4.2.1Electricity and Magnetism
Electric Charge — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions
5 min read 11 parts Lesson 10 of 65 1 quiz
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Lesson summary
4.2.1 Electric Charge — IGCSE PHYSICS notes: Describe the two kinds of charge, the forces between them, charging by friction, conductors and insulators, and electric fields.Types — there are two kinds of electric charge, positive and negative.Inside an atom — electrons are negati…
- Only electrons move
- Non-contact force
- Polythene rod and cloth
- Acetate rod and cloth
- Charge stays
- Conductor test
Section 1 of 11Two kinds of charge
Two kinds of charge
Types
- Types there are two kinds of electric charge, positive and negative.
Inside an atom
- Inside an atom electrons are negative, protons are positive and neutrons are neutral.
- An atom has equal numbers of protons and electrons, so its charges cancel and it is neutral overall.
Unit
- Unit electric charge is measured in coulombs (C) (Extended).
Only electrons move
- Only electrons move protons stay fixed in the nucleus, so charging always involves electrons being gained or lost.
Section 2 of 11Forces between charges
Rule
- Rule opposite charges attract and like charges repel.
Non-contact force
- Non-contact force the force acts at a distance, like the force between magnetic poles, but it is a different phenomenon.
| Object 1 | Object 2 | Result |
|---|---|---|
| positive | positive | repel |
| positive | negative | attract |
| negative | positive | attract |
| negative | negative | repel |
Section 3 of 11Charging by friction
1Process
- Process rubbing two different insulators together transfers electrons from one to the other.
- The object that gains electrons becomes negative.
- The object that loses electrons becomes positive.
- The two objects end with equal and opposite charges, so they attract each other.
2Polythene rod and cloth
- Polythene rod and cloth electrons move from the cloth to the rod, so the rod is negative and the cloth is positive.
3Acetate rod and cloth
- Acetate rod and cloth electrons move from the rod to the cloth, so the rod is positive and the cloth is negative.
4Charge stays
- Charge stays insulators cannot let the charge flow away, so it stays where it was made.
Section 4 of 11Conductors and insulators
Conductor
- Conductor a material that lets charge (electrons) flow through it easily, such as a metal.
- In a metal the outer electrons are delocalised (free), so they can move through the lattice of positive ions.
Insulator
- Insulator a material that does not let charge flow easily, such as plastic, rubber, glass and dry wood.
Earthing
- Earthing connecting a charged conductor to the ground lets electrons flow to or from earth and discharges it.
Section 5 of 11Testing materials and detecting charge
Conductor test
- Conductor test touch a charged gold-leaf electroscope with the material.
- A conductor makes the leaf fall quickly because charge flows away.
- An insulator leaves the leaf raised or lets it fall only slowly.
Detecting charge
- Detecting charge a charged object attracts small pieces of uncharged paper.
Finding the sign
- Finding the sign a charged rod is tested by seeing whether it is repelled by a rod of known charge.
- Repulsion is the reliable test because an uncharged object is also attracted.
Section 6 of 11Testing a rod of unknown charge in order
1
Charge a reference rod of known sign (for example polythene, negative) and hang it from a stirrup.
2
Bring the test rod near one end of the suspended rod.
3
Observe whether the suspended rod moves away or towards the test rod.
4
Conclude that repulsion means the test rod has the same sign as the reference rod.
Section 7 of 11Electric fields (Extended)
Electric field
- Electric field a region of space in which an electric charge experiences a force.
Direction
- Direction the direction of the field at a point is the direction of the force on a positive charge placed there.
Field lines
- Field lines they go outward from a positive point charge and inward to a negative point charge.
Parallel plates
- Parallel plates between two oppositely charged plates the field is uniform, with parallel, equally spaced lines from + to −.
Section 8 of 11Uses and dangers of static charge
Uses
- Uses photocopiers, printers and paint spraying rely on charged particles being attracted to oppositely charged surfaces.
Dangers
- Dangers lightning and sparks can ignite fuel, so fuel pipes and tankers are earthed.
Section 9 of 11More from the specification
More from the specification
Electric charge
- Electric charge a property of matter measured in coulombs, with like charges repelling and unlike charges attracting
Current
- Current the rate of flow of charge, so charge equals current multiplied by time
Potential difference
- Potential difference energy transferred per unit charge, linking energy, charge and voltage in a circuit
Resistance and power
- Resistance and power resistance is voltage divided by current and electrical power is voltage multiplied by current;
- a graph of current against voltage shows ohmic behaviour as a straight line
Electric charge
- Electric charge a property of matter measured in coulombs, with like charges repelling and unlike charges attracting
Current
- Current the rate of flow of charge, so charge equals current multiplied by time
Potential difference
- Potential difference energy transferred per unit charge, linking energy, charge and voltage in a circuit
Resistance and power
- Resistance and power resistance is voltage divided by current and electrical power is voltage multiplied by current;
- a graph of current against voltage shows ohmic behaviour as a straight line
Energy transferred
- W = V Q, in joules.What the symbols mean: E or W = energy transferred (J) · Q = charge (C) · V = potential difference (V) · I = current (A) · t = time (s)
Alternating current
- Current that continually reverses direction.
Electromotive force
- Energy transferred from other forms to electrical energy per unit charge.
Electrical energy
- Energy carried by electrons in a circuit.
Mains supply
- Domestic a.c. supply, rated 230 V rms at 50 Hz in the UK.
Electrical power
- Rate of energy transfer, P = I V.What the symbols mean: P = power (W) · I = current (A) · V = potential difference (V)
Current
- Flow of electrons in a circuit.
Resistance
- Opposition to current, R = V / I, in ohms (Ω).
Terminal
- Connection point of a cell, labelled + or −.
Component
- Part of a vector along one axis.
Potential difference
- Work done by a unit charge passing through a component.
Section 10 of 11Key terms
| Term | Meaning |
|---|---|
| Electric charge | Property of matter that causes electric forces; positive or negative, measured in coulombs (C). |
| Charging by friction | Rubbing two insulators so that electrons transfer and the objects gain opposite charges. |
| Conductor | Material that allows charge to flow through it easily. |
| Insulator | Material that does not allow charge to flow through it easily. |
| Earthing | Connecting a conductor to the ground so that charge flows away. |
| Electric field | Region of space in which a charge experiences a force. |
Section 11 of 11Exam technique
1
Saying positive charge moves in a solid — only electrons move, so a positive object has lost electrons.
2
Saying attraction proves two objects are oppositely charged — an uncharged object is attracted too, so only repulsion is a sure test.
3
Mixing up electric and magnetic forces — they are different phenomena.
4
Drawing electric field lines pointing the wrong way — they run from positive to negative.
Marked practiceQuizzes for this lesson
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