Chapter 7 · ApplicationsElectricity at home in KeralaExplore online

Applications & Explore Online ധാരാവൈദ്യുതി

The wiring in your home, the regulator of the ceiling fan, the inverter battery that keeps the lights on during a power cut, the two-wheeler battery, LED torches and the serial lights of Christmas and Onam, the iron box, and the multimeter an electrician carries – every one of them works by the ideas of this chapter. Each lab below works out its numbers with V = IR, the series and parallel rules and the rules for combining cells.

📖 The ideas behind every lab

Models used: meters are ideal (ammeter ≈ 0 Ω, voltmeter ≈ ∞ Ω); bulbs and appliances are treated as fixed resistances. Values marked “approx.” are typical real values and values marked “example” are made-up but sensible numbers – neither comes from the textbook.

⚠ Never experiment with the mains supply (230 V AC) at home – it can kill. Use only torch cells or a 9 V battery.

1. House wiring – everything in parallel

The KSEB supply reaches your home through the energy meter as alternating current (AC) at 230 V. From the main switch board two wires – the live (phase) wire and the neutral wire – run to every room, and each appliance is connected between them, in parallel (സമാന്തരരീതി). So every appliance gets the full 230 V, each can be switched on and off by itself, and one appliance failing does not stop the others (page 153 of the textbook). Each switch is in series with its own appliance, in the live wire. The currents of all the branches add up in the main wire, and a fuse or an MCB (miniature circuit breaker) in series with the main wire breaks the circuit if the total current becomes too large – an overload, or a short circuit (ഷോർട്ട് സെർക്കീട്ട്) where the live and neutral wires touch.

🏠 Wiring lab: switch the appliances on and off

Model: each appliance is a fixed resistance (example values): bulb 920 Ω, fan 575 Ω, TV 460 Ω, iron box 46 Ω. Real fans and TVs are not simple resistors, and a real MCB trips after a short delay on a small overload but at once on a short circuit. A 6 A MCB is typical for a lighting circuit (approx.). The arrows show the current at one instant – in AC its direction reverses many times every second.

⚠ Never experiment with the mains supply (230 V AC) at home – it can kill. Use only torch cells or a 9 V battery.

📝 In the exam this looks like: “In which way are bulbs and fans connected in household circuits? What are the advantages?” → in parallel; each gets the full 230 V, each can be switched on or off independently, and if one fails the others keep working. “A bulb of resistance 920 Ω works on 230 V. Find the current.” → I = V/R = 230 ÷ 920 = 0.25 A. “Why does the effective resistance of the house decrease when more appliances are switched on?” → more parallel paths: 1/R = 1/R1 + 1/R2 + … grows, so R falls and the total current rises.

2. Fan regulators and dimmers – a resistance in series

Let us assess Q8 asks how a fan and its regulator are connected: in series. The same current flows through both, so the regulator can control it. The old box-type regulators were simply a variable resistance – a rheostat (റിയോസ്റ്റാറ്റ്) with a few fixed steps. Turning the knob to a lower speed puts more resistance in series: the total resistance rises, the current falls (I = V/R), and the potential difference divides – more of the 230 V across the regulator, less across the fan. A dimmer for a lamp works the same way. You can safely build the second circuit below with four torch cells, a small torch bulb and a rheostat.

🌀 Regulator lab: turn the knob

Model (example): the fan is treated as a fixed 460 Ω resistance and the regulator steps are 690, 460, 230, 115 and 0 Ω; the torch bulb is 6 Ω and the rheostat steps are 9, 6, 3, 1.5 and 0 Ω. A fan motor is not really a fixed resistance and modern regulators are electronic, so real numbers differ.

📝 In the exam this looks like: Let us assess Q8 – “How are a fan and its regulator connected?” → in series, because the regulator must change the current through the fan. “What is the principle of a rheostat?” → resistance increases with length, so changing the length of wire in the circuit changes the resistance and the current. A numerical: “A 6 Ω bulb and a rheostat set at 6 Ω are in series with a 6 V battery. Find the current.” → R = 6 + 6 = 12 Ω, I = 6 ÷ 12 = 0.5 A; the bulb gets 0.5 × 6 = 3 V.

