Rubbing insulators transfers electrons and builds up charge
Definition
Static electricity
An electric charge that builds up on an object and stays in place.
- Certain insulating materials become electrically charged when they are rubbed together, because rubbing transfers electrons from one material to the other.
- The material that gains electrons becomes negatively charged.
- The material that loses electrons is left positively charged.
- Electrons carry negative charge, so gaining extra electrons gives an overall negative charge, and losing electrons leaves more positive charges than electrons, giving an overall positive charge.
- The electrons are transferred, not created: for every electron one material gains, the other loses one, so the two materials gain equal and opposite amounts of charge as long as none leaks away.
- Because the materials are insulators, the transferred electrons cannot flow away easily, so the charge stays put and builds up.
Key Idea
Only electrons move when insulators are rubbed together. The positive charges stay where they are.
Like charges repel, opposite charges attract, without touching
- Two charged objects exert a force on each other.
- Objects with the same type of charge repel, so two negatively charged objects push apart.
- Objects with different types of charge attract, so a negative and a positive object pull together.
- These are non-contact forces, because the objects do not have to touch; a charged object can start to move as another charged object is brought near, which is evidence that a force acts across the gap.
Building up charge can cause a spark
- Rubbing insulating surfaces can build up a large amount of charge.
- If a charged object comes close enough to another object, electrons can suddenly jump through the air between them.
- This sudden movement of charge is a discharge and may produce a visible spark, which transfers electrons and reduces the build-up of charge.
- A full description of sparking links the stages: two insulating surfaces are rubbed together.
- Electrons transfer from one surface to the other.
- One object becomes negatively charged and the other positively charged.
- Charge builds up because it cannot flow away easily through the insulator.
- When the build-up is large enough, electrons suddenly cross the air gap, producing a spark.
Common Mistake
- Do not say rubbing creates charge or that positive charges move; charge is transferred because electrons move.
- An object becomes positive because it has lost electrons, not because it gained positive charges.
- Remember the rule the right way round: opposite charges attract and like charges repel.
Exam technique
- When you explain charging, always name the particle and its direction: electrons move from A to B, so A becomes positive and B becomes negative.
- For a force question, give both the charge rule and the evidence: the objects move towards or away from each other without touching, so the force is non-contact.
Self review
- Which particles are transferred when two insulators are rubbed together?
- An object gains electrons; what charge does it end up with, and why?
- Why does an object that loses electrons become positively charged?
- What happens when two objects with the same type of charge are brought close?
- Why are electrostatic attraction and repulsion called non-contact forces?
- What movement of charge produces a spark?
