Simple Circuits, Symbols and Safety സെർക്കീട്ട് · സെർക്കീട്ട് ഡയഗ്രം
A torch, a doorbell, the LED at the science fair – each is a circuit (സെർക്കീട്ട്): a source, some components and wires joined in a closed loop. This page shows why the loop must be complete, what cells, switches, bulbs and LEDs do, how to draw any circuit quickly with the symbols of your textbook's Table 7.5, and how to build circuits correctly with an ammeter and a voltmeter. It ends with the most important lesson of the chapter: how to stay safe with electricity.
- Conductors and insulators (page 1).
- “A flow needs a difference – a cell keeps it going” (page 2).
1. A complete (closed) circuit
Charge can flow only round a complete loop of conductors, from one terminal (ടെർമിനൽ) of the cell, through the bulb, and back to the other terminal. This is a closed circuit. If there is a break anywhere – a switch turned off, a loose wire, a broken bulb filament – it is an open circuit and the current stops everywhere in the loop at once. A switch or key (സ്വിച്ച് / കീ) is simply a safe, easy way to make or break the loop.
Blue dots are free electrons (− to +); red arrows show the conventional current (+ to −). They move only when the loop is closed and there is a source. Notice that one break anywhere stops the flow everywhere.
2. The parts of a simple circuit
🔋 Cell and battery
A cell (സെൽ) has two terminals: the + terminal (the cap of a torch cell) and the − terminal (the flat base). A torch cell gives 1.5 V. A battery (ബാറ്ററി) is two or more cells joined to work as one source – a 9 V battery has six 1.5 V cells inside.
🔘 Switch or key
Closes (ON) or opens (OFF) the circuit. In the lab a plug key is often used: pushing the brass plug in closes the gap.
💡 Bulb
A thin coiled wire, the filament, gets white hot and glows when current flows through it. If the filament breaks (the bulb “fuses”), the circuit is open.
🔴 LED
A light-emitting diode lets current through in one direction only. Its longer leg must go towards the cell's + side. It needs a few volts and only a small current, so a small resistor is usually put in series with it – exactly the “extra component” the girl noticed at the science fair.
Sample model of a red LED: it starts to conduct at about 2 V and is happiest at about 20 mA (0.02 A). With a 3 V battery, the resistor takes the extra 1 V: I = 1 V ÷ 47 Ω ≈ 0.021 A. Without a resistor the current would be far too large and the LED could be damaged. (Real LEDs differ; a white LED needs about 3 V.)
3. Circuit symbols – Table 7.5
Drawing pictures of cells and bulbs is slow. Scientists everywhere use the same simple symbols, so a circuit diagram (സെർക്കീട്ട് ഡയഗ്രം) can be drawn in a minute and read by anyone. These are the symbols of your textbook's Table 7.5 (page 142), plus three you will meet later in the chapter:
| Component | Symbol | Remember |
|---|---|---|
| Cell സെൽ | Long thin line = +; short thick line = −. | |
| Battery ബാറ്ററി | Two or more cells joined + to −. | |
| Switch / key – OFF സ്വിച്ച് / കീ | A gap: open circuit. (Second form: plug key without its plug.) | |
| Switch / key – ON സ്വിച്ച് / കീ ഓൺ | The gap is closed: closed circuit. (Plug key with the plug in – the dot.) | |
| Bulb / lamp ബൾബ് / ലാമ്പ് | Either form is correct; this guide mostly uses the circle with a cross. | |
| LED എൽ.ഇ.ഡി | Triangle points the way the current can go; arrows = light given out. | |
| Voltmeter വോൾട്ട്മീറ്റർ | Measures potential difference; connected across (in parallel with) a component. | |
| Ammeter അമ്മീറ്റർ | Measures current; connected in line (in series). | |
| Resistor പ്രതിരോധകം / റെസിസ്റ്റർ page 9 | Zig-zag or rectangle (textbook page 146). | |
| Rheostat റിയോസ്റ്റാറ്റ് page 9 | A resistor with an arrow: its resistance can be changed. |
Twelve cards in a fixed order. Your score is counted – try to get them all right before moving on.
4. From a picture to a circuit diagram
Your textbook's Fig 7.19 shows a photo of two cells, a bulb and a switch (a), and its circuit diagram (b). The rules for drawing a diagram: use the standard symbols, draw wires as straight lines with square corners, keep the components in the same order round the loop as in the real circuit, and mark + and − where it matters.
5. Build a circuit with meters
An ammeter (അമ്മീറ്റർ) measures current, so the current must pass through it: connect it in series. It has almost no resistance. A voltmeter (വോൾട്ട്മീറ്റർ) measures the potential difference between two points, so connect it across the component, in parallel. It has a very large resistance and takes almost no current. Connect each meter's + terminal towards the cell's + terminal. Get the meters the wrong way round in the circuit and strange things happen – try it below.
Model: ideal cells (1.5 V each), bulb = 6 Ω (made for 3 V), resistor = 12 Ω, ideal ammeter (0 Ω), ideal voltmeter (very large resistance), red LED (about 2 V). Things to try: (1) the default circuit; (2) put the voltmeter in a slot (in series) – the bulb goes out; (3) put the ammeter across the bulb – a short circuit; (4) two batteries the same way, then one reversed; (5) open the switch.
Worked example 1 · A three-cell torch
Q. A long torch holds three 1.5 V cells one behind the other, the + of each touching the − of the next (in series). What total voltage do they give?
Cells joined + to − add their voltages (page 6 explains why). Multiply the voltage of one cell by the number of cells.
Worked example 2 · How many cells for the TV remote?
Q. A TV remote needs 3 V. How many 1.5 V cells must be put in it, and how should they be joined?
Number of cells = voltage needed ÷ voltage of one cell. To add up, they must be joined + to − (in series).
6. Electrical safety at home
Everything in this chapter is done with torch cells or a 9 V battery, which are safe. The supply from KSEB to your house is very different: it is 230 V AC (alternating current), about 150 times the voltage of a torch cell, and a current through the body at that voltage can stop the heart. Your textbook says it plainly (page 153): do not do these experiments with household electricity.
✋ Hands and water
Never touch switches, plugs or appliances with wet hands or while standing barefoot on a wet floor. Water with dissolved salts conducts. Never pour water on an electrical fire – switch off the mains first.
🔌 Plugs and sockets
Never poke anything into a socket. Pull a plug out by the plug, not the wire. Don't overload one socket with many adapters. Replace cracked plugs and wires with peeling insulation – only an electrician should do the repair.
🌧️ Monsoon lines
After a storm, lines may fall on roads, fences, trees and waterlogged ground. Keep far away from any fallen wire or anything touching it, keep others away, and inform KSEB at once. Never climb trees near lines or fly kites near them.
🩹 If someone gets a shock
Do not touch the person directly. Switch off the mains at once, or push them away from the wire with a dry wooden stick or plastic chair (insulators). Then call for help.
Find the main board near your front door (ask an elder to show you; do not open it). You will see the KSEB energy meter, a main switch, and a row of small switches called MCBs (miniature circuit breakers) that switch off a circuit by themselves if too much current flows. Many houses also have an ELCB or RCCB. Your textbook asks you to discuss with elders the precautions for handling household electricity – this board is a good place to start.
In a healthy circuit, the current going out along one wire comes back along the other – the two are equal. If some current leaks away through a faulty appliance or a person's body to the earth, the two are no longer equal. The RCCB senses this small difference (in homes, usually just 30 mA) and switches the supply off within a fraction of a second. It is a life-saving device – but it is not a reason to take risks.
7. Check yourself
Next: The chapter begins – charge flow and potential difference ▶