Unit of Resistance

Unit of Resistance

Edited By Vishal kumar | Updated on Jul 02, 2025 04:39 PM IST

When studying electric currents, students frequently come across the term "resistance." While there are two elements that affect electric current, conductance and resistance, we will focus on the latter on this page. When it comes to resistance, it's simply described as the impediment that a substance creates in the flow of electric current and it is denoted by the letter R. We'll look at the resistance unit and other details further down. The unit of resistance is the ohm.

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Unit of Resistance

What is the unit of resistance?

SI unit of resistance in physics or unit of resistance in physics:

The ohm, which is defined as a volt per ampere, is the si unit of resistance. In honour of German physicist George Simon Ohm, the name has been preserved. The unit is sometimes written as a word or represented by the Greek letter omega in uppercase.

Also read -

Specific resistance-

A conductor's resistance R is dependent on its length L, cross-section A, and composition. The resistance is proportional to the length of the conductor for a certain cross-section. Whereas the resistance of the cross-section for a certain length is inversely proportional. The two dependencies can be combined as,

R ∞ L/A

R = p L/A

Here constant proportionality is known as the specific resistance. The strength of a homogenous chunk of a unit-length and cross-sectional material is defined as the strength or resistance of a material. Quantitatively,

p = R A/L

The specific resistance unit SI is Ohm (mode of resistance).

Ohm’s law-

Ohm's law states that the relationship between current, voltage, and resistance. The amount of steady current through a large number of materials is directly proportional to the potential difference, or voltage, across the materials. Ohm's law may be expressed mathematically as

I∝V

The continuous proportionality of,

R = V/I

R is known as the conducting wire's resistance, depending on the physical condition and structure of the material in question.

Related Topics Link,

Si unit of resistance-

The si unit of resistance is the ohm (Ω).

International ohm unit definition-

1 Ohm is the resistance of a mercury column with a temperature of melting ice, a uniform cross-sectional area, a length of 106.3 cm, and a mass of 14.4521 g.

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Some popular SI units-

Abohm, megohm, statohm and Planck-impedance are some of the most used resistance units. Take a look at the conversion chart below.

Conservation to ohms
Abohm	1.10^{-9}ohm
Stat coulomb	0.00001c2ohms
Preece	1.106ohms
Megohm	1000000ohms

Some important points-

Temperature-dependent resistance. If there is a consistent holding of other physical amounts, resistance increases with increasing metal temperature. The resistivity decreases significantly for glass, however, at very high temperatures.
In the superconducting state, superconductors have zero resistance (at very low temperatures).
The resistance of semiconductors decreases as the temperature increases.

Also check-

NCERT Physics Notes:

Frequently Asked Questions (FAQs)

1. What is the resistance unit?

The ohm (symbol: tune), named for Georg Simon Ohm, is a SI electric resistance unit. An ohm corresponds to an ampere volt.

2. What is k ohms?

One kiloohm is 1,000 ohm

3. How are resistance units calculated?

The S.I. resistance unit is the ohm. The resistance of an object is defined as the ratio between voltage (V) and it’s current (I).

4. How can you accurately measure resistance?

The 4-wire ohms method is the most precise measurement of tiny resistances since the test lead and contact resistance are reduced. This is usually utilised in automated tests, where the multimeter and the DUT have resistive and/or long cables, several connections and switches.

5. What is the multimeter resistance symbol?

This unit, denoted by the capital omega, is measured in the unit of "Ohm. The unit symbol for resistance usually refers to the resistance function: the Greek omega-letters or the term "ohms" sometimes.

6. What is the resistance unit?

The ohm (symbol: tune), named for Georg Simon Ohm, is a SI electric resistance unit. An ohm corresponds to an ampere volt.

7. Full form of ohm?

Ohm. Abbreviation Ω – The meter-kilogram-second-system electrical resistance unit, named in honour of the German Physicist of the 19th century, Georg Ohm.

8. What is the unit of electrical resistance?

The unit of electrical resistance is the ohm (Ω). One ohm is defined as the resistance that allows one ampere of current to flow when one volt of potential difference is applied across it.

9. Why is resistance measured in ohms?

Resistance is measured in ohms to honor Georg Ohm, who discovered the relationship between voltage, current, and resistance (Ohm's Law). The ohm provides a standardized unit for quantifying how much a material opposes the flow of electric current.

10. What's the difference between resistance and resistivity?

Resistance is a property of a specific object, measured in ohms (Ω), while resistivity is a property of the material itself, measured in ohm-meters (Ω⋅m). Resistivity is independent of the object's size or shape, whereas resistance depends on both the material and the object's dimensions.

