Electricity & Magnetism
Explore the fundamentals of electric current, voltage, and resistance, apply Ohm's Law to circuits, understand series and parallel circuits, and discover the deep connection between electricity and magnetism.
Electric Charge and Current
All matter is made of atoms containing protons (+), electrons (−), and neutrons (neutral). When electrons are free to move between atoms (as in metal conductors), we get electric current — a flow of electric charge.
V
Voltage (V)
The "push" (electrical pressure) that drives charges around a circuit. Also called potential difference. Measured in volts (V) using a voltmeter placed in parallel.
I
Current (I)
The rate of flow of electric charge through a circuit. Measured in amperes / amps (A) using an ammeter placed in series. Current flows from + to − (conventional direction).
R
Resistance (R)
Opposition to the flow of current. Measured in ohms (Ω). Caused by collisions between electrons and atoms. Converts electrical energy into heat and light.
Simple Series Circuit Diagram
Conductors and Insulators
Conductors allow current to flow freely (e.g. copper, aluminium, gold). Insulators resist current flow (e.g. rubber, plastic, glass, wood). Semiconductors (e.g. silicon) conduct under certain conditions — used in transistors and computer chips.
Static vs Current Electricity
Static electricity is a build-up of charge on an object (e.g. rubbing a balloon on hair). Current electricity is the continuous flow of charge around a circuit. Static discharge can cause sparks (e.g. lightning).
Ohm's Law and Circuits
Ohm's Law (discovered by Georg Ohm in 1827) states that the current flowing through a conductor is directly proportional to the voltage across it, provided temperature remains constant. It is one of the most useful relationships in electricity.
Series Circuits
- Components connected in a single loop
- Same current flows through all components
- Voltage is shared across components (adds up to supply voltage)
- Total resistance = R₁ + R₂ + R₃ (resistances add)
- If one component fails, the whole circuit breaks
- Example: old-style Christmas lights
Parallel Circuits
- Components connected in separate branches
- Same voltage across all branches
- Current is shared between branches
- Total resistance is less than smallest individual resistance
- If one component fails, others keep working
- Example: household electrical circuits, modern lights
| Property | Series | Parallel |
|---|---|---|
| Current | Same throughout | Splits between branches |
| Voltage | Splits between components | Same across each branch |
| Total resistance | Increases (R₁ + R₂) | Decreases (less than smallest R) |
| If one breaks | All stop | Others continue |
Magnetism and Electromagnetism
Magnetism and electricity are deeply linked — they are two aspects of the same fundamental force called electromagnetism. A moving electric charge creates a magnetic field, and a changing magnetic field creates an electric current.
Permanent Magnets
- Attract materials containing iron, nickel, or cobalt
- Have two poles: north (N) and south (S)
- Like poles repel; unlike poles attract
- Surrounded by invisible magnetic field lines (from N to S outside the magnet)
- Cannot be demagnetised by simply cutting them — each half becomes a new magnet
Electromagnetism
- An electromagnet is made by coiling a wire around an iron core and passing current through it
- The magnetic field can be switched on/off with current
- Strength increased by: more turns of wire, more current, softer iron core
- Used in: electric motors, generators, MRI machines, electric bells, speakers, maglev trains
Electric Motor — Electricity → Magnetism → Motion
A current-carrying coil inside a magnetic field experiences a force. This is used to convert electrical energy into rotational mechanical energy. Motors power fans, cars, washing machines, and most modern machinery.
Generator — Motion → Magnetism → Electricity
Moving a conductor through a magnetic field (or changing the field near a conductor) induces an electric current. This is the principle behind all power stations — whether using steam (coal, nuclear) or flowing water (hydro) or wind to spin magnets inside coils of wire to generate electricity.
Key Vocabulary
Ohm's Law
V = I × R. The voltage across a conductor equals the current multiplied by the resistance. At constant temperature, current is directly proportional to voltage and inversely proportional to resistance.
Resistance (Ω)
The opposition a material offers to current flow, measured in ohms (Ω). Caused by electron collisions with ions in the material. Metals have low resistance; insulators have very high resistance.
Electromagnet
A temporary magnet created by passing an electric current through a coil of wire around a magnetic core (usually iron). Unlike permanent magnets, it can be switched on and off and its strength controlled.
Potential Difference (Voltage)
The energy transferred per unit of charge between two points in a circuit, measured in volts (V). Sometimes called "the push" that drives current around a circuit. A 9 V battery provides 9 joules per coulomb of charge.
Worked Examples
A circuit has a voltage of 12 V and a resistance of 4 Ω. What is the current?
Formula: V = I × R, rearranged: I = V ÷ R
Substitute: I = 12 V ÷ 4 Ω
Answer: I = 3 A (3 amperes). A current of 3 amps flows through the circuit.
Two resistors of 6 Ω and 4 Ω are connected in series to a 20 V battery. Find the total resistance and current.
Total resistance (series): R_total = R₁ + R₂ = 6 + 4 = 10 Ω
Current: I = V ÷ R = 20 V ÷ 10 Ω = 2 A
Voltage across each resistor: V₁ = I × R₁ = 2 × 6 = 12 V; V₂ = 2 × 4 = 8 V. Check: 12 + 8 = 20 V ✓
Explain why household circuits use parallel wiring instead of series wiring.
In series: If one appliance (e.g. a lamp) breaks or is switched off, it breaks the circuit and everything stops. Adding more appliances increases total resistance, reducing current and making all appliances dimmer/weaker.
In parallel: Each appliance receives the full mains voltage (230 V in Australia). Switching off one appliance doesn't affect the others. Adding more appliances does not change the voltage others receive.
Conclusion: Parallel circuits are safer, more reliable, and more practical for household use. Each appliance operates independently at the correct voltage.
Knowledge Check
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Key Concepts Summary
- ✓ Voltage (V) is the "push" driving current; current (I) is the rate of charge flow in amps; resistance (R) opposes flow in ohms.
- ✓ Ohm's Law: V = I × R. Rearranges to I = V/R or R = V/I.
- ✓ Series circuits: same current everywhere; voltages add; total resistance increases; one break stops all.
- ✓ Parallel circuits: same voltage across branches; current splits; total resistance decreases; one break doesn't stop others.
- ✓ Moving electric charges create magnetic fields — this is the basis of electromagnetism.
- ✓ Electric motors convert electrical energy to motion; generators convert motion to electrical energy (the reverse process).
- ✓ Australian mains voltage is 230 V. Household circuits use parallel wiring so appliances can operate independently.