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L = N Δ Φ Δ I = N μ 0 NA Δ I Δ I . size 12{L=N { {ΔΦ} over {ΔI} } =N { {μ rSub { size 8{0} } ital "NA" { {ΔI} over {ℓ} } } over {ΔI} } } {}

This simplifies to

L = μ 0 N 2 A (solenoid). size 12{L= { {μ rSub { size 8{0} } N rSup { size 8{2} } A} over {ℓ} } } {}

This is the self-inductance of a solenoid of cross-sectional area A and length . Note that the inductance depends only on the physical characteristics of the solenoid, consistent with its definition.

Calculating the self-inductance of a moderate size solenoid

Calculate the self-inductance of a 10.0 cm long, 4.00 cm diameter solenoid that has 200 coils.


This is a straightforward application of L = μ 0 N 2 A size 12{L= { {μ rSub { size 8{0} } N rSup { size 8{2} } A} over {ℓ} } } {} , since all quantities in the equation except L size 12{L} {} are known.


Use the following expression for the self-inductance of a solenoid:

L = μ 0 N 2 A . size 12{L= { {μ rSub { size 8{0} } N rSup { size 8{2} } A} over {ℓ} } } {}

The cross-sectional area in this example is A = πr 2 = ( 3 . 14 . . . ) ( 0 .0200 m ) 2 = 1 . 26 × 10 3 m 2 size 12{A=πr rSup { size 8{2} } = \( 3 "." "14" "." "." "." \) \( 0 "." "0200"`m \) rSup { size 8{2} } =1 "." "26" times "10" rSup { size 8{ - 3} } `m rSup { size 8{2} } } {} , N is given to be 200, and the length is 0.100 m. We know the permeability of free space is μ 0 = × 10 −7 T m/A . Substituting these into the expression for L gives

L = ( × 10 7 T m/A ) ( 200 ) 2 ( 1.26 × 10 3 m 2 ) 0.100 m = 0 . 632 mH .


This solenoid is moderate in size. Its inductance of nearly a millihenry is also considered moderate.

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One common application of inductance is used in traffic lights that can tell when vehicles are waiting at the intersection. An electrical circuit with an inductor is placed in the road under the place a waiting car will stop over. The body of the car increases the inductance and the circuit changes sending a signal to the traffic lights to change colors. Similarly, metal detectors used for airport security employ the same technique. A coil or inductor in the metal detector frame acts as both a transmitter and a receiver. The pulsed signal in the transmitter coil induces a signal in the receiver. The self-inductance of the circuit is affected by any metal object in the path. Such detectors can be adjusted for sensitivity and also can indicate the approximate location of metal found on a person. (But they will not be able to detect any plastic explosive such as that found on the “underwear bomber.”) See [link] .

Photograph of people around a security gate at an airport departure terminal.
The familiar security gate at an airport can not only detect metals but also indicate their approximate height above the floor. (credit: Alexbuirds, Wikimedia Commons)

Energy stored in an inductor

We know from Lenz’s law that inductances oppose changes in current. There is an alternative way to look at this opposition that is based on energy. Energy is stored in a magnetic field. It takes time to build up energy, and it also takes time to deplete energy; hence, there is an opposition to rapid change. In an inductor, the magnetic field is directly proportional to current and to the inductance of the device. It can be shown that the energy stored in an inductor     E ind size 12{E rSub { size 8{"ind"} } } {} is given by

E ind = 1 2 LI 2 . size 12{E rSub { size 8{"ind"} } = { {1} over {2} } ital "LI" rSup { size 8{2} } } {}

This expression is similar to that for the energy stored in a capacitor.

Calculating the energy stored in the field of a solenoid

How much energy is stored in the 0.632 mH inductor of the preceding example when a 30.0 A current flows through it?


The energy is given by the equation E ind = 1 2 LI 2 size 12{E rSub { size 8{"ind"} } = { {1} over {2} } ital "LI" rSup { size 8{2} } } {} , and all quantities except E ind size 12{E rSub { size 8{"ind"} } } {} are known.


Substituting the value for L size 12{L} {} found in the previous example and the given current into E ind = 1 2 LI 2 size 12{E rSub { size 8{"ind"} } = { {1} over {2} } ital "LI" rSup { size 8{2} } } {} gives

E ind = 1 2 LI 2 = 0.5 ( 0.632 × 10 3 H ) ( 30.0 A ) 2 = 0.284 J .


This amount of energy is certainly enough to cause a spark if the current is suddenly switched off. It cannot be built up instantaneously unless the power input is infinite.

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Questions & Answers

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It's Reply
what happens to the size of charge if the dielectric is changed?
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omega= omega not +alpha t derivation
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u have to derivate it respected to time ...and as w is the angular velocity uu will relace it with "thita × time""
do to be peaceful with any body
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define variable velocity
Ali Reply
displacement in easy way.
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binding energy per nucleon
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Manuel Reply
Because HE needs someone to dominate the earth (Gen. 1:26)
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Jun Reply
ok we can say body is electrically neutral ...conductor this quality is given to most metalls who have free electron in orbital d ...but human doesn't have ...so we re made from insulator or dielectric material ... furthermore, the menirals in our body like k, Fe , cu , zn
when we face electric shock these elements work as a conductor that's why we got this shock
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Practice Key Terms 6

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