Chapter 15 & 16

Electrostatics: Forces & Energy

Electric Field

Definition of the Electric Field,

If a test charge experiences a force at a given location, the electric field at that location is

Units: N/C

The electric field is the force per charge at a given location.
Video of Electric Field

Solving for the force gives us the force on a given charge:

Example 15-10

In a certain region of space, a uniform electric field has a magnitude of and points in the positive x direction. Find the magnitude and direction of the force this field exerts on a charge of (a) +2.80 µC and (b) -9.30 µC.

Solution:

(a)

(b)

Electric Field of a Point Charge

Magnitude of the Electric Field Due to a Point Charge

Example 15-11

Find the electric field produced by a 1.0 µC point charge at a distance of (a) 0.75 m and (b) 1.5 m.

Solution:

(a)

(b)

Example 15-12

Two charges, and , have equal magnitudes q and are placed as shown in the figure. The net electric field at point P is vertically upward. We conclude that (a) is positive, is negative; (b) is negative, is positive; or (c) and have the same sign.

Solution:

(b) is negative, is positive.

Example 15-13

Two charges, each equal to +2.90 µC, are placed at two corners of a square 0.500 m on a side, as shown. Find the magnitude and direction of the net electric field at a third corner of the square, the point labeled 3 in sketch.

Solution:

 Find the magnitude of : Find the magnitude of : Calculate the components of and : Find the components of : Find the magnitude of : Find the direction of:

On to Electric Field Lines