11. What is the relationship between conductance and resistance?

Conductance is the reciprocal of resistance. It measures how easily current flows through a material. The unit of conductance is the siemens (S), which is equivalent to 1/ohm. As resistance increases, conductance decreases, and vice versa.

12. How does the size of a resistor affect its resistance?

Generally, increasing the length of a resistor increases its resistance, while increasing its cross-sectional area decreases its resistance. This is because resistance is directly proportional to length and inversely proportional to area.

13. Can resistance ever be zero?

In practice, all normal materials have some resistance. However, superconductors, which are materials cooled to extremely low temperatures, can exhibit zero electrical resistance. This phenomenon allows current to flow without any energy loss.

14. How does the concept of resistance relate to Ohm's Law?

Ohm's Law states that V = IR, where V is voltage, I is current, and R is resistance. This means resistance is the ratio of voltage to current (R = V/I). Ohm's Law forms the foundation for understanding how resistance affects current flow in electrical circuits.

15. What is a potentiometer and how does it relate to resistance?

A potentiometer is a variable resistor with three terminals. It allows for adjustment of resistance by moving a sliding contact along a resistive element. Potentiometers are used to control voltage in circuits, such as volume controls in audio equipment.

16. Can resistance be negative?

In conventional resistors, resistance is always positive. However, in some special devices like tunnel diodes or in certain conditions like in plasma physics, a negative differential resistance can occur. This doesn't mean the total resistance is negative, but that the resistance decreases as current increases in a specific operating range.

17. What is the difference between AC and DC resistance?

DC resistance is the resistance to direct current, which is constant. AC resistance, also called impedance, includes resistance to alternating current and can vary with frequency due to inductive and capacitive effects in the circuit.

18. How does temperature affect resistance?

For most conductors, increasing temperature increases resistance because the increased thermal motion of atoms interferes with electron flow. However, for semiconductors, increasing temperature can decrease resistance by providing energy for electrons to move to the conduction band.

19. How do series and parallel connections affect total resistance?

In a series connection, resistances add up: Rtotal = R1 + R2 + R3 + ... In a parallel connection, the reciprocals of resistances add up: 1/Rtotal = 1/R1 + 1/R2 + 1/R3 + ... This means parallel connections always result in a lower total resistance than any individual resistor.

20. How does resistance affect the brightness of a light bulb in a circuit?

In a simple circuit with a battery and light bulb, increasing the resistance of the circuit (either by adding resistors or using a bulb with higher resistance) will decrease the current flow. This results in less power being delivered to the bulb, making it appear dimmer.

21. What is a rheostat and how does it differ from a potentiometer?

A rheostat is a two-terminal variable resistor used to control current in a circuit. Unlike a potentiometer, which is typically used as a voltage divider, a rheostat is used to directly vary the resistance in a circuit. Rheostats are often designed to handle higher currents than potentiometers.

22. What is meant by "skin effect" in relation to resistance?

Skin effect is the tendency of alternating current to flow near the surface of a conductor, effectively reducing the cross-sectional area available for current flow. This increases the effective resistance of the conductor at high frequencies. The skin effect is more pronounced in larger conductors and at higher frequencies.

23. How does resistance play a role in the functioning of a fuse?

A fuse is a safety device that uses the principle of resistance heating. It contains a metal wire or strip designed to melt and break the circuit if the current exceeds a safe level. The resistance of the fuse wire causes it to heat up as current flows, and if the current is too high, it melts, opening the circuit and protecting other components.

24. What is the significance of "critical resistance" in maximum power transfer?

Critical resistance refers to the load resistance that allows for maximum power transfer from a source to a load. Maximum power transfer occurs when the load resistance equals the internal resistance of the source. This concept is crucial in designing efficient power delivery systems and in impedance matching for signal transmission.

25. How does resistance affect the efficiency of power transmission lines?

Resistance in power transmission lines causes energy loss in the form of heat (I^2R losses). This reduces the efficiency of power transmission, especially over long distances. To minimize these losses, power companies use high voltages (which reduce current for the same power) and low-resistance materials like copper or aluminum for transmission lines.

26. What is the concept of "negative resistance" in certain electronic devices?

Negative resistance is a property of some electronic components where an increase in voltage across the device results in a decrease in current through it, over a certain operating range. This behavior is opposite to that of a normal resistor. Devices exhibiting negative resistance, such as tunnel diodes, can be used to create oscillators and amplifiers.

27. How does resistance affect the sensitivity of measuring instruments like galvanometers?

The sensitivity of measuring instruments like galvanometers is inversely related to their internal resistance. A lower resistance allows for a larger current flow for a given voltage, making the instrument more sensitive to small changes. However, this also means the instrument draws more current from the circuit being measured, potentially affecting the measurement.

