Section 1 of 5Charging by Friction
Rubbing a balloon on hair or a polythene rod with a cloth leaves both objects charged.
Insulators
- When two different insulating materials are rubbed together, electrons are transferred from the surface of one to the other.
Gain of electrons
- The material that gains electrons becomes negatively charged, because it has more electrons than protons.
Loss of electrons
- The material that loses electrons becomes positively charged, and it has an equal and opposite charge to the other.
Only electrons move
- Protons are fixed in the nuclei of atoms, so a positive charge is always caused by missing electrons.
Staying charged
- Insulators do not let charge flow away, so the charge stays on the surface until a conductor or moist air allows it to leak.
Section 2 of 5Forces Between Charges
Charges produce forces that can be seen with suspended balls or rods.
1
Two charged objects exert a force on each other even though they do not touch, so it is a non-contact force.
2
Objects with the same type of charge repel each other.
3
Objects with opposite types of charge attract each other.
4
The force is greater when the charges are larger or closer together.
5
A charged insulator can also attract a neutral one, because the nearby surface of the neutral object is pulled into a slight opposite charge.
| Charges | Force |
|---|---|
| + and + | Repulsion |
| − and − | Repulsion |
| + and − | Attraction |
Section 3 of 5Electric Fields
Electric Fields
Definition
- An electric field is a region where a charged particle experiences a force.
Field lines
- Lines drawn with arrows show the direction of the force on a small positive charge placed in the field.
Point charge
- Around a positive point charge the lines point away radially, and around a negative point charge they point inwards.
Field strength
- Where the lines are closer together the field is stronger, and the force on a charge is greater.
Section 4 of 5Uses and Hazards
Photocopiers and spray painting
- Charged particles are attracted to oppositely charged surfaces, so toner or paint is evenly deposited.
Sparks
- If charge builds up on an object, the electric field in the air can become strong enough to make air conduct and cause a spark.
Fuelling
- Aircraft and fuel tankers are earthed during refuelling, so that charge flows away safely and cannot cause a spark.
Section 5 of 5Exam Checklist & Tips
11.1.5 – Key Points for the Exam
- Static electricity Charge that builds up on an insulator and does not flow.
- Electric field A region in which a charged particle feels a force.
- Earthing Connecting a conductor to the ground so that charge flows away safely.
- Non-contact force A force that acts between objects without them touching.
Common Mistakes
- Saying that protons move when an object is charged; only electrons are transferred.
- Stating that the object that loses electrons becomes negative; it becomes positive.
- Drawing electric field lines that cross, or pointing towards a positive charge.
- Forgetting that a neutral object is attracted to a charged one because of induced charge, not because it carries a net charge.