Electricity and Magnetism
Section: Physics | Syllabus: Cambridge Primary Science (0846)
Circuit Symbols
- Scientists use standard symbols to draw circuits
Standard symbols used to draw circuit diagrams
- Cell: two lines (one long, one short) - a single cell
- a battery is two or more cells joined together, drawn as two or more cell symbols in a row
- Switch: can be open (drawn with a gap, so the circuit is broken and no current flows) or closed (drawn as a complete connection, so current can flow)
- Lamp/Bulb: a circle with an 'X' inside
- Wire: straight lines
- Buzzer: semi-circle on two legs
Conductors, Insulators and Circuit Safety
- To test if a material is an electrical conductor, connect it into a gap in a circuit that has a battery and a lamp (or buzzer)
- if the lamp lights up (or the buzzer sounds), the material is a conductor
- if nothing happens, the material is an insulator
- Conductors: metals like iron and copper
- Insulators: materials like plastic and glass
The Water-Circuit Analogy
- A circuit can be compared to a system of water pipes:
- the cell/battery is like a pump, pushing the current around
- the wires are like pipes carrying the current
- the switch is like a tap, which can block or allow flow
- a lamp is like a narrow pipe, which resists the flow
Circuit Safety
- Lamps can get hot when a circuit is switched on for a while - take care not to touch them, as they can burn your skin
Series Circuit
- A series circuit is one where the electric current has only one path to follow - one single loop
- all components are connected one after another in the same loop
In a series circuit, current has only one path to follow
- One path: the current flows through every component in turn
- Dimmer bulbs: if you add more bulbs, they all get dimmer, because they share the energy from the battery
- One fails, all fail: if one lamp stops working, the circuit is broken and none of the other lamps will light up
- Real-life example: strings of festival/party lights are often wired in series - when one bulb blows, the whole string goes out, and you have to check every single bulb to find the faulty one
Parallel Circuits
- A parallel circuit gives each component its own separate loop back to the battery
- the current has multiple paths to choose from
In a parallel circuit, each component has its own branch back to the battery
- Multiple paths: each lamp (or component) has its own branch of the circuit
- Full voltage: each pathway in a parallel circuit receives the full circuit voltage, so bulbs stay bright even when more are added
- One fails, rest stay on: if one lamp burns out, the other lamps remain alight, because their loops are still complete
- Real-life example: the lights in your home are wired in parallel - switching off one light does not turn off all the others
Worked Example: Comparing Series and Parallel Circuits
- Question: both circuits have identical components. Complete the sentences: the parallel circuit is electrical circuit............, the circuit with the brightest lamps is electrical circuit............ [1 mark]
- Answer: B; B [1]
Circuits with Multiple Switches
- A circuit can combine series and parallel sections, with different switches controlling different lamps
- to work out which lamps light up, trace the path from the battery through each closed switch - a lamp only lights if there is a complete, unbroken path to it
- a switch on its own branch only controls the lamp(s) on that branch; a switch on the main loop can control the whole circuit
- Some circuits are wired so that closing one switch changes the circuit from series to parallel (or the other way round), by opening up a new path for the current
Sharing Brightness with a Connector
- If a wire connector is added that lets current skip past one or more lamps (a short circuit around them), those lamps go out, and the remaining lamps share the circuit's brightness equally
- each remaining lamp gets 100% ÷ (number of lamps still in the circuit) of the brightness - e.g. if 2 lamps remain out of an original 4, each gets 100 ÷ 2 = 50% brightness
Magnets and Magnetism
- A magnet is an object that attracts certain metals and has two ends called poles: a North pole and a South pole
- Magnetic materials: iron, steel and nickel are attracted to magnets
- most other materials (like plastic, wood, and most metals such as copper and aluminium) are not magnetic
Attracting and Repelling
- Like poles repel: two North poles (or two South poles) push each other away
- Unlike poles attract: a North pole and a South pole pull towards each other
Like poles repel; unlike poles attract
Uses of Magnets
- A compass contains a small magnetised needle that always points towards the Earth's magnetic North pole, helping with navigation
- Magnets are also used in fridge door seals, can openers, and sorting machines that separate magnetic metals from waste
Common Mistakes
- ❌ Thinking all metals are magnetic
- ✓ only some metals - mainly iron, steel and nickel - are attracted to magnets; metals like copper, aluminium and gold are not magnetic
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