Electric fields

What you'll learn

• What an electric field is and why it matters in static electricity.
• How to draw and interpret field lines around point charges.
• How the field looks between parallel plates.
• How field ideas explain attraction, repulsion, sparks, and charged objects moving without touching.

This part of Edexcel 1PH0 is marked with P, so it is Separate Physics content.

Before electric fields: charge and static electricity

An object is charged when it has gained or lost electrons.

• Electrons have a negative charge.
• Protons have a positive charge.
• Like charges repel: positive repels positive, negative repels negative.
• Unlike charges attract: positive attracts negative.

In static electricity, charge is not flowing around a circuit. It has built up on an object, often because electrons were transferred by rubbing.

A charged object can exert a force on another charged object without touching it. To describe how that happens, we use the idea of an electric field.

What is an electric field?

An electric field is not a physical object you can see. It is a way of describing the region around a charged object where another charge would feel a force.

So if you place a small charge somewhere and it is pushed or pulled, that place is inside an electric field.

A test charge is a small imaginary positive charge used to show what direction the field acts in. We use a positive test charge by convention.

Field lines: drawing the invisible field

Electric fields are represented using field lines.

Field lines are imaginary lines that show:

• the direction of the electric field using arrows
• the shape of the field
• the strength of the field by how close together the lines are

Where field lines are close together, the field is stronger. Where they are spread out, the field is weaker.

Field lines do not cross. If they crossed, the field would have two directions at the same point, which would not make sense.

Electric field around a point charge

A point charge is a charged object treated as if all its charge is concentrated at one point. At GCSE, this is often drawn as a small circle with a plus or minus sign.

For a positive point charge:

• field lines go radially outwards
• this shows a positive test charge would be repelled away

For a negative point charge:

• field lines go radially inwards
• this shows a positive test charge would be attracted towards it

The field is strongest close to the charge because the lines are closest together there.

Field strength and line concentration

The strength of an electric field tells you how large the force would be on a charge placed there.

At GCSE, you usually judge field strength from the field-line pattern:

• more concentrated lines = stronger field
• less concentrated lines = weaker field

Around a point charge, the field gets weaker as you move further away. This is because the same pattern of field lines spreads out over a larger region.

Electric field between parallel plates

Two flat metal plates can be charged so that one is positive and the other is negative. These are called parallel plates when they face each other with a gap between them.

Between the plates, away from the edges, the electric field is almost uniform.

For parallel plates:

• field lines are straight
• field lines are evenly spaced
• arrows point from the positive plate to the negative plate
• the field is strongest if the lines are closer together
• near the edges, the field bends slightly; this is called a fringing field

How electric fields explain static electricity

Electric fields help explain how charged objects can attract or repel without touching.

A charged object creates an electric field around itself. If another charge enters that field, it experiences a force. This is why charged objects can move each other at a distance.

Repulsion

If two objects have the same type of charge, each object is in the other object’s electric field. Each one experiences a force away from the other.

For example, two negatively charged balloons repel because each balloon’s excess electrons are pushed away by the other balloon’s electric field.

Attraction

If two objects have opposite charges, each object experiences a force towards the other.

For example, a negatively charged rod attracts a positively charged object because the positive charge feels a force towards the negative rod.

Attraction of neutral objects

A charged object can also attract a neutral object. This is very important in static electricity.

A neutral object has equal amounts of positive and negative charge overall. However, charges inside it can become slightly separated when a charged object is nearby. This is called induced charge separation.

Suppose a negatively charged rod is brought near a small neutral piece of paper:

• electrons in the paper are repelled slightly away from the rod
• the side of the paper nearest the rod becomes slightly positive
• the positive side is closer to the rod than the negative side
• the attraction to the nearer positive side is stronger than the repulsion from the further negative side
• the paper is attracted overall

Sparks and discharging

If a charged object builds up a large enough charge, the electric field around it can become very strong. Normally, air is an insulator, so charge does not easily flow through it.

But in a very strong electric field, air particles can become charged and allow a sudden flow of charge. This is a spark or static discharge.

Examples include:

• a small shock after walking on carpet and touching a metal handle
• lightning during a storm
• crackling when removing some synthetic clothing

Describing field diagrams in exam answers

When asked to describe an electric field diagram, focus on three things:

1. Shape — radial around point charges; straight and parallel between plates.
2. Direction — away from positive, towards negative.
3. Strength — closer field lines mean stronger field.

Forces on charges depend on the sign of the charge placed in the field:

• positive charge: force is in the same direction as the field
• negative charge: force is opposite to the field