State the unit of electric field strength.

N C⁻¹ (equivalently V m⁻¹).

What is an electric field?

A region where a charged particle experiences a force.

What is the value of ε0?

8.85 × 10⁻¹² F m⁻¹ (permittivity of free space).

In which direction does the electric field point?

The direction of the force on a positive charge: away from positive charges, towards negative charges.

What happens to the force between two charges if their separation is doubled?

It falls to one quarter (inverse square law).

How do the forces on two point charges compare, whatever their sizes?

They are equal in magnitude and opposite in direction (Newton's third law).

How are fields from several point charges combined?

By vector addition (superposition). Fields in opposite directions subtract.

Where is the resultant field zero between two like charges?

At a point nearer the smaller charge, where the two field strengths are equal.

Electric fields and Coulomb's lawEdexcel International A Level Physics: Flashcards

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Define electric field strength.

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Define electric field strength.: The force per unit positive charge at a point: E = F/Q.
State the unit of electric field strength.: N C⁻¹ (equivalently V m⁻¹).
What is an electric field?: A region where a charged particle experiences a force.
State Coulomb's law as an equation.: F = Q1Q2/(4πε0r²).
What is the value of ε0?: 8.85 × 10⁻¹² F m⁻¹ (permittivity of free space).
State the equation for the field strength of a point charge.: E = Q/(4πε0r²).
In which direction does the electric field point?: The direction of the force on a positive charge: away from positive charges, towards negative charges.
What happens to the force between two charges if their separation is doubled?: It falls to one quarter (inverse square law).
How do the forces on two point charges compare, whatever their sizes?: They are equal in magnitude and opposite in direction (Newton's third law).
How are fields from several point charges combined?: By vector addition (superposition). Fields in opposite directions subtract.
What is the force on a charge Q in an electric field E?: F = EQ.
Where is the resultant field zero between two like charges?: At a point nearer the smaller charge, where the two field strengths are equal.

Exam questions on Electric fields and Coulomb's law

Two very small charged spheres, A and B, carry charges of +2.0 nC and +5.0 nC respectively. They are held 4.0 cm apart in air, so they may be treated as point charges in a vacuum.
Calculate the magnitude of the force between the spheres when they are 4.0 cm apart.2 marks
In a simple model of a hydrogen atom, an electron moves in a circular orbit around a single proton. The separation of the two particles is 5.3 × 10⁻¹¹ m. The magnitude of the charge on each particle is 1.60 × 10⁻¹⁹ C.
Calculate the force on the electron due to the proton and state its direction.2 marks
A small metal sphere carries a charge of +6.0 nC. Point P is 0.30 m from the centre of the sphere, in air.
Calculate the electric field strength at P and state its direction.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).