Search found 464 matches

by joe
Mon May 29, 2017 8:53 am
Forum: Physics Questions
Topic: Electrons and Gravity?
Replies: 3
Views: 139

Re: Electrons and Gravity?

If your definition of a particle is something that can be seen with the eye or photographed with an ordinary camera, then yes, an electron is not such a thing. In physics, particles have well defined position, moment and trajectory. Waves have well defined wavelength and frequency. I think that all ...
by joe
Sun May 28, 2017 12:13 pm
Forum: Physics Questions
Topic: Electrons and Gravity?
Replies: 3
Views: 139

Re: Electrons and Gravity?

A direct measurement would be a very difficult experiment, because the gravitational effect is so small and any stray electric or magnetic effects so large. However, lots of indirect experiments have been made to look at possible different gravitational effects on n, p and e. Suppose that you measur...
by joe
Thu Dec 29, 2016 7:53 am
Forum: Physics Questions
Topic: Electricity and electric current?
Replies: 17
Views: 1013

Re: Electricity and electric current?

If there are no charges present, then yes, an electric field can propagate at the speed of light. But your pieces of wire have charge in them. To change the voltage of a piece of wire, you need to transfer charge to its surface. The inverse ratio of those is the capacitance (C = Q/V). To transmit th...
by joe
Mon Dec 12, 2016 10:23 am
Forum: Physics Questions
Topic: Electricity and electric current?
Replies: 17
Views: 1013

Re: Electricity and electric current?

The single wire has a (very small) capacitance per unit length. (In other words, you need to put a tiny static charge on it to raise its voltage. That tiny charge ends up on its surface.) It also has a (very small) inductance per unit length. (In other words, when you change the current, you change ...
by joe
Fri Dec 02, 2016 8:49 am
Forum: Physics Questions
Topic: Electricity and electric current?
Replies: 17
Views: 1013

Re: Electricity and electric current?

There's a subtle point here. Before you close that switch, the left hand wire is at 10V, the RH wire at 0 V. A ms after you close it, the LH wire is at 10 V (bottom) and 5 V (top), varying smoothly between the two. The RH wire is at 0 V (bottom) to 5 V (top). (There is also a current flowing through...
by joe
Fri Dec 02, 2016 8:14 am
Forum: Physics Questions
Topic: Double-slit experiment revisited....again....
Replies: 16
Views: 6442

Re: Double-slit experiment revisited....again....

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...
by joe
Fri Dec 02, 2016 7:39 am
Forum: Physics Questions
Topic: Electricity and electric current?
Replies: 17
Views: 1013

Re: Electricity and electric current?

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).
by joe
Wed Nov 30, 2016 11:15 am
Forum: Physics Questions
Topic: Electricity and electric current?
Replies: 17
Views: 1013

Re: Electricity and electric current?

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...
by joe
Wed Nov 30, 2016 9:56 am
Forum: Physics Questions
Topic: Electricity and electric current?
Replies: 17
Views: 1013

Re: Electricity and electric current?

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.
by joe
Wed Nov 30, 2016 9:16 am
Forum: Physics Questions
Topic: Electricity and electric current?
Replies: 17
Views: 1013

Re: Electricity and electric current?

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...
by joe
Wed Nov 30, 2016 8:14 am
Forum: Physics Questions
Topic: Electricity and electric current?
Replies: 17
Views: 1013

Re: Electricity and electric current?

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...
by joe
Sun Oct 16, 2016 6:20 am
Forum: Physics Questions
Topic: Discussion of Integral calculus
Replies: 106
Views: 4057

Re: Discussion of Integral calculus

Yes, I changed the title of the thread. I don't think we have uncovered a paradox.
Joe
by joe
Mon Oct 10, 2016 11:14 am
Forum: Physics Questions
Topic: Discussion of Integral calculus
Replies: 106
Views: 4057

Re: Fundamental paradox of Integral calculus?

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'.
by joe
Mon Oct 10, 2016 9:46 am
Forum: Physics Questions
Topic: Discussion of Integral calculus
Replies: 106
Views: 4057

Re: Fundamental paradox of Integral calculus?

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...
by joe
Mon Oct 10, 2016 9:03 am
Forum: Physics Questions
Topic: Discussion of Integral calculus
Replies: 106
Views: 4057

Re: Fundamental paradox of Integral calculus?

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 ...

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