17.3 CHAPTER 17. ELECTROSTATICS
Negative charge actingon a test charge
If the charge, Q, were negative we would havethe following result.
-Q
Notice that it is almost identical to the positivecharge case. This is important – the arrows are the
same length because themagnitude of the chargeis the same and so is themagnitude of the test
charge. Thus the magnitude (size) of the force is thesame. The arrows pointin the opposite direction
because the charges nowhave opposite sign andso the positive test charge is attracted to the charge.
Now, to make things simpler, we draw continuous lines showing the paththat the test charge would
travel. This means we don’t have to work out the magnitude of the forceat many different points.
Electric field map due to a positive charge
+Q
Some important pointsto remember about electric fields:
- There is an electric field at every point in space surrounding acharge.
- Field lines are merely a representation – they are not real. When we draw them, we just pick
convenient places to indicate the field in space. - Field lines usually startat a right-angle ( 90 o) to the charged object causing the field.
- Field lines never cross.