Coulomb’s law & electric fields worksheet with answers

Free printable Coulomb’s law & electric fields worksheet: 6 problems from easy to exam level, with an answer key. Print it or save it as a PDF.

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1.Easy
++10 µC++2 µCr = ?

Point charges of +10 µC and +2 µC exert a force of 50 N on each other. How far apart r are they? Give the answer in cm. Use k = 8.99 × 10⁹ N·m²/C².

q1 = +10 µCq2 = +2 µCF = 50 N
r = cm
2.Easy
++4 µC++1 µCr = ?

Point charges of +4 µC and +1 µC exert a force of 50 N on each other. How far apart r are they? Give the answer in cm. Use k = 8.99 × 10⁹ N·m²/C².

q1 = +4 µCq2 = +1 µCF = 50 N
r = cm
3.Medium
++1 µC−−5 µC++2 µC2.5 cm + 7.5 cm

Three point charges lie on a straight line: +1 µC on the left, +2 µC on the right 10 cm away, and −5 µC between them, 2.5 cm from the left charge. Find the magnitude of the net electric force F on the middle charge. Use k = 8.99 × 10⁹ N·m²/C².

qL = +1 µCqR = +2 µCqM = −5 µCrL = 2.5 cmrR = 7.5 cm
F = N
4.Medium
++3 µC++5 µCP40 cm

A charge of +3 µC and a charge of +5 µC are 40 cm apart. Point P lies on the line between them, 20 cm from the first charge. Find the magnitude of the electric field E at P.

q1 = +3 µCq2 = +5 µCr1 = 20 cmr2 = 20 cm
E = N/C
5.Hard
5 cm8 cm+q1​ = +3 µC+q3​ = +1 µC+q2​ = +1 µC

Three positive point charges sit at the corners of a right triangle: q₃ = 1 µC is at the right-angle corner, q₁ = 3 µC is 5 cm from it along one side and q₂ = 1 µC is 8 cm from it along the other side. Find the magnitude of the net electric force F on q₃.

q1 = 3 µCq2 = 1 µCq3 = 1 µCr13 = 5 cmr23 = 8 cm
F = N
6.Hard
L = 20 cm12°qq

Two identical small balls, each of mass 8 g, hang from the same point on insulating threads 20 cm long. They are given equal charges and each thread settles at 12° from the vertical. Use g = 9.81 m/s². Find the charge q on each ball in nC. Use k = 8.99 × 10⁹ N·m²/C².

m = 8 gL = 20 cmθ = 12°g = 9.81 m/s²
q = nC

Answer key

  1. 1. r = 5.997 cm
  2. 2. r = 2.682 cm
  3. 3. F = 55.94 N
  4. 4. E = 449500 N/C
  5. 5. F = 10.88 N
  6. 6. q = 113.3 nC

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