Electric Field Lines


Is it a MUST that the field line direction be the same as the velocity of a positive charge in the field area? When we are drawing the elecrtic field lines, should we stop at the surface of the charged metal objects?

When we are drawing the gravitational field lines, we draw them into the centre of the object. I think if the object is a conductor, then we have to draw the electric field lines into its centre too. But for an insulator, the electric field lines will be stopped by the insulating surface. We should draw the electric field lines only to its surface. Is it correct?


It is the acceleration of the positive test charge that defines the direction of the field in which it finds itself, not its velocity. If the test charge started from rest then the acceleration and velocity will point in the same direction, which by definition is the direction of the electric field. If the test charge had some initial velocity, then the acceleration and velocity may not be in the same direction, unless the charge is massless. I think in introductory electricity courses we usually do not worry about the initial velocity of a test charge. In that case yes the electric field MUST be in the direction of a positive test charge velocity because that is the way the field is defined.

The presence of matter does not by itself interrupt electric fields. Steady state(not time dependent) electric fields can not exist inside conducting material because of the nature of a conductor. In conductors the electrons are free to move under the influence of any field. Therefore they will rearrange themselves in such a way as to completely cancel any internal field. So field lines should stop at any metal surface, charged or uncharged.

In insulating material the electrons are locked up in their atomic orbits so they are not free to distribute themselves throughout the material in response to any internal fields. Therefore in insulators internal electric fields can exist. Quite the opposite from your first impression.

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