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Electric and Magnetic Fields

Coulomb's law and the field of a point charge

  • 2 things to know
  • Papers 1 and 3

Overview

Recall questions on Coulomb's law, ε₀ and 1/(4πε₀), how the sign shows attraction or repulsion, and the radial field of a point charge.

What you need to know

Answer each one before you reveal it.

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Numbers

What is the equation for the force between two point charges, and what are ε₀ and the combination 1/(4πε₀)?

Numbers

How does the sign of the calculated force show attraction or repulsion, and what is the field strength due to a point charge?

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    What does Coulomb's law state?

    Model answer

    The force between two point charges is proportional to the product of the charges and inversely proportional to the square of their separation.

  2. 2

    State the equation for the force between two point charges:

    Model answer

    F = Q₁Q₂/(4πε₀r²), where Q₁ and Q₂ are the charges and r is their separation.

  3. 3

    What does the symbol ε₀ represent, and what is its value?

    Model answer

    It is the permittivity of free space, with a value of 8.85 × 10⁻¹² F m⁻¹.

  4. 4

    What is the value of the combination 1/(4πε₀)?

    Model answer

    It is approximately 8.99 × 10⁹ N m² C⁻².

  5. 5

    How does the sign of the calculated force tell you whether the charges attract or repel?

    Model answer

    A positive value means the charges repel because they are alike, and a negative value means they attract because they are unlike.

  6. 6

    State the equation for the electric field strength due to a point charge:

    Model answer

    E = Q/(4πε₀r²), where Q is the point charge and r is the distance from it.

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