Charge on a battery’s terminals

Leave a comment

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.

Holding on to radical questions


What is a radical question? Why do I get drawn to them? Why do they frustrate me?

I have a radical question when I believe that the way we generally do something is not smart, that there is a better way to do it. But we stick to the old way of doing things just because we have been doing it that way for a long time, and it’s convenient to just continue. And just keeping it going that way takes so much of our energy that we don’t look at things afresh.

This state of affairs pains me, since it pulls my energies inĀ other directions rather than focusing them on work I believe in, work that I believe is best for myself and the people I’m working with. It’s not just the idea of this waste of time and energy, there’s a real pain and frustration coming from the organism within. I feel that pursuing the radical question has the potential to make my work more of play, at the same time making it more useful for the people I’m working with. There’s a romantic notion of a more wholesome, happier life associated with the radical question.

Wherever you are, there will be some constraints which you have to accept as existential. Obviously you are not going to change the whole world! When you put on paper what are the constraints you are willing to work under to pursue your radical question, and what is the test to decide if it’s useful to hold the radical question within the constraints you have accepted, I think the radical question has the potential to become real and woven into your work. If it’s unrealistic you can drop the radical question and live with the status quo or look for another situation with a different set of constraints to pursue your radical question.

And either way you would have probably learnt a lot in the process.

Simple programs for teaching integers and overflow

Leave a comment

Click here to download

While teaching unsigned and signed binary integers, I wrote these programs so that the students can run them and see overflow in action. There are four programs- addu, adds, subu and subs- for adding and subtracting 8-bit unsigned and signed integers. The numbers to be added are given as arguments when running the program. The result is printed on the screen and also saved in a file, which can be opened with a hex editor to see the result in binary format.

While the students where running the programs they obviously had questions about the the dot-slash. That was a good opportunity to tell them that all the commands they run, ls or cd or chmod or anything for that matter, are all executable files residing somewhere in the file system. I made them type which ls and find that ls is actually /bin/ls.

I pointed out the interesting fact that ls, which was somewhere else in the filesystem, could be run by just typing ls and not necessarily /bin/ls, while the addu program which was right here in this folder you needed to specify that it is in the current folder (they knew that dot stands for the current folder). Then I showed them how by adding the current directory to the PATH variable, you could run it by just typing addu, like any other command in the system.

Current without a ‘closed circuit’?


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.