Section 1 of 4What an Electric Field Is
An electric field is a region in which a charged object experiences a force, and it is shown using field lines.
Field region
- Field region Any space around a charge where another charge feels an electrostatic force, whether it is attractive or repulsive.
Field lines
- Field lines Lines drawn from positive charges towards negative charges, with arrows showing the direction of the force on a positive test charge.
Field strength
- Field strength The field is stronger where the field lines are closer together, so a larger force acts on any charge placed there.
Uniform field
- Uniform field Between two parallel plates of opposite charge, the field lines are parallel and evenly spaced, so the field is uniform.
Section 2 of 4Static Electricity in Use
Charges can be placed on objects deliberately so that they are attracted or repelled in a controlled way.
1
Spray painting gives paint droplets a charge, so they spread evenly and are attracted to an earthed object, reducing overspray.
2
A photocopier charges a drum in patches, and charged toner powder sticks to the charged areas before it is transferred to paper.
3
Dust precipitators charge smoke particles so that they are attracted to oppositely charged plates and removed from the air.
4
Lightning occurs when charge builds up in clouds, creating a large potential difference that causes a discharge through the air.
| Charges | Force between them | Field line pattern |
|---|---|---|
| Two like charges | Repel each other | Lines point away from each positive charge |
| Two unlike charges | Attract each other | Lines run from the positive to the negative charge |
| Single positive charge | Repels a positive test charge | Lines radiate outwards in every direction |
Section 3 of 4Safety and Earthing
Safety and Earthing
Sparks
- Sparks A build-up of static charge can discharge suddenly as a spark, which is dangerous near flammable materials.
Earthing
- Earthing Fuel tankers and aircraft are earthed so that charge flows away safely rather than building up to a dangerous level.
Clothing
- Clothing Rubbing some synthetic fabrics transfers electrons, leaving charged clothing that can attract dust and cause small shocks.
Insulators
- Insulators Insulating materials hold charge in place, so the charge stays on the surface and does not flow away by itself.
Section 4 of 4Exam Checklist & Tips
2.4.4 – Key Points for the Exam
- Electric field A region in which an electric charge experiences a force because of other charges.
- Field line A line showing the direction of the force that would act on a positive charge placed in the field.
- Static electricity A build-up of electric charge on an insulator or isolated object, which does not flow away easily.
- Earthing Connecting a charged object to the ground so that excess charge can flow away safely.
- Uniform field A field with the same strength and direction at every point.
Common Mistakes
- Drawing field lines as the paths that charged particles follow;
- they show the direction of the force, not the path of a particle.
- Drawing field lines that cross each other, which cannot happen because the field has a single direction at each point.
- Stating that field lines point towards positive charges; they point away from positive charges.
- Saying static charge always flows away quickly from insulators;
- insulators hold charge in place, which is why charge builds up.