QUESTION #245

# Why the lines of force in an electric field start at the +ve charge and terminate at the -ve charge?

Asked by: Shweta Kala

### Answer

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