Electricity and electric current?

A forum set up for physics questions from students in the courses PHYS1121, 1131, 1221 and 1331 at the University of New South Wales. It is intended for questions that cannot readily be answered in class,
either because they fall outside the main syllabus and therefore would be distraction (however interesting) or for other reasons.

Moderator: msmod

User avatar
joe
Posts: 755
Joined: Fri Aug 29, 2003 11:57 am
Location: Sydney

Re: Electricity and electric current?

Post by joe »

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 the magnetic field, and the changing magnetic field produces a voltage by Faraday's law)
So even a single wire operates as a transmission line
https://en.wikipedia.org/wiki/Transmission_line
and so there is a delay between a signal input at one end and its arrival at the other. Depending on the capacitance and inductance per unit length, this can be very much slower than the speed of light.
Boris Lagutin

Re: Electricity and electric current?

Post by Boris Lagutin »

Thank you, Joe.

So the speed of your charge redistribution and the speed of the electro-magnetic field propagation are different. That's what I thought. I still have many questions, but I will ask them later.

Happy Holidays!
User avatar
joe
Posts: 755
Joined: Fri Aug 29, 2003 11:57 am
Location: Sydney

Re: Electricity and electric current?

Post by joe »

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 that charge through a finite resistance or inductance takes time. So the charging time depends on the geometry of the circuit. It is not simply related to the speed of light or any other speed.
Post Reply