again I’m counting on your help. I have taught my students not to believe anything in physics before they can see the reasoning that leads to a conclusion. In most cases high school physics can be explained satisfactorily using principles that are familiar to the students. But then I tried to teach Faradays’s ice-pail experiment (or Franklin’s; the name depends on the nationality of an author!). The experiment uses a positively charged metal ball which is lowered inside a metal can. This induces a charge on the inner surface of the can leaving the outer surface positively charged. Then the ball touches the inner surface. My students asked:
Why the positive charge is totally on the outer surface after the metal ball has touched the inner surface (the ball and the inner surface will be neutral)? Wouldn’t it make more space for the charges that repel each other if they were distributed both to the inner and the outer surface?
It is easy to accept that all excess charge resides on outer surface of a metal sphere but the can has quite different geometry. And if the can is too large when compared to the ball the experiment is supposed to give a different results (I don’t understand this part myself).
The experiment becomes more understandable if the Gauss’s law is applied. But unfortunately it is not part of the IB syllabus and it would take a bit too much time to teach it first. I have tried to argue that the charges on the outer surface don’t ”know” if the ball has touched the inner surface or not because the electric field inside the conductor (between the outer and the inner surface) is zero and that’s why the surfaces are shielded from each other. In addition the net charge inside the can is zero all the time: before and after the touch. Could you find a more convincing argument?
Regards,
Antti Savinainen
Kuopio Lyseo High School/IB
Finland