---
title: "Electric Circuits and Safety"
book: "Primary & Middle School Physics"
subject: physics
language: en
chapter: 41
exercises: 12
source: https://one-course.com/books/physics/1/en/chapter/41-electric-circuits-and-safety
---

# Chapter 41 — Electric Circuits and Safety

Three years of loops, [bulbs](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-bulb) and [branches](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel) have made you a builder of circuits. This year you become something rarer: a *writer* of circuits. Electricians in every country read and draw the same wordless language — and its grammar, plus a chapter of hard-won safety wisdom, is what stands between clever wiring and burnt fingers.

## 41.1 The wordless language

**Definition 41.1 (Circuit diagram).**

A *circuit diagram* is the standard drawing of a circuit: each device is shown by its agreed *symbol*, and the wires by straight lines drawn with square corners. A diagram shows how the circuit is *connected* — who feeds whom, what forks where — and deliberately ignores how the real wires snake across the table.

![The alphabet: six symbols carry almost every circuit of this book. The same marks are read from workshop to workshop around the world.](https://one-course.com/images/onecourse/chapters/physics-1/g6-circuits-and-safety/fig-c9961afab208.svg)

*The alphabet: six [symbols](#def-g6-circuits-and-safety-diagram) carry almost every circuit of this book. The same marks are read from workshop to workshop around the world.*

**Method 41.2 (From table to paper, and back).**

To *draw* a built circuit:

1. list the devices, and pick each one’s [symbol](#def-g6-circuits-and-safety-diagram) ;
2. follow the real loop with a finger from the [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery) ’s $+$ , noting the order of devices and every fork;
3. redraw as a tidy rectangle: [symbols](#def-g6-circuits-and-safety-diagram) spaced out, wires straight, corners square — same connections, no snakes.

To *build* from a diagram, run the translation backward — and afterward, check as always: every loop traced from $+$ must reach $-$ through at least one device.

**Example 41.3 (Same circuit, two portraits).**

On the table: a [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery), a tangle of alligator-clip wires looping over a pencil case, a lamp leaning on a book, a [switch](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#ex-g3-first-electric-circuit-switch) dangling. On paper: a calm rectangle — [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery) on the left, switch on top, lamp on the right. The tangle and the rectangle are the *same circuit*: every connection identical. To decide whether two circuits are the same, ask one question only: who is connected to whom?

**Example 41.4 (Motors and buzzers join the game).**

Two newcomers to your kit behave just like lamps in a diagram: the *motor* ([symbol](#def-g6-circuits-and-safety-diagram): M in a circle) spins whenever current flows through it — fans, toy cars, drills; the *buzzer* (a struck circle) [sounds](https://one-course.com/books/physics/1/en/chapter/19-sound-around-us#def-g3-sound-around-us-sound) whenever fed — doorbells, alarms. Series and parallel rules apply to them unchanged: a [switch](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#ex-g3-first-electric-circuit-switch) [in series](https://one-course.com/books/physics/1/en/chapter/23-series-circuits#def-g4-series-circuits-series) commands them; parallel [branches](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel) make them independent.

## 41.2 Old rules in the new language

**Example 41.5 (Reading a diagram like a pro).**

Given any diagram, interrogate it with the finger-walk of the parallel-circuits chapter: from $+$, walk every road to $-$. All devices on one road: series — one gap stops all, order free, brightness shared. Roads that fork at [junction](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel) dots: parallel — [branches](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel) independent, full brightness each, [switch](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#ex-g3-first-electric-circuit-switch) placement now a real decision. The language changed; the laws did not.

![Read it before building it: one switch on the common road, then a fork — lamp on one branch, fan motor on the other. The switch commands both; the branches ignore each other’s troubles.](https://one-course.com/images/onecourse/chapters/physics-1/g6-circuits-and-safety/fig-950ccb3e9d23.svg)

*Read it before building it: one switch on the common road, then a fork — lamp on one [branch](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel), fan [motor](#ex-g6-circuits-and-safety-motor) on the other. The [switch](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#ex-g3-first-electric-circuit-switch) commands both; the [branches](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel) ignore each other’s troubles.*

## 41.3 The chapter of scars

Electricity’s safety rules were paid for in fires and worse. Here they are, with their reasons — a professional knows both.

