Why the lines of force in an electric field start at the +ve charge and terminate at the -ve charge?
Asked by: Shweta Kala
It's purely by definition. It was originally thought that the 'body' that carried electric
charge in a current conducting wire was positive, but of course we all now know that that
body is the electron which carries a negative charge. So in fact, if we say a current is
flowing from left to right, what we actually mean is that negatively charged electrons are
moving from right to left.
Answered by: Paul Axon, None, Theoretical Physics U/G University Of Sussex, UK
Lines of force don't REALLY start at the +ve charge and end on the -ve charge! In fact, we
don't know what way they go - they might be going the opposite way. So, to make sure that
everyone agrees with them travelling in a certain direction, we define what an electric
field line is.
We define it as 'The line along which an isolated positive charge would move if it were
free to do so'. Now, if you consider the situation with like charges repelling and unlike
charges attracting, that must mean that if you start with a positive charge and a negative
charge separated by a distance, and then place another positive charge beside the first
one, it will be repelled by this 9ve charge and attracted by the negative charge. So, the
direction in which this single positive charge will move is from positive to negative.
But this is only a convention adopted by scientists so that everyone is talking about the
same thing. If you plot the magnetic field of a magnet using Iron Filings, you see 'lines'
going from one pole of the magnet around the the other. But there are no arrows showing you
the actual direction. So, scientists agreed that to keep everyone talking about the same
thing, that magnetic lines of force should travel from North to South. It is exactly the
same for electric field lines and the definition of a magnetic field line is almost exactly
the same as that of an electric field line, except it refers to isolated North Poles
instead of isolated +ve charges.
Answered by: Michael Treacy, M.S., Physics Teacher, Ireland
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