Electric Fields

In the last lesson you saw that two charged objects push or pull on each other without touching. But how does one charge "know" the other is there across an empty gap? The answer is that every charged object is surrounded by an invisible region called an electric field, and it is this field that exerts a force on any other charge placed within it. The idea of a field is one of the most powerful in physics: it lets us picture and predict forces that act at a distance. This lesson, part of Topic P3 (Electricity) of OCR Gateway Science A, explains what an electric field is, how we draw it with field lines, the shape of the field around a point charge and between parallel plates, and how a strong field can ionise the air to make a spark.

By the end of this lesson you should be able to describe the electric field around a charged object, draw and interpret electric field lines (their direction and spacing), describe the field around a point charge and between parallel plates, explain the force on a charge placed in a field, and use the idea of a field to explain why sparks occur.

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What Is an Electric Field?

An electric field is a region around a charged object in which another charge experiences a force. If you place a small charge anywhere in this region, it will feel a push or a pull; outside the field, it feels nothing. The field is what carries the influence of the charge across the space around it, which is why the forces between charges are non-contact forces — neither object has to touch the other for the field of one to act on the charge of the other.

The field is strongest close to the charged object and gets weaker as you move further away. This matches what you already know: two charges push or pull on each other more strongly when they are close together than when they are far apart, because near the object the field is more intense.

Exam Tip: Define an electric field as a region where a charge feels a force. This single idea explains why charges exert non-contact forces — each charge sits in the other's field.

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Electric Field Lines

We cannot see an electric field, so we represent it with field lines (also called lines of force). These follow a strict set of rules that you must know:

• Direction: field lines always point from positive to negative — that is, in the direction of the force on a small positive charge placed in the field. So they point away from a positive charge and towards a negative charge.
• Spacing (density): where the lines are close together, the field is strong; where they are further apart, the field is weaker. This is why the lines crowd together near the charge (strong field) and spread out far away (weak field).
• Arrows: every field line carries an arrow showing its direction.
• Field lines never cross one another.

Exam Tip: Two facts win most field-line marks: arrows point from + to − (the direction a positive charge would be pushed), and closely spaced lines mean a stronger field. Always put arrows on your field lines — a line without an arrow loses the direction mark.

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The Field Around a Point Charge

A small charged sphere (a "point charge") has a radial field: the field lines are straight and point directly outwards (for a positive charge) or inwards (for a negative charge), spreading out evenly in all directions like the spokes of a wheel. Because the lines spread apart as you move away from the charge, they are closest together near the charge, correctly showing that the field is strongest near the object and weakens with distance.

For a negative point charge the picture is identical except that every arrow points inwards, towards the charge, because that is the direction a small positive test charge would be pushed.

If you put a small positive charge into the field of this positive sphere, it would be pushed outwards along a field line (repelled). A small negative charge would be pulled inwards, against the arrows (attracted). The field lines therefore double as a map of which way a charge will be forced.

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The Field Between Two Parallel Plates

If two flat metal plates are given opposite charges (one positive, one negative) and placed close together facing each other, the field between them is completely different from the radial field of a point charge. Between the plates the field is uniform: the field lines are straight, parallel, equally spaced and point from the positive plate to the negative plate. Equally spaced lines mean the field has the same strength everywhere between the plates.

The contrast is worth learning as a pair:

Feature	Around a point charge	Between parallel plates
Shape of lines	Radial (spread out like spokes)	Straight, parallel
Spacing	Closer near the charge	Equally spaced everywhere
Field strength	Strongest near the charge, weakens with distance	Uniform (same strength everywhere)
Direction	Out of +, into −	From + plate to − plate

Exam Tip: Learn the two standard fields: a point charge has a radial field (lines spread out, stronger nearer the charge); two parallel plates have a uniform field (straight, equally spaced lines, same strength everywhere). Getting the line shape right is usually worth a mark on its own.

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The Force on a Charge in a Field

Place any charge in an electric field and it feels a force. The size and direction of that force depend on the field and on the charge:

• A positive charge is pushed in the same direction as the field lines (from + towards −).
• A negative charge is pushed in the opposite direction to the field lines (towards the +).
• The force is stronger where the field is stronger — that is, where the field lines are closer together.