Weird Science

Self-induction

Polish ver­sion is here

A Bit of The­ory

Self-induc­tion, a phe­no­me­non rela­ted to induc­tance, is a spe­cial case of elec­tro­ma­gne­tic induc­tion. This phe­no­me­non occurs when an elec­tro­mo­tive force is gene­ra­ted within the same cir­cuit that car­ries the cur­rent cau­sing the induc­tion. Accor­ding to Lenz's law, the resul­ting elec­tro­mo­tive force oppo­ses chan­ges in the elec­tric cur­rent flo­wing thro­ugh the cir­cuit.

The value of the elec­tro­mo­tive force of self-induc­tion is descri­bed by the for­mula:

Ilustracja

where:

Epsi­lon - indu­ced elec­tro­mo­tive force [V]

L - induc­tance [H]

I - cur­rent [A]

t - time [s]

Self-induc­tion stron­gly depends on the induc­tance of the cir­cuit (L), so the phe­no­me­non is most easily obse­rved in coils. To a les­ser extent, this phe­no­me­non can also occur in any con­duc­tor.

How to Obse­rve It?

Let’s take ano­ther look at the above for­mula. When the cur­rent in the cir­cuit incre­a­ses, the change in cur­rent (delta I) has a posi­tive value (final value minus ini­tial value). Due to the nega­tive sign pre­ce­ding L, the elec­tro­mo­tive force of self-induc­tion will be nega­tive. This results in a vol­tage drop in the cir­cuit by that value.

A more inte­re­sting case occurs when the cur­rent in the cir­cuit decre­a­ses. In this case, the change in cur­rent is nega­tive. As a result, the elec­tro­mo­tive force of self-induc­tion will have a posi­tive value. Its magni­tude depends on the coil's induc­tance, the cur­rent flo­wing thro­ugh the coil, and the rate at which the cur­rent decre­a­ses. The lar­ger the first two quan­ti­ties and the smal­ler the last one, the gre­a­ter the resul­ting vol­tage. Using a coil with high induc­tance and quic­kly inter­rup­ting the cir­cuit (a rapid drop in cur­rent within a short time) can pro­duce a self-induc­tion vol­tage seve­ral times higher than the vol­tage ori­gi­nally powe­ring the coil!

War­ning: Do not use cur­rents with exces­si­vely high values, as the resul­ting self-induc­tion vol­tage can be dan­ge­ro­u­sly high. In such cases, elec­tric shock may occur. When using the descri­bed coils and a 1.5V bat­tery as the power source, there is no dan­ger. Never­the­less, cau­tion sho­uld be exer­ci­sed. The author assu­mes no respon­si­bi­lity for any damage or injury that may occur. You pro­ceed at your own risk!

To obse­rve this phe­no­me­non fir­sthand, we need a small neon lamp:

Ilustracja:

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It will serve as an indi­ca­tor of the high vol­tage gene­ra­ted.

We also need an ele­ment with high induc­tance. For my expe­ri­ments, I used the fol­lo­wing coils:

Ilustracja:

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Ilustracja:

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Ilustracja:

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The first coil con­si­sts of seve­ral dozen turns of insu­la­ted wire wound on a toro­i­dal pow­de­red iron core from a dama­ged com­pu­ter power sup­ply. The second coil is from a bro­ken relay, con­ta­i­ning about 1,500 turns of thin wire. The third is a wor­king elec­tro­ma­gnet used, among other things, in my magne­tic levi­ta­tion device. All of these coils work well for this expe­ri­ment.

Con­nect the neon lamp to the coil's ter­mi­nals. Then take a 1.5V bat­tery (pre­fe­ra­bly alka­line) and con­nect it to the coil. Cur­rent will flow thro­ugh the coil, but the neon lamp will not light up. Why? This is because the neon lamp requ­i­res appro­xi­ma­tely 120V to ignite. One and a half volts is far too low to make it glow. Now quic­kly discon­nect the power source. What hap­pens? At the moment the bat­tery is discon­nec­ted, the neon lamp fla­shes!

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You can also see this effect in my video:

Notice that with high induc­tance, the self-induc­tion vol­tage is seve­ral times higher than the vol­tage ori­gi­nally powe­ring the coil. Discon­nec­ting the power sup­ply cau­ses a sud­den change in cur­rent in the cir­cu­i­t—the coil, thro­ugh self-induc­tion, "tries" to main­tain the flow of cur­rent, accor­ding to Lenz's law. This is what cau­ses the neon lamp to light up.

Since the gas inside the neon lamp only glows near the nega­tive elec­trode, we can deter­mine the pola­rity of the indu­ced vol­tage. This once again con­firms Lenz's law in prac­tice: the gas glows near the elec­trode that was pre­vio­u­sly con­nec­ted to the posi­tive ter­mi­nal of the bat­tery.

Fur­ther rea­dings:

Marek Ples

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