http://www.ck12.org Chapter 12. Electricity Version 2
Example 3
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^2Before 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