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 (equivalent to V m⁻¹).

What is the direction of an electric field?

The direction of the force on a positive charge.

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

It falls to one quarter, as F ∝ 1/r².

What happens to the force between two charges if one charge is tripled?

It is tripled, as F is proportional to the product of the charges.

Which way does the field point near a negative point charge?

Radially inwards, towards the charge.

How are the forces on two interacting point charges related?

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

How is the field from several charges found?

By adding the fields as vectors.

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

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Define an electric field.

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Define an electric field.: A region in which a charged particle experiences a force.
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 (equivalent to V m⁻¹).
What is the direction of an electric field?: The direction of the force on a positive charge.
Write the equation for the force on a charge Q in a field E.: F = QE.
State Coulomb's law as an equation.: F = Q₁Q₂/4πε₀r².
What is the value of ε₀?: 8.85 × 10⁻¹² F m⁻¹.
What happens to the force between two charges if the separation is doubled?: It falls to one quarter, as F ∝ 1/r².
What happens to the force between two charges if one charge is tripled?: It is tripled, as F is proportional to the product of the charges.
State the equation for the field strength of a point charge.: E = Q/4πε₀r².
Which way does the field point near a negative point charge?: Radially inwards, towards the charge.
How are the forces on two interacting point charges related?: They are equal in magnitude and opposite in direction (Newton's third law).
How is the field from several charges found?: By adding the fields as vectors.

Exam questions on Electric fields and Coulomb's law

A small sphere carries a charge of +2.0 nC. When it is placed at a point P in an electric field it experiences a force of 6.0 × 10⁻⁵ N.
Calculate the electric field strength at P.2 marks
Two small charged spheres, X and Y, carry charges of +3.0 nC and +5.0 nC. They are in a vacuum with their centres 0.060 m apart. The spheres may be treated as point charges.
Calculate the magnitude of the force between X and Y.2 marks
A point charge of +6.0 nC is fixed in a vacuum. A student investigates the electric field around it at a point Q that is 0.15 m from the charge.
Calculate the electric field strength at Q 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).