Static electricity
Static electricity is the build-up of electric charge on insulating objects. Unlike current electricity, the charges don't flow continuously — they sit until either grounded or discharged in a spark.
How charge builds up — friction
When two insulating materials are rubbed together, electrons (which are negatively charged) transfer from one to the other. The material that gains electrons becomes negatively charged; the one that loses them becomes positively charged.
- Polythene rubbed with a duster → polythene gains electrons → polythene −, duster +.
- Acetate rubbed with a duster → acetate loses electrons → acetate +, duster −.
Protons stay put — only electrons move during friction charging.
Forces between charges
- Like charges repel (− & −, or + & +).
- Unlike charges attract (+ & −).
The strength of the force decreases with distance.
Electric fields
A charged object creates an electric field in the space around it. A small positive test charge placed in the field experiences a force in the direction of the field line.
- Field lines point away from positive charges and towards negative charges.
- Closer field lines = stronger field.
- Two charged plates create a (roughly) uniform field between them.
Sparks and discharge
When a charged object is brought close to a conductor, the electric field can ionise the air, allowing electrons to flow as a spark. Lightning is a giant version of this — charge separates between cloud and ground until the field is strong enough to break down the air.
Earthing
A conductor connected to the ground allows excess charge to flow to or from the Earth. Many devices (filling stations, fuel trucks) are deliberately earthed to prevent dangerous static buildup. A spark in a fuel-rich atmosphere could ignite explosive vapour.
✦Worked example— Worked example — a charged balloon
A rubbed balloon attracts a stream of water from a tap. Why?
- Friction transfers electrons to the balloon — it becomes negatively charged.
- The balloon's electric field repels electrons in the water molecules, leaving the near side of the stream slightly positive.
- The (now polarised) water is attracted to the negative balloon.
⚠Common mistakes
- Saying "the charge moves" — it's the electrons that move; protons stay in nuclei.
- Confusing positive (lost electrons) with "added protons" — it's the same thing in net effect, but mechanism is electron loss.
- Forgetting field lines have direction — always positive → negative.
- Thinking insulators can't be charged — they can; in fact they retain charge better than conductors do.
➜Try this— Quick check
Two balloons, both rubbed with the same cloth, are suspended near each other. They repel. Why?
- They've gained equal electrons → both negatively charged → like charges repel.
AI-generated · claude-opus-4-7 · v3-deep-physics