What are the units of E?

N C⁻¹, equivalent to V m⁻¹.

Which way does the electric field point?

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

What do field lines look like between oppositely charged parallel plates?

Parallel, equally spaced lines at right angles to the plates: a uniform field.

Work done moving charge Q across the gap: W = Fd = QEd. Also W = QV. So QEd = QV and E = V/d.

What is the path of a charged particle entering a uniform field at right angles?

A parabola, because its velocity along the plates is constant while it accelerates uniformly across them.

What happens to E in a radial field if r doubles?

It falls to one quarter (inverse square law).

What does the spacing of field lines show?

Field strength: the closer together the lines, the stronger the field.

Which direction is the force on an electron in an electric field?

Opposite to the field direction, as its charge is negative.

Electric field strengthAQA 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 on a small test charge placed at that point, E = F/Q.
What are the units of E?: N C⁻¹, equivalent to V m⁻¹.
Which way does the electric field point?: The direction of the force on a positive test charge: away from positive charges, towards negative ones.
What do field lines look like between oppositely charged parallel plates?: Parallel, equally spaced lines at right angles to the plates: a uniform field.
Give the formula for E in a uniform field.: E = V/d.
Derive E = V/d.: Work done moving charge Q across the gap: W = Fd = QEd. Also W = QV. So QEd = QV and E = V/d.
What is the path of a charged particle entering a uniform field at right angles?: A parabola, because its velocity along the plates is constant while it accelerates uniformly across them.
What is the acceleration of a particle of charge Q and mass m in a field E?: a = QE/m.
Give the formula for E at distance r from a point charge Q.: E = Q / (4πε₀r²).
What happens to E in a radial field if r doubles?: It falls to one quarter (inverse square law).
What does the spacing of field lines show?: Field strength: the closer together the lines, the stronger the field.
Which direction is the force on an electron in an electric field?: Opposite to the field direction, as its charge is negative.

Exam questions on Electric field strength

Two horizontal metal plates in a vacuum are 0.040 m apart. They are connected to a 2.0 kV supply, producing a uniform electric field in the gap between them.
The plate separation is doubled to 0.080 m while the supply is kept at 2.0 kV. Calculate the new electric field strength and state what happens to the force on a given charge in the gap.2 marks
A small metal sphere carries a charge of +4.0 nC. Its charge may be treated as a point charge at its centre, and the sphere is isolated in air (take the permittivity of air as that of free space, ε₀ = 8.85 × 10⁻¹² F m⁻¹).
Explain, with reference to field lines, why the field strength decreases with distance from the sphere.2 marks
In a cathode ray tube, an electron travelling horizontally at 2.0 × 10⁷ m s⁻¹ enters the gap between two horizontal parallel plates. The plates are 0.050 m long and 0.040 m apart, and there is a uniform field of 1.5 × 10⁴ V m⁻¹ directed vertically downwards between them. Ignore gravity and edge effects. The electron has charge −1.60 × 10⁻¹⁹ C and mass 9.11 × 10⁻³¹ kg.
Calculate the magnitude of the vertical acceleration of the electron while it is between the plates.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).