Unit of Inductance - Definition, Types, FAQs

Edited By Vishal kumar | Updated on Jul 02, 2025 05:09 PM IST

Introduction
In this chapter, we will study: What is Inductance?, SI unit of Inductance. We will also learn about types of inductance and see their application.

What is Inductance?

Inductance is an electrical term, tendency shown by the inductor to oppose any change in Electric Current flowing in it. It is denoted by (L). It is also called specific inductance.

SI unit of Inductance

Henry is the SI unit of Inductance. It is named after American scientist Joseph Henry.

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Unit of Inductance - Definition, Types, FAQs

Definition of 1 Henry

To generate a voltage of 1 volt when current is changing 1 ampere per second, then inductance is 1 Henry.

V(t)=Ldi/dt

Where, L is inductance and current ⅆi/ⅆt is the rate of change of current.

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Types of Inductance

Self-inductance:

It is a property of a current-carrying coil, which opposes any change in current flowing in it. The EMF produced in the coil resist increases in the current if there is an increase in current and the same is true for decrease also or

Expression for self inductance is given by

v_L=- N ⅆφ/ⅆt_-

Where, v_L is equal to induced EMF

And ⅆφ/ⅆt_-is equal to rate of change of magnetic flux

SI unit of self induction is Henry.

NOTE: The negative sign indicates that EMF produced is in the opposite direction or it is resisting.

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Mutual Inductance

In mutual inductance, when two coils are brought close to each other with a magnetic field in one coil then EMF is induced in the second coil also, or

EMF induced is given by:

$-\varepsilon_{12}=N_1\frac{d\phi _{12}}{dt}$

Where, ε₁₂ is EMF produced by the first coil on 2 coils.

dΦ₁₂/ⅆt is equal to the rate of change of magnetic flux in coil 2 by magnetic field generated by coil 1.

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Frequently Asked Questions (FAQs)

1. What is the SI unit of Inductance?

Henry is the SI unit of inductance.

2. What is the value of 1 millihenry?

As we know, 1 Henry equal 1000 millihenry , so 1 millihenry equals 0.001 Henry

3. Which of the following is the unit of Inductance? A. Henry B. Farad C. Coulomb D. Ampere

As we know, the Henry is the Unit of Inductance is Henry, so the correct option is (a).

4. Which of the following is not a unit of inductance? A. H B. Henry C. F D. kgm2s-2A2

F(Faraday) is not the unit of inductance while all other units represent the unit of inductance.

5. Mention the unit of induction.

The unit of inductance is equivalent to Henries(H)

6. What are the types of inductors?

Iron-core inductor, laminated core inductors, powdered iron core inductors, air-core inductors are some of the types of inductors.

7. Is solenoid inductive?

Yes, solenoids are inductors but not all inductors are solenoids.

8. Which bridge is used to measure inductance?

Maxwell bridge measures inductance.

9. What's the difference between self-inductance and mutual inductance?

Self-inductance is the property of a single circuit or coil that causes it to oppose changes in its own current. Mutual inductance, on the other hand, occurs when changes in current in one circuit induce a voltage in a nearby circuit. Both types of inductance involve magnetic fields, but self-inductance involves a single circuit while mutual inductance involves interaction between two or more circuits.

10. How does adding a ferromagnetic core to an inductor affect its inductance?

Adding a ferromagnetic core to an inductor significantly increases its inductance. Ferromagnetic materials have a high magnetic permeability, which means they can support a strong magnetic field. This allows the inductor to generate more magnetic flux for the same current, effectively increasing its inductance. However, it's important to note that ferromagnetic cores can also introduce non-linear behavior and saturation effects.

11. What is the inductance formula for a solenoid?

The inductance (L) of a solenoid is given by the formula: L = (μ * N^2 * A) / l, where μ is the permeability of the core material, N is the number of turns in the coil, A is the cross-sectional area of the coil, and l is the length of the coil. This formula shows that inductance increases with more turns, larger area, and shorter length, assuming the core material remains the same.

12. How does an inductor behave with DC vs AC current?

With DC (direct current), an inductor initially opposes changes in current but eventually acts like a short circuit once the current stabilizes. With AC (alternating current), an inductor continuously opposes changes in current, creating a phase shift between voltage and current. The inductor's impedance increases with frequency in AC circuits, making it useful for filtering and frequency-dependent applications.

