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The Gold Leaf Electroscope

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A Gold Leaf Electroscope

When ever you come across charges, you'd (most probably) also come across the word "electroscope." So first, let's set the idea of what an electroscope really is; and why we need it.

To put it simply, an electroscope is that thing -instrument/device- which gives us a QUALITATIVE idea of presence -or absence- of charge on a body. If the person using the electroscope is smart enough, then he may also coax the electroscope into telling him about the relative magnitude and polarity (sign) of the charge.

Okay then, we've briefly -but simply- defined an electroscope... and we've also said why we use it. Now comes the interesting part: How does it work? Don't worry yet, because the working of an electroscope -as you will soon see- is utterly simple!

An electroscope works on two -very- basic ideas:1. Like charges REPEL each other (while opposite charges attract.)2. When you charge a conductor, the charge gets distributed all over the conductor*.

1. We've all been studying the "like charges repel while unlike charges attract" thing right form middle school -if not earlier.- So, if I've got a very thin piece of aluminium foil that's got positive charge on it, it would repel another piece of aluminiun foil which also has positive charge.

2. Whenever charge is introduced on a conductor, it gets distributed over the conductor. Now, such distribution of charge may not be even; it is the fact that there is some distribution that is useful in building an electroscope.

A more scientific and less jazzy Gold Leaf Electroscope

Alright then, I think we now know enough to have a look at a less jazzy and slightly more scienific schematic of the gold leaf electroscope shown alongside.

Say I've got a positively charged glass rod and I touch it to the metallic disk of the electroscope. As soon as I touch it to the metallic disk, positive charge will flow out of the rod and into the metallic disk*. Now keep in mind that the (i) metallic disk, (ii) conducting bar and (iii) gold leaves; form a single conducting system. They are all conductors and hence charge can freely flow form the disk to the leaves. Thus the three elements may be viewed as a single conductor. As the system is a conductor, positive charge introduced on the metallic disk will be distributed all over the system. More importantly, a part of this charge will be distributed on the gold leaves. Here comes the crucial part. Both the gold leaves will get positively charged and will hence repel each other!

The gold leaves will spread out to form a ^ shape as shown in the very first figure i.e. their lower ends will move away form each other on account of bearing like charge (which in our case is positive.) Well technically, the upper portion of the two leaves will also repel each other, but the repulsive force won't be strong enough to break the welding/soldering between the leaves and the leaves will hence stay attached to the rod. The result (as already stated) is that as the two leaves tend to move away form each other. By this, we get visual conformation of the presence of charge. This tells us that indeed, the glass rod with which we touched the metallic disk was charged.

However, at this point it is pertinent to note that even if had touched the metallic disk with a negatively charged object, both the gold leaves would have acquired negative charge and would have moved apart in a similar manner. The same is depicted in the following schematic:

Since we make actually CONTACT with the metallic disk of the electroscope, .. charging the electroscope in this way is know as charging by contact. One can also charge the electroscope by induction, whcich we shall discuss later.

I need you to observe two things in the above depiction.1. In the above schematic, there's just one gold leaf! Well the reason (as far as I can think) is that gold is utterly expensive and we don't want to use it all up in making electroscopes. At the same time, I want you to appreciate that despite using just one leaf, the principle on which the instrument works is precisely the same. Its just that instead of two leaves repelling each other, one leaf is being repelled by the conducting bar since the two are bear the same charge.

2. As mentioned earlier, the polarity of introduced charge doesn't determine how much the leaf deflects. Deflection of the gold leaf (or leaves) is single-handedly governed by the magnitude (i.e. amount) of charge introduced -irrespective of polarity.-

At this point, it is needless to say that if you touch the electroscope with an electrically neutral object, the electroscope will not show any response. It just won't react!

Thus now, if you scroll back up and read how we defined an electroscope, you'll understand just what I was saying. The electroscope only tells us if the body with which it is tested (i.e. the body you touch the metallic cap to) is charged or not. It doesn't tell us the polarity of the charge and neither does it tell us how much charge was there in the test object. Thus it provides for only primitive QUALITATIVE judgement and quantitative measurements.

(In our example, the positively charged glass rod was the 'test object'. I'd also like to add here that since we already knew that the glass rod was positively charge, the electroscope is technically useless to us; as all it can tell us is that the glass rod it charged. But we already know more than that. Not only do we know that the rod is charged but also that it is positively charged. However, I still used the 'positively charged glass rod' as our test object to make you understand how exactly the electroscope works.)

First, let's tackle the easy part. I've already said that the extent of deflection is somewhat proportional to the magnitude of charge introduced. i.e. if the test object has more charge, the electroscope will show more deflection.

