Charge on a battery’s terminals

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Recently when I was studying electrochemical cells, I was thinking about the fact that for a cell to drive current through an external circuit, the two electrodes must be at different potentials, and surely this means that there is a static charge on one or both of them. Otherwise how would they be at different potentials to begin with? I wanted to try and detect this charge.

I had come across this simple FET electroscope circuit some time back. It’s an amazing circuit where an LED is glowing by default, and turns off when a charged plastic scale is brought near the hanging gate of the FET. The electric field of the negative charge on the scale induces charges on the gate which turn the FET off. When you remove the charged scale from its vicinity, the LED comes back on.

The charge on the plastic scale is probably at a potential of thousands of volts, and the field is strong enough to make the FET turn off at a distance. But the charge on a battery’s terminal is obviously at a much smaller potential. So when I brought the negative terminal of a 9V battery near the gate of the FET nothing happened. But when I actually touched the negative terminal of the battery on the FET gate, the LED turned off and remained that way.

I think the LED stayed off because when I touched the gate of the FET with the negative terminal of the battery, some negative charge must have been transferred onto the gate, as opposed to the induced charges when the plastic scale was brought close. I could get the same result by rubbing the scale on the gate (not every time, because charge transfer from an insulator is not easy). If I now touch the gate with my finger the charge flows to me and the LED comes back on!

Unfortunately this doesn’t happen with single 1.5 V cells. I don’t understand why, but I read somewhere that the FET needs the field corresponding to around 7V or so of potential, to turn off. But the article linked to above says that it can detect potentials as small as one volt. But then I used a different FET to the one mentioned in the article. I need to understand FET’s better to make better sense of this.

According to what I’ve recently read in electrochemistry, it seems that even a single zinc plate dipped in acid, without a second electrode, develops an electrostatic charge, due to the different rate of oxidation of zinc atoms and reduction of hydrogen ions. It would have been amazing to be able to detect this charge.

Current without a ‘closed circuit’?

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It’s common knowledge that you need a ‘closed circuit’- an unbroken, continuous, conducting path- for an electric current to flow. If you are using a battery, this usually means an unbroken path from the positive terminal of the battery, through an LED (or whatever device you are running), all the way to the negative terminal of the battery.

But the closed loop between the terminals of the battery is strictly not necessary. What is important is that an electric current needs to flow through the LED, and for this all that is required is that the LED is connected between two points at different electrostatic potentials. The terminals of a battery contain static charges, and one could theoretically draw a small current for a small duration if we connected an LED between one terminal of the battery and a neutral object. The neutral object will act as a source or sink of electrons, depending on whether we are connecting it to the positive or negative terminal respectively. But for the chemical reactions in the battery to continue happening to provide a continuous current, the other terminal also needs to be operating (this is something that needs discussion, but I’ll do it in another post).

To test this out, I connected the positive lead of the LED to the positive terminal of a 9V battery, and held the negative lead with my fingers (myself being the neutral body). Obviously the LED didn’t light up. But then I connected the negative terminal of the battery to the earthing in an AC mains socket, so that it can act as a sink for electrons from the negative terminal of the battery. And the LED lit up! Not brightly, but that’s understandable, since my body has a large resistance.

Here’s a photograph of the LED glowing when I touch its negative terminal. The second picture shows the LED when it’s off, so that you can see the difference.