28. What is the relationship between resistance and the drift velocity of electrons in a conductor?

Resistance is inversely proportional to the drift velocity of electrons in a conductor. Higher resistance means electrons encounter more obstacles (like lattice vibrations or impurities) as they move through the material, reducing their average drift velocity. Conversely, in materials with lower resistance, electrons can move more freely, resulting in higher drift velocities.

29. How does the concept of resistance apply to superconductivity?

Superconductivity is a phenomenon where certain materials, when cooled below a critical temperature, exhibit zero electrical resistance. In this state, electrons can flow without any energy loss. This contradicts normal resistance behavior and allows for applications like loss-free power transmission and powerful electromagnets.

30. How does resistance affect the operation of a voltage divider circuit?

In a voltage divider circuit, two or more resistors are connected in series across a voltage source. The output voltage is taken across one of the resistors. The ratio of the resistances determines the ratio of the output voltage to the input voltage. By changing the resistance values, the output voltage can be controlled, making voltage dividers useful for creating reference voltages or level shifting.

31. How does resistance relate to the concept of electrical noise in circuits?

Electrical resistance is a source of thermal noise (also called Johnson-Nyquist noise) in circuits. This noise is caused by the random motion of charge carriers in a conductor due to thermal energy. The amount of noise is proportional to the resistance and temperature. Understanding and minimizing this noise is crucial in designing sensitive electronic equipment, especially in communications and measurement systems.

32. What is the difference between linear and non-linear resistors?

Linear resistors obey Ohm's Law: their resistance remains constant regardless of the applied voltage or current. Non-linear resistors, however, have a resistance that varies with the applied voltage or current. Examples of non-linear resistors include thermistors, varistors, and light-dependent resistors (LDRs). Non-linear resistors are often used in sensing and protection applications.

33. How does resistance affect the Q factor of an inductor?

The Q factor (quality factor) of an inductor is the ratio of its inductive reactance to its resistance. A higher resistance lowers the Q factor, which results in a less ideal inductor with more energy loss. Lower resistance in the windings of an inductor leads to a higher Q factor, indicating a more efficient energy storage and release in the magnetic field.

34. How does quantum tunneling affect resistance in very small electronic devices?

Quantum tunneling is a phenomenon where particles can pass through a potential barrier that they classically shouldn't be able to overcome. In very small electronic devices (nanoscale), quantum tunneling can allow electrons to "tunnel" through thin insulating layers, effectively reducing the resistance. This effect becomes significant in modern nanoscale transistors and can lead to unintended current leakage.

35. What is the ballistic resistance in nanoscale conductors?

Ballistic resistance occurs in extremely small conductors where the length of the conductor is shorter than the mean free path of electrons. In this regime, electrons can travel through the conductor without scattering, leading to a resistance that depends primarily on the quantum mechanical transmission probability rather than the material's bulk properties. This concept is important in nanoelectronics and quantum transport.

36. How does resistance affect the operation of a current mirror circuit?

In a current mirror circuit, resistance plays a crucial role in determining the accuracy of current copying. The output resistance of the current mirror affects how well it maintains a constant current despite changes in load voltage. Higher output resistance leads to better current regulation. Additionally, any mismatch in the resistances of the input and output branches can lead to errors in the mirrored current.

37. How does the resistance of a material change near its melting point?

As a material approaches its melting point, its resistance typically increases. This is due to the increased thermal vibrations of atoms, which interfere more

38. How do resistor color codes relate to resistance values?

Resistor color codes use a series of colored bands to indicate resistance value and tolerance. Each color represents a number or multiplier. For example, brown-black-orange would represent 10 x 10^3 ohms or 10 kΩ. This system allows for quick identification of resistance values without needing to measure each resistor.

39. What is the significance of the kilo-ohm (kΩ) and mega-ohm (MΩ)?

Kilo-ohms (kΩ) and mega-ohms (MΩ) are larger units of resistance. 1 kΩ = 1,000 Ω, and 1 MΩ = 1,000,000 Ω. These units are used to express larger resistance values more conveniently, just as kilometers are used for longer distances instead of meters.

40. How does resistance relate to power dissipation in a circuit?

Power dissipation in a resistor is given by P = I^2R or P = V^2/R, where P is power, I is current, V is voltage, and R is resistance. This shows that increasing resistance while maintaining constant current increases power dissipation, which is often observed as heat generation in the resistor.