3. Inverter, UPS and vehicle batteries – cells in series and parallel

A battery (ബാറ്ററി) is several cells joined to work as one source. The 12 V battery of a car, a two-wheeler or a home inverter is a lead-acid storage battery: six cells of about 2 V each in series, 6 × 2 V = 12 V (approx.). These are secondary cells (സെക്കൻഡറി സെൽ) – they can be recharged. A bigger inverter or a UPS may join two 12 V batteries in series for 24 V. To make a source last longer without changing its voltage, identical cells are joined in parallel – that is how a power bank or a laptop battery joins its lithium-ion cells (key box 9).

🔋 Battery lab: build a battery from cells

Model: identical, ideal cells; “lasts k times as long” assumes the same load and that every row shares the current equally. Cell voltages of lead-acid and lithium-ion cells are typical values (approx.).

⚠ Never short-circuit a cell or battery; never put cells in your mouth; stop if a battery gets hot; keep water and fire away; dispose of batteries (nickel, cadmium, lead) at e-waste collection points.

📝 In the exam this looks like: “A 12 V battery is made of six identical cells in series. Find the emf of one cell.” → 12 ÷ 6 = 2 V (the textbook’s 9 V battery of six cells: 9 ÷ 6 = 1.5 V). “Four 1.5 V cells – how will you get the maximum potential difference?” → all four in series: 4 × 1.5 = 6 V. “Why are cells connected in parallel in a power bank?” → the emf stays that of one cell, but the current is shared, so it lasts longer. “Two cells joined + to + give?” → 0 V – never do this.

4. LED torches and serial lights – the series resistor

The chapter opens at a school science fair (ശാസ്ത്രോത്സവം) with an LED lit by a battery – and a small component beside it. That component is a resistor (പ്രതിരോധകം) in series with the LED (page 153). An LED needs only a small potential difference (about 2 V for a red LED, about 3 V for a white one, approx.) and a small current (about 10–20 mA, approx.). The resistor takes the rest of the battery’s potential difference and limits the current: R = (Vsupply − VLED) ÷ I. This is Ohm’s law for the resistor alone. Decoration strings (serial lights) put many LEDs in series – so when one breaks, the whole string goes dark.

💡 LED lab: choose the resistor

Model: an LED that is lit keeps an almost fixed potential difference across it (approx. values above); below that it lets almost no current through. Resistors are sold in standard values (10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82 × a power of ten), so we choose the next bigger one – the current then comes out a little smaller, which is safe. An LED also has polarity: its long leg goes to the + side.

📝 In the exam this looks like: Fig 7.33 – a bulb that works at 3 V with a resistance of 6 Ω on a 9 V battery: with a 12 Ω resistor in series, R = 18 Ω, I = 9 ÷ 18 = 0.5 A and the bulb gets 0.5 × 6 = 3 V; without it the bulb gets 9 V and fuses. “Why was a resistor connected with the LED at the science fair?” → to limit the current (it takes the extra potential difference). Extended activity 2 – three 3 V LEDs on a 9 V battery → connect them in series (3 + 3 + 3 = 9 V). “Why do all the lights of a serial set go off when one fails?” → they are in series: one break opens the only path.

5. Heaters and iron boxes – the nichrome coil

An iron box, a water heater, a toaster and an electric kettle all heat with a coil of nichrome (നിക്രോം) – a metal with a high resistance. The electrons collide with the atoms of the wire again and again; those collisions are what we call resistance, and they make the wire hot. How much resistance the coil has depends on what the textbook lists: the material, the length (longer → more resistance) and the area of cross-section (ഛേദതല പരപ്പളവ് (വണ്ണം)) (thicker → less resistance), and also the temperature – a hot coil has more resistance than a cold one (page 9). On the fixed 230 V of the mains, a coil with less resistance draws more current and heats more strongly.

