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Registered Member #193
Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022
In response to an earlier thread in which it was asserted that no current would flow below 1.7 volts, I thought I'd address the question of whether or not there's a threshold below which no current flows by doing the experiment. The electrolyte is dilute (about 1M) sulphuric acid in water. The temperature was about 18C and the electrodes are nominally 1 square cm platinised platinum nominally 1 cm apart. Current was supplied from a bench PSU via a 10 turn 1 K wirewound pot adjusted to increase the voltage in convenient steps The current was measured on a Keithley 2105 and the voltage on a Solartron 7150+ The currents and voltages were not particularly steady due to polarisation effects and are thus subject to rather large error margins, but the issue was whether or not a current would flow, rather than the exact value. I have rounded the data to 2 sig figs since that's about what I think the error margins were. I'd be delighted if someone were to make better measurements to confirm what I found. The results were Volts µA 0.025 0.39 0.049 0.56 0.1 1.2 0.15 1.8 0.2 2.9 0.25 4.7 0.3 6.8 0.35 8.2 0.4 9.1 0.45 13 0.51 21 0.61 44 0.7 55 0.83 120 0.9 150 1 180
Registered Member #235
Joined: Wed Feb 22 2006, 04:59PM
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Looks good. Why stop at 1V? Can you at least go to 1.25V, or 1.7V as asserted? electrode potential is 1.23V. The 180uA is in the realm of leakage currents for larger electrolytic capacitors. I understood the assertion of no current flow was an ideal first approximation, similar to the accepted ideal diode curve.
Registered Member #193
Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022
The original assertion was " an electrolytic cell, with dilute sulphuric acid and platinum electrodes*. This is actually very similar to a LED load, as there is a quantum mechanically defined voltage below which the intended behaviour doesn't happen. Below roughly 1.7v, you cannot split a water molecule."
and, yet there's a current flow at well below half that. The 1.23 volts figure sometimes cited assumes a whole bunch of things that are not always true and, in general, you get a small current, even with very low voltages.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Adrenaline wrote ...
Does current flow always result to splitting water? Relating to the LED, does current flow always result in light output?
My understanding is that whenever an electron transits from a higher energy state to a lower energy state (ie recombination) energy is ALWAYS released as a photon. On most occasions this isn't visible, and is converted to heat because it's absorbed before the photon reaches your eye and excites another electron as it decays, but in the case of an LED or plasma it's visible because the photons aren't intercepted before they reach your eye.
(Not all photons are emitted in the visible spectrum, either)
Registered Member #193
Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022
Adrenaline wrote ...
Does current flow always result to splitting water? Relating to the LED, does current flow always result in light output?
No. At least some of it goes to reducing dissolved oxygen and some may go tot other reactions with impurities. However the fundamental point is that the voltage depends on the concentrations of the reactants (among other things). This gives you a pretty good indicationof the voltage to expect but at the outset where the concentration of hydrogen is practically nil the equation is poorly defined- you either divide by zero or take the log of it depending on how you wrote the equation.
Going back to diodes (light emitting or otherwise) ... As a quick "hand waving" argument, the number of electrons above some given energy is exponentially dependent on the temperature. Making the energy gap smaller means that (exponentially) more electrons now have an energy above that limit. So you expect the number of electrons that can jump the barrier and carry a current to vary exponentially with the voltage. That's why the log of the current is (fairly nearly) proportional to the applied voltage.
There isn't some "hard and fast" limit below which no electrons flow, though measuring the current might be difficult if it's below a few nA.
Registered Member #2463
Joined: Wed Nov 11 2009, 03:49AM
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Apart from the obvious,(Faraday was apprenticed to a platinum monger) , If the experiment could be calibrated in Faradays, i.e. charge flow, the voltage could be dispensed with, and only the current considered.
If you only have one meter, then the current and voltage cannot be measured at the same time.
Also what the series resistance of the Keithley 2105 when measuring current ? It the constant voltage source has a substantial resistance in series, it acts like a constant current source.
