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
−−4 µC++4 µCr = 5 cm

Two point charges of −4 µC and +4 µC are 5 cm apart. Find the magnitude of the electric force F between them. Use k = 8.99 × 10⁹ N·m²/C².

q1 = −4 µCq2 = +4 µCr = 5 cm
F = N
2.Easy
+q₁+q₂r

Two point charges repel each other with a force of 26 N. The first charge is halved and the distance between the charges is made four times as large. Find the new force F′.

F = 26 Nq₁′ = 0.5·q₁r′ = 4·r
F′ = N
3.Medium
++8 µC−−10 µCP60 cm

A charge of +8 µC and a charge of −10 µC are 60 cm apart. Point P lies on the line between them, 45 cm from the first charge. Find the magnitude of the electric field E at P.

q1 = +8 µCq2 = −10 µCr1 = 45 cmr2 = 15 cm
E = N/C
4.Medium
++8 µC−−3 µC++5 µC2.5 cm + 7.5 cm

Three point charges lie on a straight line: +8 µC on the left, +5 µC on the right 10 cm away, and −3 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 = +8 µCqR = +5 µCqM = −3 µCrL = 2.5 cmrR = 7.5 cm
F = N
5.Hard
L = 20 cm15°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 15° 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 = 15°g = 9.81 m/s²
q = nC
6.Hard
+ + + + + +− − − − − −e⁻2.5 × 10⁷ m/s8 cmE = 3400 N/C

An electron enters the uniform field between two parallel plates moving parallel to them at 2.5 × 10⁷ m/s. The plates are 8 cm long and the field between them is 3400 N/C. Electron charge 1.602 × 10⁻¹⁹ C, mass 9.109 × 10⁻³¹ kg; ignore gravity. How far y is it deflected sideways when it leaves the plates? Give the answer in mm.

v₀ = 2.5 × 10⁷ m/sL = 8 cmE = 3400 N/Ce = 1.602 × 10⁻¹⁹ Cme = 9.109 × 10⁻³¹ kg
y = mm

Answer key

  1. 1. F = 57.54 N
  2. 2. F′ = 0.8125 N
  3. 3. E = 4.351 × 10⁶ N/C
  4. 4. F = 321.2 N
  5. 5. q = 158.3 nC
  6. 6. y = 3.062 mm

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