**Definition 41.6 (Short circuit).**

A *short circuit* happens when a bare conducting road connects a [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery)’s (or socket’s) two sides *directly*, skipping every device. With nothing useful in its way, the current rushes wildly — wires [heat](https://one-course.com/books/physics/1/en/chapter/34-heat-and-insulation#def-g5-heat-and-insulation-heat), insulation [melts](https://one-course.com/books/physics/1/en/chapter/9-ice-water-steam#def-g2-ice-water-steam-meltfreeze), batteries are ruined, and mains wiring can catch fire. Next year’s circuits chapter studies the villain closely; this year, learn its face and keep it out of your builds.

**Remark 41.7 (The house’s bodyguards).**

Look inside the fuse box at home: rows of small [switches](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#ex-g3-first-electric-circuit-switch) — *circuit breakers* — one per [branch](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel) of the house’s great [parallel circuit](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel). Each stands guard over its [branch](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel): if a fault lets the current rush — a [short circuit](#def-g6-circuits-and-safety-short), an overloaded socket — the guard snaps itself off in a blink, cutting the [branch](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel) before wires overheat. When a breaker “trips”, it is not misbehaving: it has just done its one job. Find the fault before resetting it.

**Remark 41.8 (The rules, with reasons).**

1. *[Battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery) experiments only; the socket, never.* The mains pushes hundreds of times harder — through damp skin it can stop a heart.
2. *No bare wire straight across a [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery).* That is a [short circuit](#def-g6-circuits-and-safety-short) : ruined cells, hot wire, burnt fingers.
3. *Water and mains electricity never share a room’s reach:* no gadgets by the bath, no wet hands on plugs — damp skin is a [conductor](https://one-course.com/books/physics/1/en/chapter/22-conductors-and-insulators#def-g4-conductors-insulators-conductor) .
4. *Never poke anything into a socket.* The guard in the fuse box is fast, but you must never bet on it.
5. *Damaged cords go to the bin,* not the drawer: cracked insulation is a fence with a gap.

## 41.4 Exercises

**Exercise 41.1 ★.**

What does a [circuit diagram](#def-g6-circuits-and-safety-diagram) show faithfully, and what does it deliberately ignore?

**Solution of Exercise 41.1.**

Faithfully: the connections — which device joins which, where roads fork. Ignored: the real wires’ lengths, colors and snaking paths across the table.

**Exercise 41.2 ★.**

Draw from memory the [symbols](#def-g6-circuits-and-safety-diagram): [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery); lamp; open [switch](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#ex-g3-first-electric-circuit-switch); closed [switch](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#ex-g3-first-electric-circuit-switch); [motor](#ex-g6-circuits-and-safety-motor); [buzzer](#ex-g6-circuits-and-safety-motor). Which mark tells the [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery)’s $+$ side?

**Solution of Exercise 41.2.**

[Battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery): two unequal parallel bars; lamp: circle with a cross; open [switch](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#ex-g3-first-electric-circuit-switch): a lifted drawbridge; closed [switch](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#ex-g3-first-electric-circuit-switch): the bridge laid flat; [motor](#ex-g6-circuits-and-safety-motor): M in a circle; [buzzer](#ex-g6-circuits-and-safety-motor): a struck circle. The *long* bar marks the [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery)’s $+$.

**Exercise 41.3 ★.**

Draw the diagram of a doorbell: [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery), [push-switch](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#ex-g3-first-electric-circuit-switch) and [buzzer](#ex-g6-circuits-and-safety-motor) in one loop. Why does the bell stop when the finger lifts?

**Solution of Exercise 41.3.**

One rectangle: [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery), [push-switch](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#ex-g3-first-electric-circuit-switch) and [buzzer](#ex-g6-circuits-and-safety-motor) in a single loop. Lifting the finger opens the [switch](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#ex-g3-first-electric-circuit-switch) — a gap in the only road, and the loop law silences the [buzzer](#ex-g6-circuits-and-safety-motor) instantly.

**Exercise 41.4 ★.**

Two circuits are built from the same box of parts: are they “the same circuit”? What single question decides?

**Solution of Exercise 41.4.**

Not necessarily. The deciding question: who is connected to whom? Same devices with the same connections — same circuit, however different the tangles look.

