Electric charge
Section: Electricity & Magnetism | Syllabus: Cambridge IGCSE Physics (0625)
Types of Electric Charge
- There are two types of electric charge: positive (+) and negative (−)
- Like charges repel
- two positive charges push apart, and two negative charges push apart
- Unlike charges attract
- a positive charge and a negative charge pull together
Unit of Charge
Extended Only
- Charge is measured in coulombs (C)
- Charge on a single electron:
- e = 1.6 × 10^-19 C
Charging by Friction
- Rubbing two insulators together transfers electrons from one material to the other
- only electrons move - protons stay fixed in the nucleus
- charge is transferred, not created
- Material that loses electrons → becomes positively charged
- it is left with more protons than electrons
- Material that gains electrons → becomes negatively charged
- it now has more electrons than protons
Worked Example: Charging Polythene by Friction
Muchindu rubs a polythene rod with a dry cloth and wants to work out what charge each object ends up with.
- Electrons transfer from the cloth to the polythene rod
- The polythene rod gains electrons → becomes negatively charged
- The cloth loses electrons → becomes positively charged
Detecting Electrostatic Charge
- A gold leaf electroscope detects electric charge and shows whether an object is charged
- Structure:
- metal cap connected to a metal rod
- thin gold leaf attached to the bottom of the rod
- enclosed in a case to keep out air currents
- How it works:
- touching a charged object to the cap spreads charge to the rod and the leaf
- rod and leaf gain the same charge, so like charges repel and the leaf rises away from the rod
- a bigger charge produces a bigger deflection; an uncharged object produces none
A gold leaf electroscope detects charge by the deflection of the gold leaf
Conductors and Insulators
- An electrical conductor has free electrons that can move from atom to atom
- this lets charge flow easily
- An electrical insulator has electrons tightly bound to their own atoms
- charge cannot flow through it
| Good Conductors | Good Insulators |
|---|---|
| Metals (copper, aluminium, silver) | Plastics (polythene, PVC, Perspex) |
| Carbon (graphite) | Rubber, glass, dry air |
Experiment: Distinguishing Conductors from Insulators
- Set up a circuit with a cell and a lamp, with a gap left in the circuit for a test material
- Connect the test material across the gap
- Lamp lights → the material is a conductor; lamp stays off → the material is an insulator
Electric Fields
Extended Only
- An electric field is a region in which an electric charge experiences a force
- The direction of an electric field at a point:
- the direction of the force on a positive charge placed at that point
- Field lines point away from positive charges and towards negative charges
- Field lines meet a charged conducting surface at right angles
Field lines radiate outward from a positive point charge and from a positively charged sphere
Field Between Parallel Plates
- Between two oppositely charged parallel conducting plates, the field is uniform
- evenly spaced, parallel field lines run from the positive plate to the negative plate
- end effects at the edges of the plates will not be examined
A uniform electric field exists between two oppositely charged parallel plates
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