WHAT IS AN ELECTRIC FIELD - DEFINITION BASICS AND TUTORIALS

We characterized the electric potential as a property of the location at which a charge might find itself. A map of the electric potential would indicate how much potential energy would be possessed by a charge located at any given point.

The electric field is a similar map, but rather of the electric force (such as attraction or repulsion) that would be experienced by that charge at any location.

This force is the result of potential differences between locations: the more dramatically the potential varies from one point to the next, the greater the force would be on an electric charge in between these points. In formal terms, the electric field represents the potential gradient.

Consider the electric field created by a single positive charge, just sitting in space. Another positive charge in its vicinity would experience a repulsive force. This repulsive force would increase as the two charges were positioned closer together, or decrease as they moved father apart; specifically, the electric force drops off at a rate proportional to the square of the distance.

This situation can be represented graphically by drawing straight arrows radially outward from the first charge, as in Figure 1.1a. Such arrows are referred to as field lines. Their direction indicates the direction that a “test charge,” such as the hypothetical second charge that was introduced, would be pushed or pulled (in this case, straight away).

The strength of the force is indicated by the proximity of field lines: the force is stronger where the lines are closer together. This field also indicates what would happen to a negative charge: At any point, it would experience a force of equal strength (assuming equal magnitude of charge), but opposite direction as the positive test charge, since it would be attracted rather than repelled.

Thus, a negative test charge would also move along the field lines, only backwards. By convention, the direction of the electric field lines is drawn so as to represent the movement of a positive test charge. For a slightly more complex situation, consider the electric field created by a positive and a negative charge, sitting at a fixed distance from each other.

We can map the field conceptually by asking, for any location, “What force would be acting on a (positive) test charge if it were placed here?” Each time, the net force on the test charge would be a combination of one attractive force and one repulsive force, in different directions and at different strengths depending on the distance from the respective fixed charges.

Graphically, we can construct an image of the field by drawing an arrow in the direction that the charge would be pulled. The arrows for points along the charge’s hypothetical path then combine into continuous field lines. Again, these field lines will be spaced more closely where the force is stronger. This exercise generates the picture in Figure 1.1b.