School of Physics

We've now put a video of an experimental demonstration of Young's experiment with single photons. It's at http://www.animations.physics.unsw.edu.au/light/interference/index.html#4.6 The introduction to Young's experiment itself and to the nature of light are at http://www.animations.physics.unsw.edu...

Metals have the property that the outer electrons are 'shared' among all the atoms: they can move easily.
In insulators (like most plastics) the electrons are 'tightly bound' to their atoms. So they do not conduct electricity (at least not until the field reaches many MV per metre).

The potential difference (and thus the field) is initially across the switch. It then very quickly spreads long the wires until, in steady state, there is almost no PD across the switch and it is all across the wires (and the internal resistance of the battery, probably negligible if the wires are k...

the difference in electrical potential produces a field and that is what moves the electrons.
Initially, this PD is across the switch.
After switch closing, the PD very rapidly spreads along the wire.
I've given a more detailed description above.

Do I understand you correctly that you write that the electric field in the portion of the wire connected with the battery anode (negative lead) has already existed before switching on (my drawing)? No. I wrote: With the switch open, the wire on one side of the switch is at 10 V, and on the other i...

With the switch open, the wire on one side of the switch is at 10 V, and on the other it is zero. So 10V potential difference is across the switch. Just before the switch contacts touch (distance ~ µm), the field is large enough to ionise air, so a spark allows current to flow. This changes the pote...

Yes, I changed the title of the thread. I don't think we have uncovered a paradox.
Joe

Actually, based on your performance in the MOOC, I think that you are smart.
What this argument seems to be about is the definitions of limits, and some (understandable and common) misunderstandings about the use of 'to infinity' and 'to zero'.

You ask What is F? When I wrote Limit of f(h) goes to F as h goes to zero F was the value of the limit of f(h) as h goes to zero. When we say "The derivative of f(x) is g(x) ", we mean The limit of {f(x+h)-f(x)}/h goes to g(x) as h goes to zero. And by that we mean (quoting from above): fo...

That's why you have to be careful with limits and the language you use. The limit of N*(1/N) as N goes to infinity does not mean infinity times zero. It is not adding zeros. When we say "Limit of f(h) goes to F as h goes to zero", we define this to mean: for any epsilon, however small, we ...

summing as many zeros as possible results in zero. Consider the expression (Sum N terms) each of which is (1/N) Now, what about the limit of N*(1/N) as N goes to infinity? Would you like this to go to zero? Boris, it's very important to realise that, taking such limits, we are NOT adding zeros. The...

sounds like a classic outtake from the legacy of a Michael Faraday homopolar, or unipolar, disk/motor/generator. Homopolar generators and motors are described here on our Physclips site http://www.animations.physics.unsw.edu.au/jw/homopolar.htm Just to be clear, homopolar generators and motors do w...

Or, to put it another way, it is quite possible to integrate discontinuous functions, such as
y = 0, x < 0 and
y = 1, x >= 0.

But in your post of Mon Sep 12, 2016 6:22 am,
h is dummy variable, inside a limit statement, while delta x is a quantity that multiplies the limits. These are defined very differently.
Joe

We cannot divide nothing by nothing and get some slope

Agreed, and so we don't do that. Instead, to take a derivative, we look at the ratio delta y/delta x and examine its limit for small delta x.