Why does my LC circuit not oscillate its energy between Electric & Magnetic fields?

[James2018]

TL;DR Summary

My capacitor-inductor LC circuit does not oscillate its energy between electric and magnetic field form

My inductor has 8 turns, 4 cm diameter and 4 cm length. The capacitor I use is a 1 microfarad polyester capacitor. When the copper wire inductor is connected to DC voltage the compass needle is deflected by 90 degrees and the multimeter detects 1 A or 1000 mA of current.

When DC is switched off, the magnetic field just collapses but the capacitor, although connected to the inductor in series, does not charge and discharge periodically to restore the initial magnetic field. The digital multimeter set to ammeter mode and it detects 0 mA and the magnetic compass is no longer deflected at any angle.

Same with the charged capacitor, the multimeter detects discharging it once through the inductor I mentioned but never recharging again with a opposite polarity.

Yet physics manuals mention this happens, but for my circuit it doesn't. They even compare LC oscillation with the periodic oscillation of a pendulum.

Why doesn't my multimeter and my compass detect any currents and magnetic fields once the first discharge of the inductor's magnetic field happens?

Maybe you can help me add something to the circuit to make it work like it is mentioned in all physics textbooks.

 

[berkeman]

What do you calculate your inductance to be? What do you calculate your series resistance (of the wire, ammeter, etc.) to be? What frequency would you expect that RLC circuit to oscillate at, and for how long?

 

[Dale]

Could your multi meter even detect oscillations? Most standard multi meters can measure DC and 60 Hz and take a second or so to respond at either frequency.

What you want to use is an oscilloscope and look at the impedance as the frequency changes.

 

[Vanadium 50]

To add to @berkeman and his list of questions, how long is between removing the power source and measuring the circuit? How many thousands of miles of wire did the current travel in that time?

 

[Baluncore]

What frequency would you expect that RLC circuit to oscillate at, and for how long?

My guess is at about 120 MHz, in the VHF radio band, lasting for less than 1 us.
You might hear a momentary single click from a VHF AM radio receiver.

It would not be visible on a scanning spectrum analyser, because it is too rare, and so would probably not happen at the right time during one sweep.
You could see it on a storage or digital oscilloscope, but only with a scope bandwidth better than 1 GHz, and then only if it triggered and stored in one single sweep.

Maybe you can help me add something to the circuit to make it work like it is mentioned in all physics textbooks.

If you increase the number of turns on your coil from 8, by a factor of 10, to 80 turns, the frequency of resonance will come down by a factor of 10² = 100, into the MW AM broadcast band. You could then hear it in that band, and see it on a low-cost oscilloscope with 20 MHz bandwidth.

If you could arrange a circuit to repeat the resonance at an audio frequency, you could tune in to the resonance frequency and hear that audio tone on an AM radio.

How are you switching the current through the LC circuit ?

 

[James2018]

Perhaps I need something like a timer integrated circuit and a transistor to keep the oscillations from decay by switching the DC on and off before the oscillations decay?

 

[James2018]

how long is between removing the power source and measuring the circuit? The ammeter is connected in series in the circuit and the compass is very near the coil.

 

[James2018]

I think a superconductor would maybe sustain the LC oscillations indefinitely or maybe there would be losses from EM radiation at the resonant frequency?

 

[Baluncore]

You will probably need one thousand turns on the coil, and 100 mF of capacitance before you will see the compass or ammeter move.

Must you use the ammeter and compass?
Would you be satisfied with hearing a radio frequency oscillation?
Do you have access to an oscilloscope and/or a broadcast band AM radio ?

With an oscilloscope, you can trigger on, and observe the turn-off transient and the oscillation.

how long is between removing the power source and measuring the circuit?

There is no time between the stimulus and the start of the first cycle of oscillation.

I think a superconductor would maybe sustain the LC oscillations indefinitely or maybe there would be losses from EM radiation at the resonant frequency?

In theory, yes. But not with an ammeter in the LC circuit.

 

[Dale]

Yes, I think that is the main issue. It sounds like the OP is using a hardware store multi meter. It will never measure the oscillation, almost regardless of the other details.

 

[Baluncore]

Yes, I think that is the main issue.

