What Does Resistance Mean in Electricity?
Electrical Resistance is a barrier caused to the current flow in the circuit.
While going to any special location with your family, you might have observed when your driver drives the car fastly, on encountering the obstruction on the road, he slows down the car.
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However, during nights, when it is impossible to see the roads clearly while driving at pace, your car jumps with a high jerk suddenly.
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Do you know what this obstruction is? Well, this obstruction is the resistance, and when this obstruction occurs to the flow of current in an electric circuit, it becomes electrical resistance.
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In this article, we are going to discuss, what is electrical resistance and the factors that affect electrical resistance.
What is Electrical Resistance of a Conductor?
The electrical resistance of a conductor is the obstacle posed by the conductor to the current flowing through it.
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We define the resistance of a conductor as the ratio of the potential difference ‘V’ applied across the ends of a conductor to the current ‘I’ flowing through its ends. The formula for the electrical resistance is:
R = V/I
(The resistance is symbolized by a letter ‘R’)
The S.I. unit of the resistance is Ohm, where:
1 Ohm = 1 Volt/ 1 Ampere = 1 V/1A
Thus 1Ω is defined as the resistance of a conductor through which one ampere of current flows through the conductor when a potential difference of 1 V is applied to its ends.
The dimensional formula for the resistance is: [M1L2T-3A-2].
Factors Affecting Electrical Resistance of Conductor
The resistance of a conducting wire is because of the collision of free electrons in the conductor while drifting towards its positive end.
The resistance of a material viz: wire, conductor depends on the following factors:
Length of the material
Area of the material
Temperature
Resistance on Increasing or Decreasing the Length
Consider two identical slabs of conductors each of length ‘l’ and cross-sectional area ‘A’. Let ‘V’ be the potential difference applied across either of the slabs/conductors and ‘I’ be the current flowing through it.
So, the resistance of the conductor is:
R = V/I
Now, placing the two identical conductors side-by-side, the total length becomes l + l = 2l. If the same potential difference is applied across both the slabs, the current becomes I/2. The resistance of the arrangement becomes:
R’ = V/I/2 = 2V/I = 2R…..(1)
Equation (1) states that on doubling the length of the wire or any conductor, the resistance also doubles, i.e., R ∝ I.
Area of the Conductor
Let’s consider a slab and cut it into two halves each of length ‘l’, and a cross-sectional area of ‘A/2’. When the potential difference is applied across the ends of a conductor and the current flowing through it is I/2, then the resistance becomes:
R’ = V/I/2 = 2 V/I = 2R…..(2)
Here, we can see from equation (2) that on dividing the conductor slab into two halves, i.e., on halving the area of cross-section of a conductor, the resistance doubles, therefore, R ∝ 1/A.
Nature of the Material
Conductors
Conductors have very low resistance. One must note that copper has very low resistance but its conductance is very high, that’s why copper is used as a connecting wire. While there are other conductors like gold and silver, they also conduct electricity.
If R is the resistance, then conductance ‘G’ is:
G = 1/R
Insulators
The resistance offered by insulators is very high. In between the conductor and the insulator, there are pure semiconductors, having very high resistance.
Alloys
Alloys like Manganin and Constantan offer low resistance, their smaller lengths are required for the wires of a given diameter in making the standard resistances.
Temperature of the Material
When the temperature of the material increases, the thermal energy of the material also increases because of which ions/atoms of a conductor start vibrating with higher amplitudes and frequencies.
As the free electrons start drifting towards the positive end of the conductor, the relaxation time reduces. This, in turn, increases the resistance of the conductor.
If R0 was the resistance of the material at 0℃ and Rf is the current temperature, then the rise in resistance with the rise in the temperature by t℃ is given by:
Rf = R0 (1 + ∝t + βt2)
Here, ∝ & β are the temperature coefficients of resistance whose values vary from metal to metal. The unit of 1/K or 1/℃.
In practical applications, is given by:
∝ = \[\frac{R_{f}-R_{0}}{R_{0} \times t}\] = \[\frac{increase \; in \; resistance}{original \; resistamce \times rise \; in \; temparature}\]
So, we define temperature coefficient as the increase in resistance per unit original resistance per degree rise in temperature.