13. What is the quality factor (Q) of an inductor and why is it important?

The quality factor (Q) of an inductor is a measure of its efficiency, defined as the ratio of its inductive reactance to its resistance at a given frequency. A higher Q indicates a more efficient inductor with lower energy losses. The Q factor is important in resonant circuits, filters, and oscillators, where it affects the sharpness of tuning and the amount of energy stored versus energy dissipated.

14. What is inductance and why is it important in electrical circuits?

Inductance is the property of an electrical circuit that opposes changes in current flow. It's important because it allows energy to be stored in magnetic fields, which is crucial for many electrical and electronic devices like transformers, motors, and filters. Inductance helps control current flow and can be used to create time delays or oscillations in circuits.

15. What is the unit of inductance and what does it represent?

The unit of inductance is the henry (H), named after American scientist Joseph Henry. One henry represents the inductance of a circuit in which an electromotive force of one volt is induced when the current changes at a rate of one ampere per second. It essentially measures how much magnetic energy is stored for a given current.

16. What is the relationship between inductance and magnetic flux?

Inductance is directly related to magnetic flux. The inductance of a circuit is defined as the ratio of the magnetic flux linkage to the current producing it. In other words, inductance measures how much magnetic flux is produced for a given current. A higher inductance means more magnetic flux is generated for the same current change.

17. How does the size of an inductor affect its inductance?

Generally, larger inductors have higher inductance. This is because inductance depends on factors like the number of turns in a coil, the cross-sectional area of the coil, and the length of the coil. A larger inductor typically has more turns or a larger cross-sectional area, allowing it to store more magnetic energy for a given current.

18. Can inductance be negative?

No, inductance cannot be negative. Inductance is always a positive quantity because it represents the ability to store energy in a magnetic field. A negative inductance would imply creating energy out of nothing, which violates the law of conservation of energy. However, in some circuit analyses, we may encounter terms that look like negative inductance, but these are usually the result of specific circuit configurations or mathematical manipulations.

19. How does temperature affect inductance?

Temperature can affect inductance in several ways. For air-core inductors, the effect is usually small, mainly due to thermal expansion changing the coil's dimensions slightly. For inductors with ferromagnetic cores, the effect can be more significant. As temperature increases, the permeability of the core material typically decreases, which reduces the inductance. Some special core materials are designed to have stable inductance over a wide temperature range.

20. How does skin effect influence inductance at high frequencies?

Skin effect is the tendency of alternating current to flow near the surface of a conductor at high frequencies. This effectively reduces the cross-sectional area of the conductor, increasing its resistance. For inductors, skin effect can decrease the effective inductance at high frequencies because it changes the current distribution in the wire. This effect is more pronounced in larger diameter wires and at higher frequencies.

21. What is the time constant of an RL circuit?

The time constant (τ) of an RL (resistor-inductor) circuit is given by τ = L/R, where L is the inductance and R is the resistance. This time constant represents the time it takes for the current to reach approximately 63.2% of its final value when a voltage is applied to the circuit. It's a measure of how quickly the circuit responds to changes in voltage or current.

22. How does inductance affect the rise time of current in a circuit?

Inductance opposes changes in current, so it increases the rise time of current in a circuit. When voltage is applied to an inductive circuit, the current doesn't immediately reach its maximum value. Instead, it rises gradually, following an exponential curve. The larger the inductance, the longer it takes for the current to reach its steady-state value.

23. What is the energy stored in an inductor?

The energy stored in an inductor is given by the formula: E = (1/2) * L * I^2, where E is the energy in joules, L is the inductance in henries, and I is the current in amperes. This equation shows that the energy stored increases with the square of the current, making inductors useful for storing and releasing energy in pulsed power applications.

24. What is the difference between lumped and distributed inductance?

Lumped inductance refers to inductance concentrated in a specific component, like a coil or inductor. Distributed inductance occurs along the length of conductors in a circuit, such as wires or PCB traces. At low frequencies, distributed inductance is often negligible and circuits can be analyzed using lumped elements. However, at high frequencies or in large circuits, distributed inductance becomes significant and can affect circuit behavior.