I remember a similar experiment that I tried, which failed. Anyone who attempts an experiment like this, and questions the result, deserves every support. All the indications are, that they have the critical thinking skills needed, to make a great engineer or physicist one day.

 

[Dale]

Agreed, but it just isn't going to work with a hardware store multimeter. The device's instruction manual should include a statement on the response time for the device.

 

[DaveE]

The device's instruction manual should include a statement on the response time for the device.

It should, but only the good ones do. You get what you pay for.
In any case a DMM is the wrong instrument for transient oscillations. Even if, by some miracle, you get a number, you won't know what it means.

 

[James2018]

Maybe if a transistor can switch on and off the DC voltage at the resonant frequency of the LC circuit, perhaps the LC oscillations can be sustained. I think this is why they include transistors in radio circuitry?

 

[James2018]

Still, someone told me, that even with superconductors, perpetual exchange of energy forever between capacitor and inductor is impossible but he didn't mention why. Perhaps the energy is eventually radiated as far-field radio waves?

 

[Baluncore]

Must you use the ammeter and compass?
Would you be satisfied with hearing a radio frequency oscillation?
Do you have access to an oscilloscope and/or a broadcast band AM radio ?

You need to answer some questions before we can advise you on changing the circuit.

 

[James2018]

You need to answer some questions before we can advise you on changing the circuit.

Must you use the ammeter and compass?
Yes, it is the only way to know if there is a magnetic field in the coil that can be recharged by the capacitor.

Would you be satisfied with hearing a radio frequency oscillation?
That would tell me about the fact that oscillation is occuring, but not about the magnitude of the magnetic field, like a compass does when I measure the deflection angle. And the resonant frequency of my circuit seems to be around 25-35 KHz. Well, I guess I could use a smaller inductor for a larger resonant frequency.

Do you have access to an oscilloscope and/or a broadcast band AM radio ?
I have access to a radio that is AM and FM, I can switch between modes.

 

[Baluncore]

And the resonant frequency of my circuit seems to be around 25-35 KHz. Well, I guess I could use a smaller inductor for a larger resonant frequency.

The original resonant frequency of your 8 turns with 1 uF is about 122 MHz. With 80 turns, that would come down to 1.22 MHz in the broadcast band.

Must you use the ammeter and compass?
Yes, it is the only way to know if there is a magnetic field in the coil that can be recharged by the capacitor.

Then you must change the value of the L and C to make the resonant frequency less than 1 Hz.

Look for a transformer to use as an inductor. Maybe you could find a transformer from a heavy dead microwave oven. Use the high-voltage secondary winding.

 

[James2018]

The original resonant frequency of your 8 turns with 1 uF is about 122 MHz. With 80 turns, that would come down to 1.22 MHz in the broadcast band.


Then you must change the value of the L and C to make the resonant frequency less than 1 Hz.

Look for a transformer to use as an inductor. Maybe you could find a transformer from a heavy dead microwave oven. Use the high-voltage secondary winding.

I did switch my circuit on and off and at the lowest end of AM radio band, something like a few hundred kiloHertz the radio receiver antenna picked up a short disturbance, something like a "P-r-r-r..." sound lasting a second. Only when I connect and disconnect the wire from the negative terminal of the 3 V coin battery this happens. The capacitor and inductor remain connected in series. And I did use a smaller coil.

 

[berkeman]

I did switch my circuit on and off

How are you turning your power supply "off"? If it is still connected to the circuit with power "off", it will represent a dissipative load.

I did switch my circuit on and off and at the lowest end of AM radio band

How are you turning a power supply on and off at that rate?

 

[Baluncore]

How are you turning a power supply on and off at that rate?

The turn-off step includes VHF ringing, taking about 1 usec. That is generating a key-click across the MW spectrum. If it is a mechanical switch, then the switch bounce will appear in the audio.

 

[James2018]

You could also use a Quartz oscillator and some transistors to switch on and off very fast

 

[James2018]

The turn-off step includes VHF ringing, taking about 1 usec. That is generating a key-click across the MW spectrum. If it is a mechanical switch, then the switch bounce will appear in the audio.

