Question

Difficulty: MediumElectrostatics and Electric Charges

A point charge of +6.0×106 C+6.0 \times 10^{-6}\text{ C} experiences an attractive electrostatic force of 0.54 N0.54\text{ N} when placed at a distance of 1.0 m1.0\text{ m} from a second point charge in a vacuum. Taking Coulomb's constant k=9.0×109 Nm2C2k = 9.0 \times 10^9\text{ N}\cdot\text{m}^2\text{C}^{-2}, what is the magnitude of the second charge in microcoulombs (μC\mu\text{C})?

Answer: 10 µC

Answer

The magnitude of the second charge is 10 µC.
Using Coulomb's law F=kq1q2r2F = \frac{k |q_1 q_2|}{r^2}, substituting F=0.54 NF = 0.54\text{ N}, q1=6.0×106 Cq_1 = 6.0 \times 10^{-6}\text{ C}, r=1.0 mr = 1.0\text{ m}, and k=9.0×109 Nm2C2k = 9.0 \times 10^9\text{ N}\cdot\text{m}^2\text{C}^{-2} yields q2=1.0×105 C|q_2| = 1.0 \times 10^{-5}\text{ C}, which equals 10 μC10\text{ }\mu\text{C}.

Step-by-Step Solution

1
State Coulomb's Law formula
F=kq1q2r2F = \frac{k |q_1 q_2|}{r^2}
Coulomb's Law describes the electrostatic force between two point charges.
2
Substitute given parameters into the equation
0.54=9.0×109×6.0×106×q21.020.54 = \frac{9.0 \times 10^9 \times 6.0 \times 10^{-6} \times |q_2|}{1.0^2}
Knowns: F=0.54 NF = 0.54\text{ N}, q1=6.0×106 Cq_1 = 6.0 \times 10^{-6}\text{ C}, r=1.0 mr = 1.0\text{ m}, k=9.0×109 Nm2C2k = 9.0 \times 10^9\text{ N}\cdot\text{m}^2\text{C}^{-2}.
3
Solve for the unknown charge magnitude q2q_2
q2=1.0×105 C|q_2| = 1.0 \times 10^{-5}\text{ C}
Rearranging yields q2=0.545.4×104=1.0×105 C|q_2| = \frac{0.54}{5.4 \times 10^4} = 1.0 \times 10^{-5}\text{ C}.
4
Convert the value from Coulombs to microcoulombs
10 μC10\text{ }\mu\text{C}
1 μC=106 C1\text{ }\mu\text{C} = 10^{-6}\text{ C}, so 1.0×105 C=10 μC1.0 \times 10^{-5}\text{ C} = 10\text{ }\mu\text{C}.

Key Concept

Coulomb's Law
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