All Students READ THIS

One of the major problems students ( and the rest of you) seem to have in electrostatics and magnetics problems is distinguishing fields established by sources and the fields the test charges and test currents create. A charged particle exerts no force on itself.


The question determines which charges or currents are viewed as source(s) and whch as test(s).

The force comes from the field produced by other ( source charges). For example, if I tell you an electron is placed in a field of 300 N/C and ask you for the force on the electron, acceleration of the electron, etc., you have no business fidng the field created by that electron. If you multiply a charge by the field that same charge creates you are finding the force of the charge on itself, and doing it incorrectly since the field right a at charge must be infinite(1/ zero squared, since the ditance between the charge and itself is zero). [ Note that with charge distributions an individual charge can feel a force from the rest of the charges in the distribution but there is no net force on the entire distribution from itself or any single charge from that charge itself.]

The same applies to currents, source currents create magnetic fields and test charges or test currents experience the field and feel a force.


[A wire can exert force on itself if it is in a coil or other curved shape, BUT NO NET FORCE ( THE LEFT SIDE CAN PUSH OR PULL THE RIGHT SIDE BUT THE RIGHT SIDE PULLS THE LEFT SIDE JUST AS HARD IN THE OPPOSITE DIRECTION.]