State Coulomb's law as an equation for the force between two point charges in a vacuum.

F = (1/4πε₀) × (Q₁Q₂/r²), where F is the electrostatic force, Q₁ and Q₂ are the charges, r is the separation, and ε₀ is the permittivity of free space.

In Coulomb's law, what does each symbol represent in the equation F = (1/4πε₀) × (Q₁Q₂/r²)?

F is the electrostatic force between the charges, Q₁ and Q₂ are the magnitudes of the two point charges, r is the distance between the centres of the charges, and ε₀ is the permittivity of free space.

Calculate the electrostatic force when charge 1 = 2.0 × 10⁻⁶ C, charge 2 = 3.0 × 10⁻⁶ C, separation = 0.44 m.

Electrostatic force = (2.0 × 10⁻⁶ × 3.0 × 10⁻⁶) ÷ (4π × 8.85 × 10⁻¹² × 0.44²) = 0.279 N.

What is the permittivity of free space (ε₀) and what is its approximate value?

The permittivity of free space is a constant that appears in Coulomb's law, with a value of approximately 8.85 × 10⁻¹² F m⁻¹.

Why can air be treated as a vacuum when applying Coulomb's law to calculate electrostatic forces?

Because the permittivity of air is very close to the permittivity of free space, so the difference in calculated force is negligible.

AQA A-Level Physics 7408

Measurements and Their Errors

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Fields and Their Consequences

Coulomb's law

3 things to know
Papers 2 and 3

Overview

The force between point charges, permittivity of free space, and comparison with gravitation.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Process

What units does permittivity of free space take, and how strong is the electrostatic force between particles?

Numbers

How does the sign of the charges determine whether the force attracts or repels?

Numbers

What is the value and unit of 1/4πε₀, and what information is needed for an estimate?

Questions from this subtopic

Each one with the model answer underneath.

1
State Coulomb's law as an equation for the force between two point charges in a vacuum.
Model answer
F = (1/4πε₀) × (Q₁Q₂/r²), where F is the electrostatic force, Q₁ and Q₂ are the charges, r is the separation, and ε₀ is the permittivity of free space.
2
In Coulomb's law, what does each symbol represent in the equation F = (1/4πε₀) × (Q₁Q₂/r²)?
Model answer
F is the electrostatic force between the charges, Q₁ and Q₂ are the magnitudes of the two point charges, r is the distance between the centres of the charges, and ε₀ is the permittivity of free space.
3
Calculate the electrostatic force when charge 1 = 2.0 × 10⁻⁶ C, charge 2 = 3.0 × 10⁻⁶ C, separation = 0.44 m.
Model answer
Electrostatic force = (2.0 × 10⁻⁶ × 3.0 × 10⁻⁶) ÷ (4π × 8.85 × 10⁻¹² × 0.44²) = 0.279 N.
4
What is the permittivity of free space (ε₀) and what is its approximate value?
Model answer
The permittivity of free space is a constant that appears in Coulomb's law, with a value of approximately 8.85 × 10⁻¹² F m⁻¹.
5
What are the units of the permittivity of free space?
Model answer
F m⁻¹ (farads per metre).
6
Why can air be treated as a vacuum when applying Coulomb's law to calculate electrostatic forces?
Model answer
Because the permittivity of air is very close to the permittivity of free space, so the difference in calculated force is negligible.

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