Coulomb's Law Physics Example 4

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Example 4

hard
Two charges q1=+4×10−6 Cq_1 = +4 \times 10^{-6} \text{ C} and q2=−2×10−6 Cq_2 = -2 \times 10^{-6} \text{ C} are separated by 0.1 m0.1 \text{ m}. (a) Calculate the force between them. (b) Is the force attractive or repulsive? Use k=9×109 N m2/C2k = 9 \times 10^9 \text{ N m}^2/\text{C}^2.

Solution

  1. 1
    (a) F=k∣q1∣∣q2∣r2=9×109×4×10−6×2×10−6(0.1)2F = k\frac{|q_1||q_2|}{r^2} = 9 \times 10^9 \times \frac{4 \times 10^{-6} \times 2 \times 10^{-6}}{(0.1)^2}.
  2. 2
    F=9×109×8×10−120.01=9×109×8×10−10=7.2 NF = 9 \times 10^9 \times \frac{8 \times 10^{-12}}{0.01} = 9 \times 10^9 \times 8 \times 10^{-10} = 7.2 \text{ N}.
  3. 3
    (b) The charges have opposite signs (++ and −-), so the force is attractive.

Answer

F=7.2 N, attractiveF = 7.2 \text{ N, attractive}
Coulomb's law gives the magnitude of the electrostatic force. Opposite charges attract; like charges repel. The inverse-square dependence means the force increases rapidly at smaller distances.

About Coulomb's Law

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.

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More Coulomb's Law Examples

Example 1 medium

Two charges [formula] and [formula] are separated by [formula]. What is the electrostatic force betw

Example 2 hard

Two identical charges experience a force of [formula] when separated by [formula]. What is the magni

Example 3 medium

If the distance between two charges is tripled, by what factor does the electrostatic force change?

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