Practice Coulomb's Law in Physics

Use these practice problems to test your method after reviewing the concept explanation and worked examples.

Worksheet PDF Free Answer-key PDF Family

Quick Recap

Coulomb's law gives the electric force between two point charges. The force gets larger when the charges are larger and gets smaller with the square.

Like gravity between masses, but for charges. Double the distance and the force drops to one quarter. Double either charge and the force doubles.

Showing a random 20 of 50 problems.

Example 1

medium
A charge +q+q at x=0x=0 and +4q+4q at x=Lx=L. Find where on the segment the net force on a test charge is zero.

Example 2

challenge
A charge +Q+Q sits at the origin and +9Q+9Q at x=Lx = L. A third charge qq is placed somewhere between so it experiences zero net force. Find xx.

Example 3

hard
Two charges +3 μC+3\,\mu\text{C} and +12 μC+12\,\mu\text{C} are 0.6 m0.6\text{ m} apart. Where on the segment between them is the electric field zero?

Example 4

hard
Three charges in a line: +q+q at x=0x=0, −2q-2q at x=Lx=L, and a test +q+q at x=2Lx=2L. Find the net force on the test charge in terms of kq2/L2kq^2/L^2.

Example 5

easy
If both charges in a pair are negative, the force is ___.

Example 6

easy
Two +1 μC+1\,\mu\text{C} charges are 0.1 m0.1\text{ m} apart. Find the force. Use k=9×109k = 9\times 10^9 N·m²/C².

Example 7

easy
Charges q1=5 Cq_1=5 \text{ C}, q2=2 Cq_2=2 \text{ C}, separation r=10 mr=10 \text{ m}. Find the force. Use k=9×109k=9\times10^9.

Example 8

easy
Two charges q1=2 Cq_1=2 \text{ C}, q2=3 Cq_2=3 \text{ C} are r=3 mr=3 \text{ m} apart. Find the force. Use k=9×109k=9\times10^9.

Example 9

medium
Two charges repel with 8 N8 \text{ N} at 2 m2 \text{ m}. What is the force at 1 m1 \text{ m}?

Example 10

medium
Two charges q1=3×10−6 Cq_1 = 3 \times 10^{-6} \text{ C} and q2=5×10−6 Cq_2 = 5 \times 10^{-6} \text{ C} are separated by 0.2 m0.2 \text{ m}. What is the electrostatic force between them? Use k=9×109 N m2/C2k = 9 \times 10^9 \text{ N m}^2/\text{C}^2.

Example 11

medium
Two protons are 10−10 m10^{-10}\text{ m} apart. Find the magnitude of the Coulomb force. Use k=9×109k = 9\times 10^9 and e=1.6×10−19 Ce = 1.6\times 10^{-19}\text{ C}.

Example 12

challenge
A charge +q+q at x=0x=0 and +4q+4q at x=Lx=L. Where on the segment is the net force on a test charge zero?

Example 13

easy
What is the value and units of Coulomb's constant kk?

Example 14

hard
Two identical pith balls (each 5 g5\text{ g}) hang from 0.5 m0.5\text{ m} silk threads and repel until they are 0.04 m0.04\text{ m} apart, hanging at θ\theta from vertical. Estimate the charge on each. Use g=10 m/s2g = 10\text{ m/s}^2, k=9×109k = 9\times 10^9.

Example 15

easy
The electric force between two point charges varies as the inverse ___ of the distance.

Example 16

easy
Two 1 C1 \text{ C} charges are 1 m1 \text{ m} apart. Find the force. Use k=9×109k=9\times10^9.

Example 17

medium
Two charges feel 20 N20 \text{ N}. If both charges double, find the new force.

Example 18

easy
Two charges q1=2 μCq_1 = 2\,\mu\text{C} and q2=3 μCq_2 = 3\,\mu\text{C} are separated by 0.3 m0.3\text{ m}. Find the magnitude of the force. Use k=9×109k = 9\times 10^9 N·m²/C².

Example 19

medium
Two equal charges qq are 3 m3 \text{ m} apart and repel with 10 N10 \text{ N}. Find qq. Use k=9×109k=9\times10^9.

Example 20

medium
Coulomb's law applies to point charges or to ___ charge distributions.
← Back to Coulomb's Law Worked Examples Formula Explained