Electric Fields

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Edexcel IAL Physics · Fields & Forces

Electric Fields

The Big Idea: any charged object creates an invisible "zone of influence" around itself — an electric field — and any other charge that enters that zone feels a push or a pull, whose strength depends on how much charge is involved and how far apart they are.

📋 Summary — What This Chapter Covers

Defining an electric field — a region where a charged particle feels a force, created by all charges around it.
Electric field strength (E) — force per unit charge on a positive test charge; a vector, points away from + and towards −.
Coulomb's Law — the force between two point charges: proportional to the charges, inversely proportional to the square of the distance.
Field due to a point charge — radial field, obeys an inverse-square law (1/r²).
Field & potential relationship — E is the gradient of the potential-distance graph.
Uniform fields between parallel plates — E = V/d, constant field strength everywhere between the plates.
Electric potential (V) in a radial field — work done per unit charge bringing a test charge from infinity; follows a 1/r relationship (not 1/r²).
Representing fields — field lines (direction + strength) and equipotential lines (surfaces of equal potential, always perpendicular to field lines).

1 · Defining an Electric Field

What actually is an electric field?

Think of any charged object — a balloon you've rubbed on your jumper, an electron, a proton — as constantly "broadcasting" its presence into the space around it. That broadcast is the electric field. Formally:

Definition An electric field is a region of space in which a charged particle experiences a force.

Crucially, the charged particle sitting in that field doesn't need to be moving — it feels the force whether it's stationary or in motion. This is a really important distinction from magnetic fields, where a charge only feels a force if it's .

Every charged particle creates its own field, and that field exerts an electrostatic force (F_E) on any other charged particle that happens to be within range. The two golden rules of how charges interact:

Like charges repel (+ and +, or − and −) — the force pushes them apart.
Opposite charges attract (+ and −) — the force pulls them together.

And the size of that force isn't fixed — it changes with distance. Charges close together feel a much stronger push or pull than charges far apart. (We'll quantify exactly how much in Coulomb's Law below.)

Watch out Don't mix up electric and magnetic fields! A charge sitting still in an electric field still feels a force. A charge sitting still in a magnetic field feels nothing — it only feels a force once it starts moving.

Practice Question

A student says "electric fields only affect moving charges, just like magnetic fields." Explain why this statement is incorrect.

2 · Electric Field Strength (E)

Measuring "how strong" a field is at a point

electric field strength

Definition The force per unit charge acting on a positive test charge placed at that point.

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Also in the full note

Quantifying attraction and repulsion
The radial field
Linking force-based and energy-based views
The uniform field
Potential around a point charge
Field lines and equipotentials

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