Electrical Resistance - Definition, Formula, FAQs
Resistance is a basic and very important topic in electricity. When electric current flows through a wire, it does not move freely. The wire offers some opposition to the flow of current, and this opposition is called electrical resistance. You can understand it like water flowing through a pipe. Narrow pipes oppose water flow more than wide pipes. In the same way, different materials oppose electric current differently. Resistance helps us control current in electrical circuits and is used in many devices like bulbs, heaters, fans, and fuses. In this article, you will learn what resistance is, its formula, factors affecting resistance, applications, resistivity, and the difference between resistance and resistivity, explained in very simple and student-friendly.
This Story also Contains
- What is Resistance?
- Applications of Resistance
- What is Resistivity?
- Solved Examples on Resistance
What is Resistance?
It is the property of a material that opposes the flow of electric current through it. It decides how easily or with difficulty electric charges are moved within the conductor and there is energy dissipated as heat. The resistance unit is measured in ohms, (Ω). Some factors that affect resistance depend on the material, length, cross-section area, and the temperature of the conductor.
What is Electrical Resistance?
Resistance meaning in physics is a measure of the opposition to current flow in an electric circuit (also known as ohmic or electric resistance). Ohm is the unit of resistance denoted by the Greek character Omega (Ω). The greater the resistance, the greater the flow barrier.
The flow rate of the electrons and electric current is reduced due to colliding or obstacles. Therefore, we might say that the passage of electrons or current is opposed. This barrier to the flow of electric current offered by a substance is therefore called electrical resistance.
The resistance of conductors is estimated to be
1. The electrical resistance of the material is directly proportional to the length of the material.
2. The electrical resistance of the material is inversely proportional to the material's cross-sectional area.
3. The electrical resistance of the material is dependent on the material's composition.
4. The temperature is a factor.
The Resistance Formula
$
\begin{aligned}
R & \propto \frac{l}{a} \\
R & =\rho \frac{l}{a} \Omega
\end{aligned}
$
where,
R denotes the conductor's resistance.
I is the conductor's length.
$\mathrm{a}=$ conductor's cross-sectional area.
$\rho=$ the material's proportionality constant, often known as its specific resistance or resistivity.
The Ohm is the unit of electrical resistance.
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Factors Affecting Resistance
The resistance of a conducting wire is caused by free electrons colliding in the conductor as they drift toward the positive end.
The electrical resistance of a material, such as a wire or a conductor, is determined by the following variables:
1. The material's length.
2. The material's surface area.
3. Temperature.
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Nature of the Material
- Conductors: The resistance of conductors is quite low. It's important to remember that copper has a very low resistance but a very high conductivity, which is why it's utilized as a connecting wire. Other conductors, such as gold and silver, can conduct electricity as well.
- Insulators: Insulators provide extremely high resistance.
- Pure semiconductors, which have extremely high resistance, exist between the conductor and the insulator.
- Alloys: Manganin and Constantan alloys have low resistance; therefore their lower lengths are required to make standard resistances for wires of a particular diameter.
The Temperature of the Material
As a material's temperature rises, its thermal energy rises as well, causing ions/atoms in a conductor to vibrate at larger amplitudes and frequencies. The relaxation time decreases when the free electrons begin to wander towards the conductor's positive end. The electrical resistance of the conductor rises as a result of this.
Applications of Resistance
- Used to control the flow of electric current in a circuit
- Used in heating appliances like heater, iron, and toaster
- Used to protect electrical devices from excess current (fuse)
- Used to divide voltage in electrical circuits
- Used to control the speed of fans and electric motors
What is Resistivity?
Electric resistivity is defined as the Electrical resistance offered per unit length and unit cross-sectional area at a given temperature. Specific Electrical resistance is another name for Electrical resistivity. Electrical resistivity is measured in ohms-meters, which is the SI unit.
Difference Between Resistance And Resistivity
| Parameters | Resistance | Resistivity |
| Definition | When the flow of electrons is opposed in a material is known as resistance |
When resistance is offered |
| Formula | $ R=\frac{V}{I} $ | $ \rho=\frac{E}{J} $ |
| Sl unit | $\Omega$ | $\Omega.m$ |
| Symbol | R | $\rho$ |
| Dependence | Dependent on the length and cross-sectional area of the conductor and temperature | Temperature |
Solved Examples on Resistance
Example 1: A wire has a resistance of $\mathbf{1 0} \boldsymbol{\Omega}$. A potential difference of $\mathbf{5 V}$ is applied across it. Find the current.
Using Ohm's law, $I=\frac{V}{R}=\frac{5}{10}=0.5 \mathrm{~A}$.
Current $=0.5 \mathrm{~A}$
Example 2: A current of 2 A flows through a conductor when 8 V is applied. Find the resistance.
$R=\frac{V}{I}=\frac{8}{2}=4 \Omega$.
Resistance $=\mathbf{4} \boldsymbol{\Omega}$
Example 3: The resistance of a wire is $\mathbf{5} \boldsymbol{\Omega}$ and the current is $\mathbf{3}$ A. Find the potential difference.
$V=I R=3 \times 5=15 V$.
Potential difference $=15 \mathrm{~V}$
Frequently Asked Questions (FAQs)
The resistivity of pure metals increases with increasing temperature. The reason for this is because as the number of electrons in the conduction band grows, the mobility of those electrons decreases, increasing resistance.
Superconductors
When an electric current passes through a conductor, the ions/atoms collide with one other at high amplitudes and frequencies, forming a barrier to current flow. This barrier creates resistance in metals, circuits, and other materials.
No, when the temperature rises, the resistance of a semiconductor lowers. The electrons in the valence band gather enough thermal energy to move to the conduction band as the temperature rises. The conductivity increases as the number of electrons in the conduction band grows, and the electrical resistance decreases.
The resistance of insulators diminishes as temperature rises. The reason for this is that when the energy gap between these two bands is considerable, electron transport from the conduction to the valence band increases. As a result, resistance lowers while conductance rises.
Conductivity is the reciprocal of resistivity.