How does the electric field strength from a point charge vary with distance?

It follows an inverse square relationship with distance from the charge.

Write the equation for electric field strength at distance r from a point charge Q.

E = Q/(4πε₀r²), where E is electric field strength, Q is the point charge, ε₀ is the permittivity of free space, and r is the distance from the charge.

What information is needed to calculate the electric field strength at a point near a point charge?

The magnitude of the point charge (Q), the distance from the charge (r), and the permittivity of free space (ε₀).

Calculate the electric field strength when charge = 2.0 × 10⁻⁹ C, distance = 0.72 m.

Electric field strength = 2.0 × 10⁻⁹ ÷ (4π × 8.85 × 10⁻¹² × 0.72²) = 34.7 N C⁻¹.

State Coulomb's law in words.

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

Write the equation for Coulomb's law and define each term.

F = Qq/(4πε₀r²), where F is the electrostatic force, Q and q are the two point charges, ε₀ is the permittivity of free space, and r is the separation between the charges.

OCR A-Level Physics A H556

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Electric Fields

Coulomb's law and electric field strength

3 things to know
Papers 1, 2 and 3

Overview

Recall questions on Coulomb's law and force calculations, the permittivity of free space, electric field strength of a point charge and its calculation, and comparing Coulomb's law with Newton's law of gravitation.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

What property determines gravitational field strength, and what property determines electric field strength?

Key term

When using Coulomb's law to calculate force, what quantities must you know?

Key term

Name the types of field that give rise to forces.

Questions from this subtopic

Each one with the model answer underneath.

1
How does the electric field strength from a point charge vary with distance?
Model answer
It follows an inverse square relationship with distance from the charge.
2
Write the equation for electric field strength at distance r from a point charge Q.
Model answer
E = Q/(4πε₀r²), where E is electric field strength, Q is the point charge, ε₀ is the permittivity of free space, and r is the distance from the charge.
3
What information is needed to calculate the electric field strength at a point near a point charge?
Model answer
The magnitude of the point charge (Q), the distance from the charge (r), and the permittivity of free space (ε₀).
4
Calculate the electric field strength when charge = 2.0 × 10⁻⁹ C, distance = 0.72 m.
Model answer
Electric field strength = 2.0 × 10⁻⁹ ÷ (4π × 8.85 × 10⁻¹² × 0.72²) = 34.7 N C⁻¹.
5
State Coulomb's law in words.
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.
6
Write the equation for Coulomb's law and define each term.
Model answer
F = Qq/(4πε₀r²), where F is the electrostatic force, Q and q are the two point charges, ε₀ is the permittivity of free space, and r is the separation between the charges.

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