25. How does mutual inductance affect transformer operation?

Mutual inductance is crucial for transformer operation. In a transformer, changing current in the primary coil induces a voltage in the secondary coil through mutual inductance. The ratio of turns between the primary and secondary coils determines the voltage transformation. Higher mutual inductance leads to more efficient energy transfer between the coils, which is why transformers often use ferromagnetic cores to increase coupling.

26. What is a Henry in terms of more basic units?

One Henry can be expressed in terms of more basic SI units as: 1 H = 1 kg⋅m²⋅s⁻²⋅A⁻². This shows that inductance is related to mass, length, time, and current. Alternatively, it can be expressed as 1 H = 1 Wb/A (weber per ampere) or 1 H = 1 V⋅s/A (volt-second per ampere), which relates it directly to magnetic flux and electromotive force.

27. How does inductance affect the bandwidth of a circuit?

Inductance can limit the bandwidth of a circuit by opposing rapid changes in current. In combination with capacitance, inductance creates frequency-dependent behavior. Higher inductance tends to reduce high-frequency response, effectively lowering the circuit's bandwidth. This property is used intentionally in low-pass filters but can be a limitation in high-speed circuits where wide bandwidth is desired.

28. What is the relationship between inductance and impedance?

The inductive reactance (XL), which is the imaginary part of impedance due to an inductor, is given by XL = 2πfL, where f is the frequency and L is the inductance. This shows that the impedance of an inductor increases linearly with both frequency and inductance. At DC (f = 0), an ideal inductor has zero impedance, while at very high frequencies, its impedance can become very large.

29. How does core saturation affect inductance?

Core saturation occurs in inductors with ferromagnetic cores when the magnetic field becomes so strong that further increases in current do not produce proportional increases in magnetic flux. As the core approaches saturation, the inductance of the coil decreases. This non-linear behavior can lead to distortion in audio applications and limits the maximum current an inductor can handle in power applications.

30. What is the difference between series and parallel inductance?

When inductors are connected in series, their inductances add up, similar to resistors. The total inductance is the sum of individual inductances: Ltotal = L1 + L2 + L3 + ... For parallel connections, the reciprocal of the total inductance equals the sum of reciprocals of individual inductances: 1/Ltotal = 1/L1 + 1/L2 + 1/L3 + ... This is analogous to parallel resistance calculations.

31. How does inductance contribute to resonance in LC circuits?

In LC (inductor-capacitor) circuits, inductance and capacitance work together to create resonance. At the resonant frequency, the inductive and capacitive reactances are equal in magnitude but opposite in phase, resulting in a purely resistive circuit. The resonant frequency is given by f = 1 / (2π√(LC)). Inductance thus plays a crucial role in determining the frequency at which resonance occurs.

32. How does inductance affect the power factor in AC circuits?

Inductance causes current to lag behind voltage in AC circuits, reducing the power factor. The power factor is the cosine of the phase angle between voltage and current. In a purely inductive circuit, the current would lag the voltage by 90°, resulting in a power factor of zero. In practical circuits with both resistance and inductance, the power factor is between 0 and 1, with higher inductance generally leading to a lower power factor.

33. What is magnetic coupling and how does it relate to inductance?

Magnetic coupling refers to the interaction between two or more inductors through their magnetic fields. When inductors are magnetically coupled, a change in current in one inductor induces a voltage in the other. The strength of this coupling is described by the coupling coefficient (k), which ranges from 0 (no coupling) to 1 (perfect coupling). Magnetic coupling is the basis for mutual inductance and is crucial in devices like transformers.

34. How does the shape of an inductor affect its inductance?

The shape of an inductor significantly affects its inductance. For example, a long, thin solenoid will have different inductance than a short, wide one with the same number of turns. Toroidal inductors often have higher inductance for their size due to better containment of the magnetic field. The shape also affects factors like self-capacitance and coupling to nearby components, which can indirectly impact the effective inductance in a circuit.

35. What is the difference between air-core and iron-core inductors?

Air-core inductors use air as the core material, while iron-core inductors use iron or ferromagnetic materials. Air-core inductors have lower inductance but are more linear and stable across a wide range of frequencies and currents. Iron-core inductors can achieve much higher inductance in a smaller size but are subject to core saturation, hysteresis losses, and non-linear behavior at high currents or frequencies.

