Electric Circuits and Conduction
FoundationA complete circuit needs a power source, wires, a switch and an appliance; series circuits have one path and parallel circuits have many; metals conduct and plastics insulate; never touch appliances with wet hands — stay safe with electricity.
- A complete circuit has a power source, wires, a switch and an appliance — all needed.
- Series circuits have one path; parallel circuits have branches; homes are wired in parallel.
- Metals conduct; plastic, rubber and glass insulate.
文體小知識：電學實驗說明文帶我們接駁電路、測試導電，一步一步認識電的世界和安全用電。
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1Lights shine and fans turn only with a complete circuit. A circuit has four basic parts: a power source such as a battery supplies the energy; wires let the current flow; a switch turns the current on and off; and appliances such as bulbs or motors change the energy into light or motion. If any part is missing or broken, no current flows and the bulb stays dark.
2There are two ways to connect appliances. In series they are joined one after another with a single path for current: take out one bulb and all go out. In parallel each appliance sits on its own branch: one bulb goes out and the rest stay lit. Home appliances are wired in parallel, so the lights stay on when the television is switched off.
3Not every material conducts. Materials that let current pass easily are conductors: metals such as copper, iron and aluminium are excellent, which is why wires are made of copper. Materials that hardly conduct are insulators: plastic, rubber, glass and dry wood. Wires are sheathed in plastic and sockets have plastic cases. A simple circuit can test it: put an object in the circuit — if the bulb lights, it is a conductor; if not, an insulator.
4Electrical safety must be remembered: never touch switches or appliances with wet hands, for water makes skin conductive; never push metal objects into sockets; switch appliances off when not in use and unplug those idle for long; ask an adult to replace damaged wires at once; never shelter under a tree in a thunderstorm. Electricity serves us only when handled with care.
1Summary 重點摘要
A complete circuit needs a power source, wires, a switch and an appliance; series circuits have one path and parallel circuits have many; metals conduct and plastics insulate; never touch appliances with wet hands — stay safe with electricity.
2Vocabulary cards Vocabulary
The complete, closed path that electric current follows.
The part that supplies electrical energy, such as a battery.
The device that turns the current on and off.
The flow of electric charge around a circuit.
Appliances joined one after another with a single path for current.
Appliances on separate branches, each working independently.
A material that lets current pass easily, such as copper and iron.
A material that hardly lets current pass, such as plastic and glass.
Pulling the plug from the socket to cut the power completely.
3Step by step Steps
- Step 1: Learn the four parts of a circuitA complete circuit needs four parts: a power source, wires, a switch and an appliance. For example, join a battery, a switch and a bulb in a loop with two wires; close the switch and the bulb lights — the simplest complete circuit. The battery gives energy, the wires let the current flow, the switch turns the circuit on and off, and the bulb changes energy into light. All four are needed; without any one of them, the circuit cannot work.
- Step 2: Check whether the circuit is closedA circuit must be closed before the current can flow and the bulb can light. For example, when a bulb stays dark, check in turn whether the battery is flat, whether a wire has slipped off, whether the contact is good, and whether the bulb has burnt out, and find the break. After fixing it, close the switch and test again — the light means success. This orderly way of checking, ruling out one cause at a time, is an important way of thinking for solving problems, useful in other learning too.
- Step 3: Build a series circuitA series circuit joins appliances end to end, giving the current only one path. For example, join two bulbs end to end to a battery; both light but dimmer than a single bulb, because the energy is shared between them, and unscrewing one puts out the other, because the circuit is broken and the current has nowhere to go. Series circuits are simple to wire, but when one part fails, every appliance on the circuit is affected.
- Step 4: Build a parallel circuitA parallel circuit gives each appliance its own branch, so the current has several paths. For example, give each bulb its own branch back to the battery; both shine more brightly than in series, and the light is more even too, and unscrewing one leaves the other lit, because the other path is still closed. Homes wire appliances in parallel exactly so that each can be switched on and off on its own, without interfering with the others — convenient and safe.
- Step 5: Test conductors and insulatorsMaterials that let the current pass easily are conductors, and those that do not are insulators; a simple circuit can test them. For example, put a copper key into the circuit and the bulb lights — copper is a conductor; swap in a plastic ruler and the bulb stays dark — plastic is an insulator. Most metals are conductors and most plastics and rubbers are insulators, so wires have a metal core to carry the current and a plastic coat outside to prevent shocks.
- Step 6: Explain the circuits at homeThe circuits at home use the principles we have learned. For example, lights and appliances at home are all in parallel, switching on and off independently without affecting one another; wires are sheathed in plastic, an insulator against shocks; the switch on the wall is in series with the lamp, so one press cuts the current and the lamp goes out. Knowing the principles behind these designs helps us treasure electricity, and shows that every design puts safety first.
- Step 7: Practise electrical safetyElectricity brings convenience, but careless use causes shocks or fires, so safety must be learned young. For example, dry your hands before touching switches after washing; switch off and unplug the fan when it is not needed; report any cracked wire or hot socket to an adult at once. Remember: never touch appliances with wet hands, never overload the sockets, and switch off what is not in use — be a little master of electrical safety.
4Diagrams Diagrams
5Worked example Example
Example: Kit wires two bulbs in series; they glow dimly, and when he unscrews one bulb the other goes out too. Explain the phenomena, and suggest a way to let the two bulbs light independently.
- A series circuit has only one path for the current; unscrewing a bulb breaks the circuit, no current flows, and so the other bulb goes out too.
- In series the two bulbs share the energy, so each receives less than a single bulb and glows dimmer.
- The fix: rewire them in parallel, giving each bulb its own branch path back to the battery.
- The effect: each bulb receives enough energy and shines more brightly; if one fails, the other stays lit.
- Answer: In series there is a single path, so one break darkens both, and the shared energy makes them dim. In parallel each bulb has its own path: brighter, independent, unaffected by the other — just how household circuits are wired.
6Common mistakes Mistakes
7Quick check Quick check
Q1What are the four parts of a complete circuit?
Q2How do series and parallel circuits differ?
Q3Why are wires copper inside and plastic outside?
Q4How do we test whether an object is a conductor or an insulator?
Q5How are home appliances connected, and why?
8Recap Recap
- A complete circuit has a power source, wires, a switch and an appliance — all needed.
- Series circuits have one path; parallel circuits have branches; homes are wired in parallel.
- Metals conduct; plastic, rubber and glass insulate.
- A test circuit shows conduction: bulb lit means conductor, dark means insulator.
- Stay safe: dry hands only, no metal in sockets, and replace damaged wires at once.