PHYSICS · Stage 7 · Chapter 4: Electricity
Physics 4.1 Charging up — Static Electricity, Electrons and Electrostatic Forces
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Lesson summary
4.1 Charging up — Static Electricity, Electrons and Electrostatic Forces — Alhussan International Academy — Physics (Stages 7–9) notes: Name the two types of charge, explain why objects become charged, and explain the difference between conductors and insulators.Phenomenon = some…
- Forces between charges
- Charged balloon
- Origin of the word
- Atom model
- Neutral atom
- Which particles move
Section 1 of 7Electrostatic phenomena
- Phenomenon = something that you can observe.
- Examples
- a balloon rubbed on clothes sticks to a wall, deflects a stream of water from a tap and picks up small pieces of paper.
- A comb rubbed through dry hair can attract a thin stream of water or small pieces of paper.
- Clothes crackle when pulled apart because charge builds up by friction and then discharges.
- Cause
- these things happen because rubbing charges the balloon.
- Static
- it means stationary or standing still, so electrostatic phenomena are caused by charges that are not moving.
Section 2 of 7What charge is
- Charge
- it is a property of a particle or object, like mass.
- Two types
- positive (+) charge and negative (-) charge.
- Forces between charges
- when charges are brought together, electrostatic attraction or repulsion occurs.
- Like charges repel, so a positive charge repels another positive charge and a negative charge repels another negative charge.
- Unlike charges attract, so a positive charge attracts a negative charge.
- Charged balloon
- it can repel the negative charge in scraps of paper, running water or a wall, leaving a positive charge that produces attraction.
- Origin of the word
- in 600 BCE a Greek scientist picked up leaves with rubbed amber, and the Greek word for amber is elektron.
Section 3 of 7Charges in atoms
- Atom model
- protons and neutrons make up the central nucleus, and electrons orbit the nucleus.
- Neutral atom
- the number of electrons (negative) always equals the number of protons (positive), so the atom has no net charge and is neutral.
- Which particles move
- only electrons move from one object to another, and protons stay in the nuclei.
- Gaining or losing
- gaining electrons makes an object negative and losing electrons makes it positive.
Section 4 of 7How rubbing charges an object
- Polythene rod
- when it is rubbed with a cloth, electrons are transferred from the cloth to the rod.
- The rod has a net negative charge, because there is not enough positive charge to neutralise the negative charge.
- The cloth has a net positive charge, because there is not enough negative charge to neutralise the positive charge.
- Perspex rod
- when it is rubbed, electrons are transferred from the rod to the cloth, so the rod has a net positive charge and the cloth a net negative charge.
- Only some electrons
- not all electrons are transferred, just some.
- Conservation of charge
- the total amount of charge in the rod and the cloth is always the same, since charge cannot be created or destroyed, just like energy or mass.
| Object | Electron change | Net charge |
|---|---|---|
| Polythene rod (rubbed with cloth) | Gains electrons | Negative |
| Cloth rubbed on polythene | Loses electrons | Positive |
| Perspex rod (rubbed) | Loses electrons | Positive |
Section 5 of 7Testing with rods
1Two polythene rods2 points
- Kunala rubbed a polythene rod and hung it by a string, then brought a second rubbed rod close, and the first rod was repelled.
- Both rods are negatively charged, so the net charge on both is the same.
2Perspex rod2 points
- when a rubbed Perspex rod is brought near the polythene rod, it is attracted.
- The rods have unlike charges, and the net charge on the Perspex rod is positive.
Section 6 of 7Electric field
- Electric field the region around any charge where another charge experiences a force.
Section 7 of 7Insulators and conductors
- Insulator
- electrons stay on the rod once you have rubbed it, so the charge stays in place, as with polythene and Perspex.
- Conductor
- electrons can move in it, as in metals.
- A metal rod rubbed with a cloth also becomes charged,
- but extra electrons move through the metal to your hand and through you to earth,
- so the rod seems uncharged.
- Insulated person
- if charge is transferred to a person insulated from the ground, the charge stays on them.
- Their hairs all gain the same charge, so they repel each other and the hair stands away from the body.
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