In parallel, each appliance gets full voltage and independent control. What would happen with series wiring.

Why Do We Use Parallel Circuits in Houses? — Series vs Parallel Explained

Question

Why are household electrical appliances connected in parallel, not in series? What would happen if they were connected in series instead?

This is a classic NCERT Class 10 question, and CBSE loves asking it in 3-mark and 5-mark formats. The answer isn’t just “parallel is better” — we need to explain why using current and voltage arguments.

Solution — Step by Step

In a series circuit, the supply voltage gets divided among all components. If you have three bulbs in series on a 220V line, each bulb gets roughly 73V — not the 220V it was designed for.

In a parallel circuit, every branch connects directly across the supply. Each appliance sees the full 220V, regardless of how many other appliances are on.

Every appliance — your fan, your fridge, your TV — is rated for a specific voltage, typically 220V in India. The manufacturer designed it to operate at exactly that voltage.

If an appliance gets less voltage (as in series), it either works poorly or doesn’t work at all. A bulb rated 220V at 60W gives far less brightness at 73V.

In a series circuit, all appliances form one single loop. If one device fuses or is switched off, the loop breaks — current stops flowing everywhere. Every appliance in the house goes off.

In parallel, each appliance has its own independent path. Switching off the fan doesn’t affect the TV, because current has other routes available.

Adding more appliances in series increases total resistance: $R_{total} = R_1 + R_2 + R_3 + ...$

By Ohm’s law, higher resistance means lower current: $I = \frac{V}{R_{total}}$ . As you plug in more devices, every device gets even less current. Your house would literally get dimmer each time someone turns something on.

In parallel, total resistance decreases: $\frac{1}{R_{total}} = \frac{1}{R_1} + \frac{1}{R_2} + \frac{1}{R_3}$

Each branch draws its own current independently. A 1000W heater drawing $\approx 4.5$ A doesn’t affect the 60W bulb drawing $\approx 0.27$ A — they’re on separate branches. The supply simply provides more total current as needed.

Why This Works

The fundamental reason is that voltage is what appliances need, and parallel connections preserve voltage across every branch. Series connections distribute voltage; parallel connections distribute current paths.

Think of it this way: voltage is like pressure in a water pipe. In parallel, every tap connects directly to the main line — full pressure at every tap. In series, the pressure drops across each tap in sequence, so the last tap has barely any pressure left.

There’s also a safety and control argument. Parallel wiring lets us put a separate fuse and switch on each circuit. Your bedroom fan has its own switch; your kitchen light has its own. Independent control is only possible because each appliance is on its own parallel branch.

Alternative Method

The argument from power dissipation:

We know $P = \frac{V^2}{R}$ . For an appliance to deliver its rated power, it must receive its rated voltage.

In series, if we have $n$ identical appliances each with resistance $R$ , the voltage across each is $\frac{V_{supply}}{n}$ . Power delivered to each:

P_{series} = \frac{(V_{supply}/n)^2}{R} = \frac{V_{supply}^2}{n^2 R}

In parallel, each appliance receives full $V_{supply}$ :

P_{parallel} = \frac{V_{supply}^2}{R}

So $P_{parallel} = n^2 \times P_{series}$ . With just 3 appliances in series, each gets only $\frac{1}{9}$ th of its rated power. That’s why nothing works properly.

CBSE often asks: “Why does the brightness of a bulb decrease when more bulbs are added to a series circuit?” — this is exactly the same concept. More bulbs → more total resistance → less current → less power in each bulb. Use $P = I^2R$ to show it.

Common Mistake

Saying “parallel is used because resistance is lower.”

Lower total resistance is a consequence of parallel connection, not the reason we use it. The examiner wants: (1) each appliance gets full voltage, (2) each appliance can be independently switched, (3) failure of one doesn’t affect others.

If you write “resistance is lower in parallel,” that’s incomplete and loses marks. Start with the voltage argument — that’s what NCERT Chapter 12 emphasises.

Also watch out for this in numericals: some students apply $R_{total} = R_1 + R_2$ (series formula) when the circuit diagram clearly shows a parallel arrangement. Always identify the circuit type before writing any formula.

Question

Solution — Step by Step

Why This Works

Alternative Method

Common Mistake

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