Peoples Physics Concepts

http://www.ck12.org Chapter 12. Electricity

charge will be radially symmetric i.e., the strength of the field only depends on the distance,r, from the charge, not

the direction; the lengths of the arrows represent the strength of the field.

Example 1

Question: Calculate the electric field a distance of 4.0mm away from a− 2. 0 μC charge. Then, calculate the force

on a− 8. 0 μC charge placed at this point.

Answer: To calculate the electric field we will use the equation

E=

kq

r^2

Before we solve for the electric field by plugging in the values, we convert all of the values to the same units.

4 .0mm×

1m

1000mm

=.004m

− 2. 0 μC×

1C

1000000 μC

=− 2. 0 × 10 −^6 C

Now that we have consistent units we can solve the problem.

E=

kq

r^2

=

9 × 109 Nm^2 /C^2 ×− 2. 0 × 10 −^6 C

(.004m)^2

=− 1. 1 × 109 N/C

To solve for the force at the point we will use the equation

F=Eq