Originally posted by: Backintime
Originally posted by: acsinuk
... I do not agree that current can be defined as charges moved in unit time. ...
I thought charges are equivalent to electrons which are energy carrying particles. I have a few question here:
a. What am I measuring when I put a clampmeter around a single 1.5mm2 conductor which has a volt-drop of 4.15V (the clampmeter says 13A and the length of the conductor is 22m long) ?
The clamp meter is actually measuring the magnetic field corresponding to the current flowing in the wire.
b. Why am I getting almost zero current reading when I put a clampmeter on a single phase circuit conductors (phase and neutral)?
The phase conductor generates a magnetic field corresponding to the current in the phase conductor.
Similarly, the Neutral does the same.
Since the Phase and Neutral currents are (should be) as near as dammit the same, the magnetic fields are the same size, BUT the current in the Neutral is going "back" to the supply down the cable, whereas the phase current is coming "from" the supply up the cable, so the magnetic fields are also in opposite directions.
In theory, therefore, they cancel each other out.
c. How do I explain to my apprentice electricians what is 'current'?
Simplest is the amount of electrons moving past a point in the circuit. Use the analagy of water flow in a pipe - great for d.c.
But how can we use water analagy to see what's going on with a.c.?
Think about water flowing past a water wheel. Send water top left to bottom right of the wheel, so wheel goes clockwise, and drives machinery to do work.
After a short time, change direction so water flows top right to bottom left, wheel going anti-clockwise.
It can still drive machinery and do work.
After same time, reverse it again.
And you've got "a.c. water".
EUR ING Graham Kenyon CEng MIET TechIOSH
Principal and Proprietor,
G Kenyon Technology