Then , in this system, unlike a LED, which is not able easily to suck up (light) energy, hold it , and remember it, and skew results like a memristor , you could describe what is happening in terms of charge flow.
Voltage in past eras was measured with a potentiometer, which simply balanced when the current was determined not to be flowing.
You data could also look at the possibility that the plates in solution can condition themselves with repeated charge / discharge cycles.
Registered Member #193
Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022
radiotech wrote ...
Apart from the obvious,(Faraday was apprenticed to a platinum monger) , If the experiment could be calibrated in Faradays, i.e. charge flow, the voltage could be dispensed with, and only the current considered.
If you only have one meter, then the current and voltage cannot be measured at the same time.
Also what the series resistance of the Keithley 2105 when measuring current ? It the constant voltage source has a substantial resistance in series, it acts like a constant current source.
Then , in this system, unlike a LED, which is not able easily to suck up (light) energy, hold it , and remember it, and skew results like a memristor , you could describe what is happening in terms of charge flow.
Voltage in past eras was measured with a potentiometer, which simply balanced when the current was determined not to be flowing.
You data could also look at the possibility that the plates in solution can condition themselves with repeated charge / discharge cycles.
Measuring charge flow does not help if you are trying to plot current vs voltage. I have two meters and so I was able to measure I and V at the same time. It's more usual to specify the ammeter in terms of "voltage burden" rather than resistance. In this case it's 150 mV or better on the 10 mA range and 30 mV or better on the 100 mA range. But it doesn't matter because the voltage was measured directly across the cell. The cell does not know if it has 1 mA flowing through it because it is connected to about 10 volts through a 10K resistor or to about 10,000 volts through a 1 M resistor or anything else. So the voltage drop across the ammeter isn't important.
I do have a potentiometer, but it is time consuming to set up and does not offer the same precision of measurement as the digital meter. I also rather doubt it would draw less current
Registered Member #2463
Joined: Wed Nov 11 2009, 03:49AM
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Posts: 1546
The cell does not know if it has 1 mA flowing through it because it is connected to about 10 volts through a 10K resistor or to about 10,000 volts through a 1 M resistor or anything else. So the voltage drop across the ammeter isn't important.
The cell, or any load connected to a voltage source through a high (with respect to the load resistance) doesn't care as long as the load resistance remains constant.
The circuit current, with a load largely fractional compared to the total of the load and the series resistance remains essentially constant.
But in an experiment, where the source is 10 volts, and the cell voltage is in microvolts, the voltage across the cell, can 'sputter' i.e fluctuate because of surface chemistry. The excursions from higher to lower may be very fast, and may alter or change the results from what they might have been, had the voltage been held to strict limits.
You can ruin a storage battery with a trickle charge that has a voltage source, not clamped to a limit, because the terminal voltage will climb to whatever it can.
Registered Member #193
Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022
radiotech wrote ...
The cell does not know if it has 1 mA flowing through it because it is connected to about 10 volts through a 10K resistor or to about 10,000 volts through a 1 M resistor or anything else. So the voltage drop across the ammeter isn't important.
The cell, or any load connected to a voltage source through a high (with respect to the load resistance) doesn't care as long as the load resistance remains constant.
The circuit current, with a load largely fractional compared to the total of the load and the series resistance remains essentially constant.
But in an experiment, where the source is 10 volts, and the cell voltage is in microvolts, the voltage across the cell, can 'sputter' i.e fluctuate because of surface chemistry. The excursions from higher to lower may be very fast, and may alter or change the results from what they might have been, had the voltage been held to strict limits.
You can ruin a storage battery with a trickle charge that has a voltage source, not clamped to a limit, because the terminal voltage will climb to whatever it can.
Did you read the bit where I said "The currents and voltages were not particularly steady due to polarisation effects "?
Anyway, since the point of the exercise was to demonstrate that a current will flow with a voltage that's a lot lower than 1.7 volts, none of that stuff matters. And none of it is affected by the tiny voltage dropped across the ammeter.
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Voltage current curve for dilute sulphuric acid
