Theory question again

[Volta]

GAR
these are great little teaching tool KEEP THEM COMING apprentices (and others) love hands on things to learn. Science, ohms law, meter use, all wrapped up in one

Yup, I'll second that.

 

[winnie]

Interesting. So as this is a closed loop, do we need to consider that there are precisely zero volts present in the system now (no matter how the current was initially injected)?
This circuit seems short. My simple mind at least needs to assume that voltage was present to start this flow.
Do you know who is running this experiment?

As an example of a practical application of this effect, the magnets used for various spectroscopy systems and for MRI imaging pretty much all use persistent currents in superconducting solenoids.

The technique is to have a coil of superconducting wire. A short segment of this wire has two terminals and a heater. The heater keeps the short segment of wire above its superconducting temperature, so that short length has a resistance. This short length of wire with a heater is the 'superconducting switch'.

An external power supply is connected to the two wires. Voltage gets applied, which causes the current to ramp up; while the current is changing you still need to overcome the inductance of the wire. After the current is stable at the desired temperature, the heater is turned off and the short length of wire starts to superconduct, closing the loop.

-Jon

 

[Volta]

. . . An external power supply is connected to the two wires. Voltage gets applied, which causes the current to ramp up; while the current is changing you still need to overcome the inductance of the wire. After the current is stable at the desired temperature, the heater is turned off and the short length of wire starts to superconduct, closing the loop.

-Jon

So the persistant current is / can be AC?

No voltage is present at that time?

 

[gar]

100214-1255 EST

ibew501ed:

Following are some measured values:

Sensing lamp 75 W reflector flood. Room temperature resistance is 13.1 ohms at 68 deg F. Note a lamp bulb is a good thermometer. Outside today I measured 12.2 ohms at about 32 deg F.

Excitation lamp 250 W reflector flood at 124 V.

Fluke 27 meter in ohms position.

At about 30" face to face 13.6 ohms.
At about 12" 16.2 ohms.
Quickly returns to about 13.4 ohms, then over a long time returns to 13.1 ohms.

.

 

[mivey]

I know you were joking, but my suggested experiment was not very clear.

I was, and I thought it was clear.

 

[LarryFine]

Actually I can take a second 250 W flood and point it at the one of which I am measuring and see the resistance change.

"And, for next week's show, boys and girls, we'll feed amplified music into one bulb, connect the other one to a second amplifier, and send sound across space with light waves."

 

[winnie]

So the persistant current is / can be AC?

The persistent current itself is DC. But with zero resistance and a very large inductance, you need to think in terms of extremely low frequency AC for the ramp up. The power supply voltage and the rate that the current ramps up is calculated using the inductance of the coil, and my understanding is that this can take hours or days.

In theory you could have a superconducting coil connected to a capacitor, and get an oscillation that lasts a very long time...but if you have AC then you will get dissipation by electromagnetic radiation, so this would not be persistent.

-Jon