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?
