4.2Electricity and Magnetism
Electrical Quantities — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions
30 min read 44 parts Lesson 9 of 65 Unit quiz
Lesson summary
4.2 Electrical Quantities (Part 1) — IGCSE PHYSICS notes: Explain how insulators become charged by friction through electron transfer and predict attraction or repulsion between charges.Positive and negative — there are two types of electric charge.Inside an atom — electrons carr…
- Unit (Extended)
- Not magnetism
- What happens
- Only electrons move
- Polythene rod and cloth
- Acetate rod and cloth
Part 1 of 5Two kinds of charge
Two kinds of charge
Positive and negative
- Positive and negative there are two types of electric charge.
Inside an atom
- Inside an atom electrons carry negative charge, protons carry positive charge and neutrons are neutral.
- An atom has equal numbers of protons and electrons, so the charges cancel and the atom is neutral overall.
Unit (Extended)
- Unit (Extended) electric charge is measured in coulombs (C).
Part 2 of 5Forces between charges
Effect
- Effect two charged objects close together exert a force on each other; it is either attractive or repulsive.
Rule
- Rule opposite charges attract and like charges repel.
- The saying "opposites attract" is a quick way to remember this.
Not magnetism
- Not magnetism electrostatic forces are similar to magnetic forces but they are a different phenomenon and must not be confused.
Part 3 of 5Attraction or repulsion summary
| Object 1 | Object 2 | Result |
|---|---|---|
| positive | positive | repel |
| positive | negative | attract |
| negative | positive | attract |
| negative | negative | repel |
Part 4 of 5Charging insulators by friction
1What happens
- What happens when two different insulating solids are rubbed together they can become charged.
- The charges stay on the insulators because they cannot flow away.
- One insulator ends with a net positive charge and the other with a net negative charge.
2Only electrons move
- Only electrons move an object that gains electrons becomes negatively charged;
- one that loses electrons becomes positively charged.
- Positive charge never moves in a solid, so always explain charging in terms of electrons.
3Polythene rod and cloth
- Polythene rod and cloth electrons transfer from the cloth to the rod.
- The rod becomes negative and the cloth becomes positive, so they attract each other.
4Acetate rod and cloth
- Acetate rod and cloth electrons transfer from the rod to the cloth.
- The rod becomes positive and the cloth becomes negative.
Part 5 of 5More from the specification
More from the specification
Electric current and charge transfer
- Electric current (I) in a circuit is the rate of flow of electric charge (I = Q / t),
- where free electrons flow through metal conductors from negative to positive terminals.
Potential difference, resistance, and electrical energy
- Potential difference (V = W / Q) is the electrical energy transferred per unit charge across a component, related to resistance (R = V / I) in ohms (Ω).
Electrical power in a circuit
- Electrical power (P = IV = I²R) represents the rate at which electrical energy is converted into other energy forms by circuit components per second, measured in watts (W).
Electrostatic field and gold-leaf electroscope
- An electric field is a region where an electric charge experiences a force;
- an electroscope with a metal plate and stem detects static charge through leaf repulsion.
Electric current and charge transfer
- Electric current (I) in a circuit is the rate of flow of electric charge (I = Q / t),
- where free electrons flow through metal conductors from negative to positive terminals.
Potential difference, resistance, and electrical energy
- Potential difference (V = W / Q) is the electrical energy transferred per unit charge across a component, related to resistance (R = V / I) in ohms (Ω).
Electrical power in a circuit
- Electrical power (P = IV = I²R) represents the rate at which electrical energy is converted into other energy forms by circuit components per second, measured in watts (W).
Electrostatic field and gold-leaf electroscope
- An electric field is a region where an electric charge experiences a force;
- an electroscope with a metal plate and stem detects static charge through leaf repulsion.
Electromotive force
- Energy transferred from other forms to electrical energy per unit charge.
Gold-leaf electroscope
- A device in which a charged gold leaf falls when photoelectrons discharge the plate.
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)
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.
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)
Electric field
- Region of space in which a charge experiences a force.
Alternating current
- Current that continually reverses direction.
Field lines
- Lines showing the direction and strength of a magnetic field.
Direct current
- Current that flows in one direction only.
Current
- Flow of electrons in a circuit.
Resistance
- Opposition to current, R = V / I, in ohms (Ω).
Electroscope
- Instrument with a metal cap, stem and gold leaf used to detect charge.
Conductor
- Material that allows current to flow.
Key terms
| Term | Meaning |
|---|---|
| Electric charge | A property of matter that causes electric forces; positive or negative, measured in coulombs. |
| Electron | A negatively charged particle in atoms that can be transferred between objects. |
| Charging by friction | Giving insulators opposite charges by rubbing them together so electrons transfer. |
| Insulator | A material that does not let charge flow easily. |
| Coulomb (C) | The unit of electric charge. |
| Net charge | The overall charge left on an object after electrons are gained or lost. |
Avoid these mistakes
1
Saying positive charge moves in a solid — only electrons move, so the positive object has lost electrons.
2
Thinking a neutral atom has no charges — it has equal numbers of positive protons and negative electrons.
3
Mixing up attract and repel — unlike charges attract, like charges repel.
4
Confusing electrostatic and magnetic forces — they are different phenomena.
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