18.5 Electric Field Lines: Multiple Charges

18.5 Electric Field Lines: Multiple Charges

• It is positive because it has a direction away from the charge.

• Since we know the electric field strength and the charge in the field, the force on that charge can be calculated.

• The force is directed opposite to the field because it is negative.

• The force is attractive.
• The modest attractive force obtained is similar to the forces experienced in static cling and similar situations, and the charges in this example are typical of common static electricity.

• You can add charges to the Field of Dreams to see how they react.
• The direction and magnitude can be adjusted by turning on a background electric field.

• The electric field has both magnitude and direction.

• There are two representations of the same electric field created by a positive point charge.
• Field lines are a map of force.

• The strength of the electric field is related to the closeness of the lines.
• A test charge placed anywhere will feel a force in the direction of the field line that is proportional to the density of the lines.

• The field lines point away from a positive charge and toward a negative charge if the electric field is defined for a positive test charge.
• The electric field strength is determined by the number of field lines per unit area and the magnitude of the electric field for a point charge.
• The strength is represented by the number of lines crossing a unit area and the direction of the field lines.

• Three different point charges are surrounded by an electric field.

• There are many charges.
• The total electric field is the sum of the individual fields created by each charge.
• This example shows how to add electric field vectors.

• We add electric fields with the same techniques used for other types of vectors.
• The origin of the coordinate system is determined by the electric field at the point of interest.

• Four digits have been retained to show that it is twice the magnitude.

• The direction of the electric field is that of the force on a positive charge, so both arrows point away from the positive charges that create them.

• The Pythagorean theorem can be used to add the arrows to a right triangle.

• This example shows the total electric field at one point in space.
• The same technique must be used for each point in the region to find the total electric field.
• The task of calculating the total field at representative points can be avoided by using some of the unifying features noted next.

• Figure 18.25 shows how the electric field from two point charges can be drawn by finding the total field at representative points and drawing electric field lines consistent with those points.
• The electric fields from multiple charges are simpler to notice than the single charges.

• As shown by the lines being farther apart in that region, the field is weaker between like charges.

• The electric field is shown in Figure 18.26(b).
• Between the charges the field is stronger.
• The fields from each charge are in the same direction, and so their strengths add.
• The field of two unlike charges is weak at large distances because the individual charges are in opposite directions.
• The field of two unlike charges looks similar to a single charge at large distances.

• The electric field shown is a result of two positive point charges.
• Field lines are drawn following the rules outlined in the text after the field is calculated.