The temperature of is different for different temperatures. Now, if the temperature varies, i.e., if the temperature ranges from t1℃ to t2℃, then ∝ is:
∝ = \[\frac{R_{f}-R_{0}}{R_{0} \times (t_{2}-t_{1})}\]
FAQs on Electrical Resistance
1. What is electrical resistance and how is it quantitatively expressed as per the CBSE 2025-26 syllabus?
Electrical resistance is the property of a material that opposes the flow of electric current through it. Quantitatively, it is expressed as R = V/I, where R is resistance, V is the potential difference, and I is the current. The SI unit of resistance is ohm (Ω), and 1 Ω = 1 V / 1 A.
2. What are the key factors that affect the resistance of a conductor?
Resistance in a conductor depends on the following factors:
- Length (l) of the conductor: Resistance increases with length (R ∝ l).
- Area of cross-section (A): Resistance decreases as area increases (R ∝ 1/A).
- Nature of the material: Different materials offer different resistances.
- Temperature: For most conductors, resistance increases with temperature.
3. How does temperature impact the electrical resistance of metals and semiconductors?
For most metals, resistance increases as temperature rises due to increased atomic vibrations. In contrast, for semiconductors, resistance decreases with temperature because more free charge carriers are generated at higher temperatures, improving conductivity.
4. Why do copper and other metals with low resistance get preferred for electrical wiring?
Copper has very low resistance and high conductivity, which means it allows electric current to flow efficiently with minimal energy loss. This makes it ideal for electrical wires, reducing heat generation and power dissipation during transmission.
5. What is the physical meaning of the SI unit ‘ohm’ for resistance?
The SI unit ohm (Ω) represents the resistance of a conductor when a current of 1 ampere flows through it under a potential difference of 1 volt. In other words, 1 Ω = 1 volt / 1 ampere.
6. How can you show mathematically that doubling the length of a wire doubles its resistance?
According to the formula R = ρl/A (where ρ is resistivity, l is length, A is area), if you double the length (making it 2l) and keep area the same, the resistance becomes R' = ρ(2l)/A = 2R. Thus, doubling the length doubles the resistance.
7. What happens to the resistance if you halve the area of cross-section of a conductor?
If the area of cross-section is halved (becomes A/2), the resistance becomes R' = ρl/(A/2) = 2ρl/A = 2R. Therefore, halving the area doubles the resistance.
8. How do the temperature coefficients α and β influence the resistance of a material?
The temperature coefficients α and β determine how much a material’s resistance increases with temperature. The relation is Rf = R₀ (1 + αt + βt²), where R₀ is resistance at 0°C and t is temperature rise. α and β vary for different metals and impact how quickly resistance changes as temperature changes.
9. What distinguishes conductors, insulators, and alloys based on their resistance?
- Conductors (e.g., copper, silver): very low resistance, high conductivity.
- Insulators (e.g., glass, plastic): extremely high resistance, do not conduct current.
- Alloys (e.g., manganin, constantan): moderate, predictable resistance, used for making standard resistors.
10. Why does resistance exist in a physical conductor at the atomic level?
Resistance is due to the collision of free electrons with atoms or ions in the conductor as current flows. These collisions impede the movement of electrons, causing resistance and resulting in energy loss as heat.
11. How is resistance different from conductance in an electric circuit?
Resistance (R) is the property that opposes the flow of current. Conductance (G) is the ability to allow current to flow, and is the reciprocal of resistance: G = 1/R. High resistance means low conductance, and vice versa.
12. Can electrical resistance have practical uses, or is it always a disadvantage?
Electrical resistance can be both useful and disadvantageous:
- Useful: In devices like heaters, light bulbs, and fuses, resistance converts electrical energy to heat or controls current flow.
- Disadvantage: In transmission lines, resistance causes energy loss and heating, which is undesirable.
13. If two identical wires are connected in series, what is the total resistance and why?
When two identical wires (each with resistance R) are connected in series, their resistances add up: R_total = R + R = 2R. This is because current must pass through both wires sequentially, facing the opposition of both.
14. Explain with examples how electrical resistance is observed in daily life.
Examples of resistance in daily life include:
- Electric heaters: High-resistance coils produce heat.
- Speed breakers: They resist the motion of vehicles, similar to resistors opposing current.
- Wires in electronics: Excessive resistance leads to warming of wires in high-current devices.
15. What are some misconceptions students have about how resistance changes with temperature?
Many students believe resistance always increases with temperature, but while metals do show increased resistance, semiconductors actually decrease their resistance as temperature rises due to more charge carriers becoming available.
