identifying process determines the value of resistance and tolerance of a its value.
We can calculate the value of resistance for each resistor from the color pattern.
Is using color code Worthy to use?
Trust me, I found It a boring thing to measure each resistor to know its value while you have a lot of them on the working table. Color code is the magic stick that saves time and effort.
The electrical components are often coded with colors or numbers to identify their functions and values.
The same is the case for resistors. The rating and values are often written on the resistor as a color code, you can use the resistor color code method to find the value of the resistor quickly without the need for any multimeter.
How to Find the value of a resistor using color code?
Here’s a clearer version of the steps:
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Identify the type of resistor: First, determine whether the resistor is a 4-band, 5-band, or 6-band resistor. This will help you understand the number of color bands and their meaning.
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Find the direction: The first band is always closest to the lead (the metal legs of the resistor). The gold or silver band, which is used for tolerance, will be at the end of the resistor.
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Hold the resistor correctly: Place the resistor in your left hand, with the lead facing you. Look at the colors of the bands in sequence from left to right, and write them down.
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Use the color code table: Refer to the color code table to identify the value each color represents.
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Understand the multiplier: The band just before the tolerance band represents the multiplier (it tells you how to adjust the value you’ve already found).
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Interpret the tolerance band: The last band indicates the tolerance, usually gold or silver. This shows how much the resistor’s actual value can vary from its nominal value in percentage:
- If the last band is gold, the tolerance is ±5%.
- If the last band is silver, the tolerance is ±10%.
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Memorize the color values: You can use a mnemonic or a sentence to help you remember the values of the color bands.
BB ROY of Great Britain had a Very Good Wife”. The Color code table is given below.
(Black, Brown, Red, Orange, Yellow, Green, Blue, Violet, Gray and White)
Should I use color code or Ohmmeter?
Resistors are used in almost all electronic circuits like capacitors and transistors.
Not like other electronic components, there are many values of resistors. So, its much easier for you to use color code than using an Ohmmeter to measure each single resistor individually.
Ohmmeter use is easy but, in the case of making a large electronic engineering project which has many resistors I highly recommend using color code to identify the resistors.
Knarfili, CC0, via Wikimedia Commons
Do resistors reduce current or voltage?
Resistors affect both current and voltage in different manner. The current in a series circuit will be the same at any point, while the voltage will drop at the end of the circuit.
If the resistors are connected in parallel, they will reduce the passing current while the voltage across them still the same.
I have written a detailed article about series and parallel connection of resistors, I highly recommend reading it from here.
How does temperature Affect resistance of a conductor?
The higher a conductor’s temperature, the higher its resistance. As the conductor heats up, its atoms vibrate more, making it harder for electrons to flow, which increases resistance.
This is why the current-carrying capacity (ampacity) of power cables depends on the laying method. Effective cooling increases ampacity, while poor cooling reduces it.
To account for this, we apply derating factors, including temperature derating, when selecting a cable to ensure it can safely handle the required load.
I have written a detailed article about Cables Derating Factors, you can check it for more information.
Light Dependent Resistor – LDR
Some resistors, made from semiconductors, show photoconductivity, meaning their resistance decreases as light increases.
Therefore, they are called light dependent resistors (LDR), and are used in light detecting circuits. And the most common usage of them is in light auto control circuits.
LDR Symbol
The symbol of LDR in electrical circuit wiring diagram.
An LDR (Light Dependent Resistor), also known as a photoresistor, is a light-sensitive device whose resistance depends on the light falling on it. Made from high-resistance semiconductor materials, it’s also called a photoconductive cell or photocell.
What does a light dependent resistor do?
A Light Dependent Resistor (LDR) detects light levels and is commonly used in applications like automatic streetlights and factory lighting systems to control when lights turn on or off.
In low light or darkness, its resistance is high, limiting current flow. As light increases, its resistance decreases, allowing more current to pass.
Value of light dependent resistor in dark and light
Depending on the light and darkness, a photoresistor resistance can vary from about 100Ω in the day light up to 10MΩ in complete darkness.
How to use LDR?
The resistance of photo-resistor (LDR) drops in light, which means the current to increase (I=V/R), while in darkness LDR resistance increases.
This variation in value is used in electronic circuits to control a relay which is used to switch on and off the lights.
Does an LDR have polarity?
No, an LDR (Light Dependent Resistor) does not have polarity. This means it does not have a positive (+) or negative (-) side, and it can be connected in any orientation within a circuit without affecting its functionality.
Why LDRs Have No Polarity:
- Resistor Characteristics:
- Resistors are passive components that oppose the flow of electrical current.
- Unlike components such as diodes or capacitors (which may have polarity and behave differently based on how they are connected), resistors do not depend on the direction of current flow.
- LDR as a Special Resistor:
- An LDR is a type of variable resistor whose resistance changes based on the intensity of light falling on it.
- Since it is essentially a resistor, it inherits the fundamental property of resistors: lack of polarity.
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