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HSC (New South Wales) Physics — practice by topic

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Physics topics

Electrical Circuits (27)Static Electricity (13)

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A dust particle picks up a net charge of -13 nC. Using the elementary charge e = 1.602 x 10^-19 C, about how many excess electrons does the particle carry?Read the answer and worked solution →In Coulomb's law, F = k q1 q2 / r^2, what is the value of Coulomb's constant k in SI units?Read the answer and worked solution →Two point charges q1 and q2 exert a force F on each other. If the value of q1 is doubled while q2 and the separation r stay fixed, what happens to the force F?Read the answer and worked solution →Two charges repel each other with a force of 2.0 N. If the distance between them is tripled while the charges stay the same, what is the new force between them?Read the answer and worked solution →

Worked practice questions

A dust particle picks up a net charge of -13 nC. Using the elementary charge e = 1.602 x 10^-19 C, about how many excess electrons does the particle carry?

  1. 8.1 x 10^10 electrons
  2. 8.1 x 10^19 electrons
  3. 2.1 x 10^9 electrons
  4. 1.3 x 10^-8 electrons
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Correct answer: A. 8.1 x 10^10 electrons

1. Convert the charge to SI units: -13 nC = -13 x 10^-9 C. 2. Divide by the charge of one electron: n = (13 x 10^-9 C) / (1.602 x 10^-19 C) = 8.11 x 10^10. 3. This rounds to about 8.1 x 10^10 electrons. 4. Option B misplaces the exponent by nine powers of ten, and option D never converts the ratio into a whole electron count, leaving units of charge rather than a pure number. _Source: OpenStax Physics (CC BY 4.0), Ch 18 "Static Electricity", section Transfer and Separation of Charge_

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In Coulomb's law, F = k q1 q2 / r^2, what is the value of Coulomb's constant k in SI units?

  1. 8.99 x 10^9 N m^2/C^2
  2. 6.67 x 10^-11 N m^2/C^2
  3. 1.602 x 10^-19 N m^2/C^2
  4. 8.85 x 10^-12 N m^2/C^2
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Correct answer: A. 8.99 x 10^9 N m^2/C^2

1. Coulomb's law defines the constant of proportionality as k. 2. In SI units, k = 8.99 x 10^9 N m^2/C^2. 3. 6.67 x 10^-11 is the gravitational constant G, 1.602 x 10^-19 is the elementary charge e, and 8.85 x 10^-12 is the permittivity of free space so none of those are Coulomb's constant. _Source: OpenStax Physics (CC BY 4.0), Ch 18 "Static Electricity", section 18.2 Coulomb's law_

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Two point charges q1 and q2 exert a force F on each other. If the value of q1 is doubled while q2 and the separation r stay fixed, what happens to the force F?

  1. F is cut by half
  2. F doubles in size
  3. F stays unchanged
  4. F quadruples in size
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Correct answer: B. F doubles in size

1. Coulomb's law states that F is proportional to the product q1 q2, not to either charge squared. 2. Doubling q1 while holding q2 and r fixed doubles the product q1 q2. 3. Since F is directly proportional to that product, F also doubles. 4. Quadrupling only happens if the distance is halved, since distance enters as an inverse square so option D mixes up the charge and distance dependence, which is the trap to watch for when scaling Coulomb's law. _Source: OpenStax Physics (CC BY 4.0), Ch 18 "Static Electricity", section 18.2 Coulomb's law_

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