🔥 Heater lab: design the coil
🔭 Go further (not in the textbook)

The three factors combine into one formula, R = ρL/A, where ρ (rho, the resistivity) depends only on the material: about 1.1 × 10−6 Ω m for nichrome and 1.7 × 10−8 Ω m for copper (approx.) – nichrome resists about 65 times as much. The heat produced each second is the power P = V × I. Nichrome is also chosen because it does not burn away (oxidise) easily when red-hot and melts only at a very high temperature (approx. 1400 °C).

⚠ Never touch the coil or the sole plate of an iron box, never open a heater, and always switch off and unplug before cleaning – ask an adult.

📝 In the exam this looks like: Let us assess Q10 – “A 50 cm wire of 5 Ω is stretched to double its length. What happens to its area of cross-section and its resistance?” → the area becomes half; the resistance becomes 5 × 2 × 2 = 20 Ω. Table 7.9 – “Through which wire is the current least?” → the longest, thinnest nichrome wire (D): the largest resistance. “When is the resistance of a heater coil greater – hot or cold?” → hot. “Why is copper not used for heating coils?” → its resistance is very low.

6. Using a multimeter – V, A and Ω

An electrician’s multimeter (മൾട്ടിമീറ്റർ) is a voltmeter, an ammeter and a resistance meter in one box (Fig 7.24) – a knob chooses the mode. The rules are the ones of Table 7.6: as a voltmeter (വോൾട്ട്മീറ്റർ) it is connected in parallel (across) the component; as an ammeter (അമ്മീറ്റർ) the circuit is broken and the meter is put in series; and resistance is measured across a component only with the power switched off. The red probe is +: it goes to the side nearer the + terminal of the cell.

🧰 Multimeter lab: choose the mode and where to connect it

Model: ideal meters and ideal cells (3 V = two 1.5 V cells); R1 = 10 Ω and R2 = 20 Ω (example). A real meter on the 200 mA range has a small fuse inside that blows on a short circuit.

📝 In the exam this looks like: Table 7.6 – voltmeter: symbol Ⓥ, measures potential difference, connected in parallel; ammeter: symbol Ⓐ, measures current, connected in series; + terminal towards the + of the cell. Let us assess Q3 (Fig 7.34) – pick the circuit in which both meters are placed correctly and the right way round. A numerical: “10 Ω and 20 Ω in series on 3 V – what do the ammeter and a voltmeter across 10 Ω read?” → I = 3 ÷ 30 = 0.1 A, V = 0.1 × 10 = 1 V.

✅ Check yourself

🌐 Explore online – resources hub

Everything in this guide works offline. The links below are extras – websites and simulations from trusted sources (PhET and others), grouped by topic. They need internet and open in a new tab, so you will not lose your place here. Ask a parent or teacher before you go online. The PhET Circuit Construction Kit named in the textbook runs directly in your browser – you can build every circuit of this chapter with batteries, bulbs, resistors and meters, with nothing to install and no danger.

Electric charge and electrons

🌐 Electric charge and electrons needs internet · opens in a new tab

Potential difference and emf

🌐 Potential difference and emf needs internet · opens in a new tab

Cells, batteries and their safe disposal

🌐 Cells, batteries and their safe disposal needs internet · opens in a new tab

Electric current and ammeters

🌐 Electric current and ammeters needs internet · opens in a new tab

Circuit diagrams and circuit builders

🌐 Circuit diagrams and circuit builders needs internet · opens in a new tab

Ohm's law

🌐 Ohm's law needs internet · opens in a new tab

Resistance, resistors and rheostats

🌐 Resistance, resistors and rheostats needs internet · opens in a new tab

Series and parallel circuits

🌐 Series and parallel circuits needs internet · opens in a new tab

Electrical safety at home

🌐 Electrical safety at home needs internet · opens in a new tab

Volta, Ampère and Ohm

🌐 Volta, Ampère and Ohm needs internet · opens in a new tab

More to explore

🌐 More to explore needs internet · opens in a new tab
🧭 Next

Now pull the whole chapter together: Summary & Quick Quiz ▶