A Moving Charge in an Electric Field

Remember that electric field lines tell us the direction and relative magnitude of the electric field at a particular point.

Field lines do not tell us:
  • ...what the electric force on a charge at that point is. It is true that the magnitude of the force will be greater where the field is stronger, but the actual size and direction of the force depends on the nature of the charge as well as the nature of the field.
  • ...which way the charge will move. This depends on the nature of the charge and its current motion as well as on the field.

    • The physlet below will help you to understand the interaction between electric field, force, and the motion of charges.

    Situation 1

    The animation below shows the electric field around two oppositely charged point charges. The red dot represents a small, positive test charge in the field.

    Run the applet and observe the motion of the test charge.

    You can drag the test charge around the field. Move it to different positions, re-running the applet each time, and observing its motion.

    What does the arrow pointing out of the test charge represent? (Answer is at the bottom of this page.)

    Situation 2

    Click the button below to make the test charge a negative test charge. (The test charge will turn green when it is negative.)

    Play with the applet again. Does the arrow pointing from the test charge still represent the same things that it did in situation 1? (Answer is at the bottom of this page.)

    Use the button below if you want to explore with the positive test charge again. You can change the sign of the test charge at any time using the relevant button.



    Answers

    Situation 1

    Note that the arrow always points in the direction of the field at that point. Its gets longer where the field is stronger, and shorter where the field is weaker. It could, therefore, be a vector representing the electric field at the position of the test charge.

    That is not all it could be, however. By convention, the direction of an electric field is defined as the direction of the electric force acting on a small, positive test charge. The arrow could also represent the electric force acting on the test charge at its current position. Note that the charge is not necessarily moving in the direction it is being pushed, but its motion is changing in that direction.

    Finally, since forces produce accelerations that are proportional to and in the same direction as the force, the arrow could be representing the acceleration of the test charge.


    Situation 2

    The arrow is now pointing in the opposite direction to the field lines, so it cannot represent the electric field at that point. It could, however, be representing either the electrical force on, or the acceleration of, the negative test charge.

    Script by Scott Bonham