What is crucial to observe is that in the above experiment, we can't tell if P and Q are oppositely charged or otherwise. One can only comment on the relative magnitudes of charge on P and Q. If the charge on P is '+3 units' while that on Q is '-12 units,' the electroscope will show more deflection for Q than for P, thereby telling us that the magnitude of charge on Q is more.

Okay then, we now have clearly understood that the relative magnitude of charges on two bodies can be easily judged form the extent of deflection of the gold leaf. However, what about polarity???

While it comes to polarity, we may want to use the electroscope differently in different situations. Let's first again assume that the charge on P is '+3 units' while that on Q is '-12 units.' Let's now see how the electroscope can be used to tell that P and Q are oppositely charged.

Like last time, we'll first touch the metallic disk of the electroscope P. We'll observe certain deflection. Now, WITHOUT DISCHARGING THE ELECTROSCOPE, i.e without refreshing the electroscope, we'll touch it's metallic disk with Q. What should we observe?

Okay, let's go through this slowly and step-by-step. As soon as we touch P to the metallic disk, the gold leaves will acquire positive charge and deflect away form each other. Keep in mind that the charge on the gold leaves is positive. If Q is now touched to the metallic disk, there will be inward flow of negative charge. This new incoming negative charge will first neutralise the initially existing positive charge on the gold leaves and then impart new negative charge to them. Thus, while the positive charge on the gold leaves is being neutralized, we'll see that that gold leaves MOVE TOWARDS each other as the force of repulsion between the two leaves will decrease on account of decrease in magnitude of like charge*. But the leaves will move towards each other for a very short period of time as once they are completely neutralized, they will both acquire negative charge and hence again repel each other again! Thus we'll observe that as soon as we touch Q to the metallic disk of the electroscope, the leaves will collapse for a very short interval of time and then again deflect away form each other. Wait......there's more! Since the magnitude of charge on Q was '-12 units' while that on P was '+3 units,' after neutralisation, there will still be net '-9 units' of charge on the electroscope! This is evidently greater in magnitude than the initial '+3 units' i.e. the magnitude of charge on the electroscope will be greater than it previously was. Thus, after collapsing the two leaves will now deflect to a greater extent!

Ahh! that was one nasty experiment! What the hell was happening? The leaves first deflected away form each other.... then they collapsed for a very short time interval.... and then they showed deflection to a larger extent! Phew. Okay.... I know this could be a little hard on you. But look at the bright side. That electroscope which we previously labeled as 'unable to detect polarity of charge,' in fact told you that P and Q are oppositely charged! If it weren't so, you'd never see the leaves collapse! What's more, the same damn electroscope also told us that the magnitude of charge on Q is more than that on P! Hmm, the electroscopes utility seems to have suddenly risen.

Okay, at this point, I am heavily tempted to end this article. However my conscious won't let me. I haven't yet told you how to find the exact polarity of charge on a test object! I haven't uttered a word about charging by induction.... "charging by induction" may sound scary but it's not. Amm, but I understand that you probably aren't exactly in the right state to listen about how that is done anymore. No problem... I'll soon write another article explaining it and link to it from here.... For now, I'll allow you to simply digest this much.

I also promise you that if you've understood most of what is written in this article and the article to follow, electroscopes will never be an issue in the exam. ... It won't even come close to bothering you.

___________________________________________________________________________________Foot Notes:* Gauss theorem tells us that charge never resides inside a conductor but only on it's surface and the net E-field inside the conductor has to be zero. This argument is equivalent to the argument that the conductor is an equipotential. Thus when I say "charge gets distributed all over the conductor," I essentially mean that it gets distributed all over the conductors surface.For those interested, Gauss law states that:The total electric flux passing through a closed surface equals 1/ε times the net charge enclosed in the surface.

* We know that positive charges (protons) never flow in conductors. It is the electrons (negative charge) that flow and hence also constitute electric current. However, it is essential to note that the flow of negative charge in one direction is equivalent to

the flow of positive charge in the opposite direction. To put it more precisely, whenever a positively charged object (such as a glass rod) is touched to the metallic disk of the electroscope, electrons form the lower parts of the conducting system will be attracted to the metallic disk. Thus there will be deficiency of electrons in the gold leaves and thus they would acquire positive charge. This is equivalent to saying that positive charge will flow form the glass rod to the gold leaves.

* This can be easily explained on the bases of Coulomb's law. To roughly put it, the law states that the force of repulsion between like charges is directly proportional to the magnitude of charge on them. This of course is not the exact statement of the law but it serves our purpose. But for those of you interested, here's the real Coulomb's law:The electrostatic force of attraction or repulsion between two isolated point charges is directly proportional to the product of the magnitudes of the two charges and is inversely proportional to the square of the distance between them. The force acts along the straight line joining the two charges.

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Posted by Samuel Burke on Sunday, November 27, 2011

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