GCSE · Physics · AQA · Spec 8463

Electric fields (physics only)

Hold a charged balloon near someone’s hair and the hair lifts towards it before anything touches. Nothing you can see crosses the gap. So what is doing the pulling?

Electric fields · See the pattern

The field around a charged sphere

View: Positive sphere. Showing 1 layer: Positive sphere and its field

View

Explore

tap a feature to see what it means

Straight lines point out from the sphere in every direction. See how crowded they are near the surface, and how they spread apart further out.

Switch views to flip the charge on the sphere, or to move a small positive charge close to it and then far away.

What do you really think?

What is actually around a charged sphere?

A small metal sphere has been given a positive charge. It sits on an insulating stand, with nothing else anywhere near it.

Which of these is closest to what you think is around the sphere right now?

An electric field along the lines you would draw, but nothing in the gaps between them.Nothing yet. A field only appears once another charged object comes near.An electric field filling the space all around it, which would push or pull any charge placed there.Positive charge streaming out of the sphere along the field lines.

How sure are you?

I'm sureI think soI'm guessing

An electric field along the lines you would draw, but nothing in the gaps between them.

This is a common misconception

The lines are only a drawing tool. The field fills all the space around the sphere, gaps included. We draw a few lines to show which way it acts, and how close together they are shows how strong it is. A charge placed in a gap still feels a force.

Nothing yet. A field only appears once another charged object comes near.

This is a common misconception

The field is already there, whether or not anything is in it. A second charged object doesn’t create the field. It just shows you the field is there, because it feels a force as soon as you put it in.

An electric field filling the space all around it, which would push or pull any charge placed there.

This matches the evidence

Yes. The sphere’s charge creates a field in all the space around it. Put any other charged object anywhere in that field and it feels a force, biggest close to the sphere.

Positive charge streaming out of the sphere along the field lines.

This is a common misconception

Nothing flows along field lines. The sphere’s charge stays on the sphere. The arrows only show which way the force on a small positive charge would act if you put one there.

An electric field is the region around a charged object where another charged object feels a force. It is there whether anything is in it or not, and it fills the gaps between the lines you draw.

Put it to work

Where is the push biggest?

A small positive charge is placed at each of these points around a positively charged sphere. Rank the points by the size of the force on the small charge.

1 · Biggest force5 · Smallest force

18 cm above the sphere
12 cm to the left of the sphere
25 cm below the sphere
3 cm above the sphere
6 cm to the right of the sphere

Sparking

Reason it through

Why can a spark jump from a charged object to an earthed conductor held very close to it?

Link 1 of 4

First link · your turn

An insulated metal dome has been charged. Nothing is touching it and nothing else is nearby. What is in the space around it?

Locked — reveal the link above first

Now write it yourself

Explain a push without a touch

Two small, light balls both have a positive charge. They hang on threads close to each other and swing apart without touching. Explain, in terms of electric fields, how the balls can push each other apart without touching. Then state what would happen to the force if the balls were moved closer together. [4 marks]

Your answer

0 words · your answer stays on this page and is not sent anywhere.

The mark scheme

Each charged ball has an electric field around it. (1 mark)
Each ball is in the electric field of the other ball. (1 mark)
So each ball experiences a force from the other ball’s field, across the gap, without touching (a non-contact force). (1 mark)
Closer together, the force would be bigger (because each ball’s field is stronger closer to it). (1 mark)

Model answer

Each charged ball creates an electric field in the space around it, so each ball is sitting in the other ball’s field. A charged object in an electric field experiences a force, so each ball is pushed away by the other across the gap, without the balls touching: a non-contact force. If the balls were moved closer together, the force would be bigger, because each ball’s field is stronger closer to it.

Four moves make the explanation complete: each ball has a field, each is in the other’s field, being in a field means a force, and closer means a bigger force.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Every charged object is surrounded by an electric field. Put another charged object in that field and it feels a force, and the closer it gets, the bigger that force.

What you need to know

What an electric field is, and that every charged object has one around it.
How the field, and the force on a second charged object, change with distance.
How to draw the field pattern for an isolated charged sphere, positive or negative.
How the electric field explains the non-contact force between charged objects.
How the electric field explains sparking.

The big picture

A charged object creates an electric field in the space around it. The field is strongest close to the object and gets weaker further away. A second charged object placed in the field experiences a force, and that force gets bigger as the distance between the objects decreases. Around an isolated charged sphere the field lines are straight, pointing away from a positive charge or towards a negative one. The field explains why charged objects can push or pull without touching, and why a strong enough field close to a charged object can make a spark jump.

Key points

1A charged object creates an electric field around itself. The field is strongest close to the object and gets weaker further away.

2A second charged object placed in the field experiences a force. The force gets stronger as the distance between the objects decreases.

3Around an isolated charged sphere the field lines are straight and evenly spaced, meeting the surface at right angles. Arrows point away from a positive sphere and towards a negative one, and lines closer together show a stronger field.

4Charged objects exert a non-contact force on each other because each one is in the other’s electric field.

5If the field close to a charged object becomes strong enough, charge can move through the air to a nearby earthed conductor, and this is seen as a spark.

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Worked example

Problem

Draw the electric field pattern around an isolated sphere that has a negative charge. Then mark the point where a small positive charge would feel the biggest force.

⚠ Watch out

Drawing the arrows on a negatively charged sphere pointing outwards. Field-line arrows show the direction of the force on a small positive charge, so they point away from a positive charge and towards a negative one.

🧠

Memory hook

Close lines, strong field, big force. Spread lines, weak field, small force. And the arrows go the way a small + charge would be pushed or pulled: out from +, in to −.

✓

Check yourself

A small positive charge sits near a negatively charged sphere. Which way is it pulled, and what happens to the force if it moves further away? (Towards the sphere; the force gets smaller.)

Flashcards

(10)

What is an electric field?

The region around a charged object in which another charged object experiences a force.

Where is the electric field around a charged object strongest?

Closest to the object. It gets weaker further away.

What happens to the force on a second charged object as it moves closer to a charged object?

The force gets bigger.

What shape are the field lines around an isolated charged sphere?

Straight lines coming out from the centre, evenly spaced, meeting the surface at right angles.

Every field-line arrow around a charged ball aims outward. Is its charge + or −?

It is positive. Arrows aim outward from positive charge and inward to negative charge.

On a field diagram, what does the spacing of the lines tell you?

How strong the field is: lines closer together mean a stronger field.

What does the arrow on a field line show?

The direction of the force on a small positive charge placed there.

Is there an electric field in the gaps between the lines you draw?

Yes. The field fills all the space around the charged object; the lines are just a way of showing it.

How can two charged objects push or pull each other without touching?

Each is in the other’s electric field, so each experiences a force: a non-contact force.

What causes a spark to jump from a charged object?

The field close to the object becomes strong enough to make charge move through the air to a nearby earthed conductor.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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Electric fields (physics only)

What is actually around a charged sphere?

Where is the push biggest?

Explain a push without a touch

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