41. What is the concept of equivalent resistance?

Equivalent resistance is the single resistance value that could replace a combination of resistors in a circuit while maintaining the same overall current flow. It's particularly useful for simplifying complex circuits with multiple resistors in series and parallel combinations.

42. How does the concept of resistance apply to human body safety with electricity?

The human body has its own electrical resistance, typically between 1,000 to 100,000 ohms depending on conditions. This resistance can limit current flow through the body, but it's not always enough to prevent injury. Understanding body resistance helps in designing safety measures for electrical systems and in comprehending the dangers of electric shock.

43. What is meant by "internal resistance" in a battery?

Internal resistance in a battery refers to the opposition to current flow within the battery itself. It's caused by the electrochemical processes and physical structure of the battery. As internal resistance increases (often with battery age or use), the battery's ability to deliver current decreases, reducing its effective voltage under load.

44. How does resistance relate to the concept of electrical insulators?

Electrical insulators are materials with very high resistance to electric current flow. They have resistances typically greater than 10^8 Ω⋅m. This high resistance prevents significant current flow, making insulators useful for protecting against electric shock and for separating conductive components in electrical systems.

45. What is a thermistor and how does it utilize the principle of resistance?

A thermistor is a type of resistor whose resistance changes significantly with temperature. There are two types: NTC (Negative Temperature Coefficient) where resistance decreases with increasing temperature, and PTC (Positive Temperature Coefficient) where resistance increases with temperature. Thermistors are used in temperature sensing and control applications.

46. How does resistance affect the charging and discharging of a capacitor?

The resistance in a circuit with a capacitor determines the rate at which the capacitor charges or discharges. A higher resistance results in a slower charging/discharging rate, while a lower resistance allows for faster charging/discharging. This relationship is described by the time constant τ = RC, where R is resistance and C is capacitance.

47. How does the resistance of a wire change if its length is doubled but its diameter is halved?

If a wire's length is doubled and its diameter is halved, its resistance increases by a factor of 8. This is because resistance is directly proportional to length and inversely proportional to cross-sectional area. Doubling the length doubles the resistance, and halving the diameter quarters the cross-sectional area, further quadrupling the resistance.

48. What is a strain gauge and how does it use the principle of resistance?

A strain gauge is a sensor that measures strain (deformation) of an object by detecting changes in electrical resistance. When the gauge is stretched or compressed, its resistance changes slightly. This change in resistance is proportional to the strain, allowing for precise measurements of stress, weight, pressure, or other forces causing deformation.

49. What is contact resistance and why is it important in electrical connections?

Contact resistance is the additional resistance that occurs at the interface between two conducting surfaces in an electrical connection. It's important because it can lead to voltage drops and power losses, especially in high-current applications. Minimizing contact resistance through proper connection techniques and materials is crucial for efficient electrical systems.

50. What is the significance of the resistance-temperature coefficient?

The resistance-temperature coefficient describes how the resistance of a material changes with temperature. It's usually expressed as a percentage change per degree Celsius. Materials with a positive coefficient (like most metals) increase in resistance as temperature rises, while those with a negative coefficient (like some semiconductors) decrease in resistance. This property is crucial in designing temperature-sensitive devices and compensating for temperature effects in circuits.

51. How does resistance affect the time constant in RC circuits?

In an RC (Resistor-Capacitor) circuit, the time constant τ is given by τ = RC, where R is the resistance and C is the capacitance. This time constant determines how quickly the capacitor charges or discharges. A larger resistance increases the time constant, resulting in slower charging and discharging, while a smaller resistance decreases the time constant, leading to faster charging and discharging.

52. What is the concept of "dynamic resistance" in diodes?

Dynamic resistance, also called AC resistance or small-signal resistance, is the instantaneous rate of change of voltage with respect to current in a diode. Unlike the static resistance (V/I ratio), dynamic resistance (dV/dI) is relevant for small AC signals superimposed on a DC bias. It's important in analyzing diode behavior in AC circuits and in small-signal amplifier design.

53. What is the significance of sheet resistance in thin film technology?

Sheet resistance is a measure of resistance of thin films that are nominally uniform in thickness. It's expressed in ohms per square (Ω/□) and is independent of the size of the square. This concept is crucial in semiconductor manufacturing, particularly in the production of integrated circuits, where it's used to characterize the resistance of thin conducting layers.

54. What is the concept of "resistance matching" in audio systems?

Resistance matching, more broadly known as impedance matching, is crucial in audio systems for maximizing power transfer and minimizing signal distortion. It involves ensuring that the output impedance of a source (like an amplifier) is properly matched to the input impedance of a load (like speakers). Proper matching ensures efficient power transfer and maintains the intended frequency response of the system.