Yes i did switch on and off manually but i am planning to switch on and off very fast using the frequency of a Quartz oscillator

 

[berkeman]

Yes i did switch on and off manually

Switch what on and off? What is your "power supply" and how is it connected in the circuit?

 

[Vanadium 50]

I think a superconductor

Do you have a superconducting magnet? If not, isn't this a little silly?

Honestly, I would reread everything people have said before posting again. That will help you better focus your questions.

I wish you had taken my suggestions more seriously. In one second, the current travels about 150000 miles. The resistance of wire is about 500 ohms per mile. So we are talking ~75 MΩ. That means, for any reasonable voltage, you are talking nanoamps by the time you start to measure it.

There is no way you will measure the magnetic field from nanoamps with a compass. What you propose is impossible from the get go.

 

[Baluncore]

In one second, the current travels about 150000 miles. The resistance of wire is about 500 ohms per mile. So we are talking ~75 MΩ.

What has the length of a wire got to do with an LC resonant circuit?

There is no way you will measure the magnetic field from nanoamps with a compass.

The amp⋅turns will be sufficient if the compass is within the inductor.

 

[Vanadium 50]

What has the length of a wire got to do with an LC resonant circuit?

But it's not an LC circuit. It's an RLC circuit. And for this to work as intended, it needs a very high Q, one inconsistent with the parameters given.

 

[James2018]

This is the circuit which perturbs the radio antenna at 530 - 550 KHz AM with a p-r-r sound. The capacitor is unpolarized and has 1 microfarad. Every time I graze and brush the wire very closely to the positive terminal of the battery, the antenna picks up an interference louder than its normal signal. But I am planning to use transistors and quartz crystal in the near future to switch the DC voltage from the battery on and off automatically at a very high frequency.

I think they use even smaller coils and smaller capacitances inside a FM radio.

 

[James2018]

What has the length of a wire got to do with an LC resonant circuit?


The amp⋅turns will be sufficient if the compass is within the inductor.

It depends on the battery, if it is alkaline it delivers 1 A if it is not alkaline delivers about 50 mA. So the magnetic field measured by the compass either deflects its needle by 90 degrees for 1 A or 15 degrees for 50 mA. I was wondering why using non-alkaline batteries merely deflected the compass by 10 to 15 degrees.

 

[Vanadium 50]

If the resonant frequency is ~100 MHz, it means the field direction reverses every few nanoseconds. The compass needle won't move, and even if it did, your eye can't detect it.

 

[DaveE]

Every time I graze and brush the wire very closely to the positive terminal of the battery, the antenna picks up an interference louder than its normal signal.

This sounds more like a spark gap transmitter than a resonant circuit.

 

[James2018]

This sounds more like a spark gap transmitter than a resonant circuit.

True, remember the radio interference caused by the fluorescent tube starters? This is why they added a nanoFarad range capacitor inside the starter, a "radio interference suppression" capacitor.

Also when I move the wire away from the battery using my hand, there is a very short tap sound heard in the radio antenna. Even shorter than when I brush the wire against the battery terminal.

My guess is that a transient LC oscillation and a spark sound the same way, containing a wide range of frequencies because of the time-frequency uncertainty principle which links short durations of a wavepacket to a wide range of frequencies in the spectrum. The only way I would know if it's due to LC oscillation is if I sustain the oscillations with something that provides gain.

 

[James2018]

If the resonant frequency is ~100 MHz, it means the field direction reverses every gew nanoseconds. The compass needle won't move, and even if it did, your eye can't detect it.

Same with 500 KHz. Inertia prevents the needle from moving that fast. But my guess is that the charge stored in a 1 microfarad capacitor is not enough to discharge into the inductor and recharge a magnetic field strong enough to deflect the compass by 90 degrees.

 

[James2018]

This is what the simulation shows, a decaying oscillation when I disconnect the switch

 

[James2018]

Now, I connected the collector of a transistor in series with an inductor to the positive terminal of the battery. I connected in series the base of the transistor with a resistor and with the 1000 MHz quartz oscillator to the negative terminal of the battery. I connected the emitter of a transistor to the negative terminal of the battery also. And I connected a capacitor in parallel to the quartz oscillator. Can this circuit sustain LC oscillations?