Simple Circuits
AdvancedA source, wires, an appliance and a switch join into a closed loop; only then can current flow and the lamp light up. A break stops the light — check step by step to find the fault.
- Four elements of a circuit: source, wires, appliance, switch.
- Current flows from the positive terminal through the appliance back to the negative, round and round.
- A break anywhere makes an open circuit and the appliance stops at once.
文體小知識：說明文把電路的規則講得一清二楚。這一篇教你接駁簡單閉合電路，還教你在電燈不亮時逐步找出問題。
§Text Passage
1The lamps, fans and televisions at home all work by electric current passing through. Current is like a runner circling a track: there must be a start, a path and a finish joined into a loop before the running never stops. This complete closed pathway is called a circuit. If the circuit is broken, current cannot circulate and the appliance will not work.
2A simple circuit needs four elements: a power source to supply electricity, such as a battery; wires to carry the current, such as copper wire covered in plastic; an appliance to turn electric energy into light, sound or movement, such as a small bulb; and a switch to open and close the circuit. Joined end to end into a loop by wires, the circuit is closed.
3Current sets out from the battery's positive terminal, flows along the wire through the bulb, and returns to the negative terminal; only when this path is clear will the bulb light. A bulb has two contact points: the metal thread on its side and the metal dot at its base, and the current must pass in and out through the filament. Break anywhere — removing one wire, for instance — and the circuit becomes an open circuit and the lamp goes out at once.
4A switch is the gate of the circuit: press it and the gate opens, the circuit closes, and current flows freely; lift it and the gate shuts, the circuit breaks, and current stops. The light switch on the wall and the button on the TV are switches. With a switch we can control appliances at any moment without taking the circuit apart — saving power and convenient too.
5When a lamp will not light, investigate like a detective: is a battery reversed or flat? Are the wires tight? Is the bulb loose or burnt out? Is the switch on? Find the cause, fix it, and the circuit works again. Remember the safety rules: experiments use only dry cells, never wall sockets; never touch appliances or switches with wet hands — electrical safety always comes first.
1Summary 重點摘要
A source, wires, an appliance and a switch join into a closed loop; only then can current flow and the lamp light up. A break stops the light — check step by step to find the fault.
2Vocabulary cards Vocabulary
The complete closed pathway that current follows.
A circuit joined everywhere so current can circulate.
A circuit with a break somewhere, so current cannot pass.
The flow of electricity around a circuit that makes appliances work.
A power source with a positive and a negative terminal.
A line that carries current, usually copper inside and insulating plastic outside.
An appliance that turns electric energy into light, with two contact points.
A device that opens and closes the circuit.
A material that lets current pass, such as copper and iron.
A material that blocks current, such as plastic and rubber, used to protect us.
3Step by step Steps
- Step 1: Where electricity comes fromPress a torch and it lights; switch on a fan and it spins — current is at work behind them. A dry cell is a power source turning chemical energy into electrical energy: the end marked plus is the positive terminal, the end marked minus the negative. A battery gives current its push, like a pump pushing water; without a source, however well the wires are joined, the bulb stays dark. Wall sockets are power sources too, but their voltage is far higher — primary students must never touch them; experiments use only dry cells.
- Step 2: The four elements of a circuitA working simple circuit must have four elements. First, the power source, such as a battery, giving current its push. Second, the wires that carry the current — conductive copper inside, non-conductive plastic outside. Third, the appliance — a small bulb, a motor or a buzzer — turning electric energy into light, movement or sound. Fourth, the switch, controlling whether the circuit is open or closed. Joined end to end into a loop, all four are indispensable; think this through before assembling any circuit.
- Step 3: Closed circuit and open circuitCurrent is like a runner on a relay: out from the battery's positive terminal, along the wire into the bulb, through the filament and out, back along another wire to the negative terminal — then round again. Break the path anywhere, even loosening one clip, and the current stops at once: an open circuit. Only when the path is joined everywhere can current keep flowing: a closed circuit. So when a lamp will not light, the first question is whether the circuit is tightly joined everywhere into one complete loop.
- Step 4: How to connect a bulbA small bulb has two contact points: the metal thread around its side and the metal dot at its base, joined inside by a fine filament. To light, current must enter at one point, pass through the filament, and leave at the other; connect only one point and the circuit is unfinished — the bulb stays dark. Screw the bulb into a holder and lead two wires out of the holder, and both contact points are connected. Look at the base and side of a bulb holder: can you trace where each of its two wires goes?
- Step 5: What a switch doesA switch is the gate on the circuit. It has two contact points and a movable metal strip: pressed down, the strip bridges the points, the circuit closes and current flows; sprung up, the strip lifts away, the circuit opens and current stops. Place the switch between any two wires and it controls the whole circuit, because one break anywhere stops all the current. The light switch on the wall and the button on a fan work exactly like the small switch in the laboratory.
- Step 6: Checking a lamp that will not lightA dark lamp calls for detective-style checking, in order. First the power: is a battery reversed? Do the terminals line up with the markings? Is it flat? Try fresh cells. Second the wiring: is every clip gripping the metal, not the plastic? Is any wire broken? Third the bulb: loose? Screw it tighter. Burnt out? Test it on another working circuit. Fourth, is the switch pressed on? Checking each item in order, the fault soon shows itself.
- Step 7: Using electricity safelyElectricity serves us well, but misused it is dangerous. Remember the rules: experiments use dry cells only — never connect homemade circuits to wall sockets; never touch switches, plugs or appliances with wet hands, because water conducts; never use damaged wires, and tell an adult at once if metal core is exposed; do not cram one socket full of plugs, or it may overload; avoid using appliances in thunderstorms. With safe habits, current works obediently for us and harms no one.
5Worked example Example
Example: A science group connects a battery, wires and a bulb, but the bulb will not light. The leader finds three possible problems: one battery is inserted backwards; one crocodile clip grips only the plastic coating of a wire; the bulb is half a turn loose. Check in order and make the bulb light.
- First the power: take out the reversed battery and reinsert it as marked, positive to positive, and check it is not flat.
- Next the wiring: release the clip on the plastic coating and regrip the metal core of the wire so current can pass.
- Then the bulb: screw the half-loose bulb in gently until both contact points touch the holder well.
- Finally close the switch; current should flow from the positive terminal through the filament back to the negative, and the bulb should light. If it still does not, try a new bulb.
- Answer: Check in order: first correct the reversed battery, then move the clip from the plastic to the metal core, and finally tighten the bulb. With all three fixed, the circuit closes again, current flows, and the bulb lights. If it still does not, test the cells' power and the bulb's condition.
6Common mistakes Mistakes
7Quick check Quick check
Q1What are the four elements of a simple circuit?
Q2What is it called when a break stops the lamp from lighting?
Q3What does a switch do?
Q4How many contact points does a bulb have?
Q5What power source should circuit experiments use?
8Recap Recap
- Four elements of a circuit: source, wires, appliance, switch.
- Current flows from the positive terminal through the appliance back to the negative, round and round.
- A break anywhere makes an open circuit and the appliance stops at once.
- To check a circuit: battery, wiring, bulb, switch, one by one.
- Safety first: dry cells only for experiments; never touch switches with wet hands.