36. How does inductance affect the phase relationship between voltage and current?

In an ideal inductor, the voltage leads the current by 90 degrees. This phase shift occurs because the inductor opposes changes in current, causing the current to lag behind voltage changes. In real circuits with resistance, the phase shift is less than 90 degrees. The exact phase relationship depends on the ratio of inductive reactance to resistance in the circuit.

37. What is the role of inductance in electromagnetic wave propagation?

Inductance plays a crucial role in electromagnetic wave propagation. Along with capacitance, it determines the propagation speed and impedance of transmission lines. In free space, the distributed inductance and capacitance of the medium determine the speed of light. In waveguides and antennas, inductance affects the resonant frequency and radiation patterns. Understanding inductance is essential for designing RF and microwave circuits.

38. How does inductance contribute to eddy current losses?

Inductance contributes to eddy current losses indirectly. When a changing magnetic field passes through a conductive material, it induces circulating currents called eddy currents. These currents create their own magnetic fields that oppose the original field, effectively increasing the apparent inductance. However, eddy currents also cause resistive heating, leading to energy losses. This is why laminated cores are often used in transformers and motors to reduce eddy current losses.

39. What is the relationship between inductance and magnetic permeability?

Inductance is directly proportional to the magnetic permeability of the core material. The formula for inductance often includes μ (mu), which represents permeability. Higher permeability materials, like ferromagnetic substances, greatly increase inductance compared to air. This relationship is why adding an iron core to an air-core inductor dramatically increases its inductance.

40. How does inductance affect the rise and fall times of digital signals?

Inductance in digital circuits can increase rise and fall times of signals by opposing rapid changes in current. This can lead to signal distortion, ringing, and overshoot. In high-speed digital design, managing inductance in traces and connections is crucial to maintain signal integrity. Techniques like proper routing, ground planes, and minimizing loop areas are used to control inductance effects.

41. What is the concept of self-resonance in inductors?

Self-resonance in inductors occurs due to the interaction between the inductor's inductance and its parasitic capacitance. Every inductor has some inherent capacitance between its windings. At a certain frequency, called the self-resonant frequency (SRF), this capacitance resonates with the inductance. Above this frequency, the inductor begins to behave more like a capacitor. The SRF limits the useful frequency range of an inductor.

42. How does inductance affect the efficiency of wireless power transfer?

Inductance is crucial for wireless power transfer systems, which often use resonant inductive coupling. The efficiency of power transfer depends on the mutual inductance between the transmitting and receiving coils, as well as their individual inductances. Higher mutual inductance and quality factors (Q) of the coils lead to more efficient power transfer. Designers must optimize coil geometries and materials to maximize inductance and minimize losses.

43. What is the difference between differential and common-mode inductance?

Differential inductance refers to the inductance experienced by differential signals (opposite currents in a pair of conductors), while common-mode inductance affects common-mode signals (currents in the same direction). In twisted pair cables or differential PCB traces, the differential inductance is often lower than the common-mode inductance due to partial cancellation of magnetic fields. Understanding and managing both types is important for signal integrity in high-speed designs.

44. How does inductance contribute to electromagnetic interference (EMI)?

Inductance can contribute to EMI in several ways. Rapidly changing currents in inductive elements can create electromagnetic fields that radiate and interfere with nearby circuits. Additionally, the inductance of cables and PCB traces can cause ringing and overshoot in signals, potentially creating high-frequency components that radiate EMI. Proper management of inductance through shielding, filtering, and layout techniques is essential for EMI control.

45. What is the concept of incremental inductance?

Incremental inductance refers to the change in inductance with respect to a small change in current, particularly in non-linear inductors like those with ferromagnetic cores. It's defined as the slope of the flux linkage vs. current curve at a specific operating point. Incremental inductance is important in analyzing circuits with varying currents or DC bias, where the inductance may change significantly with the operating conditions.

46. How does inductance affect the behavior of transmission lines?

In transmission lines, inductance (along with capacitance) determines the characteristic impedance and propagation velocity of signals. The distributed inductance of the line contributes to signal delay and can cause reflections if not properly matched. At high frequencies, the inductance of connectors and transitions becomes significant, affecting signal integrity. Understanding and controlling line inductance is crucial for high-speed digital and RF design.