**Exercise 41.5 ★.**

In the chapter’s lamp-and-motor diagram, what happens to the fan if the lamp burns out? And to both if the [switch](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#ex-g3-first-electric-circuit-switch) opens? Which old rules answer?

**Solution of Exercise 41.5.**

The fan spins on: the lamp’s death darkens only its own [branch](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel) — parallel independence. The open [switch](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#ex-g3-first-electric-circuit-switch) sits on the common road before the fork, so it stops both: the command-post rule.

**Exercise 41.6 ★.**

What is a [short circuit](#def-g6-circuits-and-safety-short), and why does it [heat](https://one-course.com/books/physics/1/en/chapter/34-heat-and-insulation#def-g5-heat-and-insulation-heat) wires? Name two of its victims.

**Solution of Exercise 41.6.**

A bare conducting road joining the source’s two sides directly, skipping every device: the unhindered current rushes and the wires [heat](https://one-course.com/books/physics/1/en/chapter/34-heat-and-insulation#def-g5-heat-and-insulation-heat). Victims: the [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery) (drained and ruined), the wire’s insulation — and, on the mains, sometimes the whole house.

**Exercise 41.7 ★.**

A circuit breaker in the fuse box has tripped. What has it just done for the house? What should be done before resetting it?

**Solution of Exercise 41.7.**

It cut its [branch](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel) in a blink because the current began to rush — a fault, not a whim. Find and fix the fault (unplug the suspect, inspect the [branch](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel)) before resetting the guard.

**Exercise 41.8 ★.**

Give the reason behind each rule: no gadgets near the bath; damaged cords to the bin; [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery) games yes, socket never.

**Solution of Exercise 41.8.**

Bath: damp skin conducts, and mains push through a wet body can be deadly. Damaged cords: cracked insulation is a fence with a gap — fingers can meet copper. [Battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery) yes, socket never: the [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery)’s push is tiny; the socket’s is hundreds of times mightier.

**Exercise 41.9 ★★.**

Translate to a diagram (in words or drawing): a [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery); from its $+$, a wire to a closed [switch](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#ex-g3-first-electric-circuit-switch); from the [switch](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#ex-g3-first-electric-circuit-switch) a fork — one [branch](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel) through a [buzzer](#ex-g6-circuits-and-safety-motor), one through a lamp; [branches](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel) rejoin and return to $-$. Predict what is heard and seen, and what changes if the [buzzer](#ex-g6-circuits-and-safety-motor)’s [branch](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel) alone gets its own extra [switch](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#ex-g3-first-electric-circuit-switch), opened.

**Solution of Exercise 41.9.**

Diagram: [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery), closed switch on the common road, then a fork — [buzzer](#ex-g6-circuits-and-safety-motor) [branch](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel) and lamp [branch](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel) — rejoining to $-$. Predicted: the lamp shines *and* the [buzzer](#ex-g6-circuits-and-safety-motor) [sounds](https://one-course.com/books/physics/1/en/chapter/19-sound-around-us#def-g3-sound-around-us-sound), independently. With an opened extra switch on the [buzzer](#ex-g6-circuits-and-safety-motor)’s [branch](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel): silence there, but the lamp shines on — [branch](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel) [switches](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#ex-g3-first-electric-circuit-switch) command only their [branch](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel).

**Exercise 41.10 ★★.**

A toy fan is wired: [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery) $+$ to [motor](#ex-g6-circuits-and-safety-motor), [motor](#ex-g6-circuits-and-safety-motor) to [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery) $-$, and — “for solidity” — one extra bare wire directly from $+$ to $-$. The fan barely turns and the extra wire grows hot. Diagnose with [Definition 41.6](#def-g6-circuits-and-safety-short): what road is the current preferring, and why is the build dangerous?

**Solution of Exercise 41.10.**

The extra wire is a [short circuit](#def-g6-circuits-and-safety-short): a bare direct road from $+$ to $-$. Current prefers the easy empty road to the hard-working [motor](#ex-g6-circuits-and-safety-motor), so the fan starves while the shortcut wire carries the rush and [heats](https://one-course.com/books/physics/1/en/chapter/34-heat-and-insulation#def-g5-heat-and-insulation-heat) — draining the [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery) and inviting burns. Remove the “solidity” wire.

**Exercise 41.11 ★★.**

Grandpa’s workshop trick: before touching any [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) fixture, he switches off its wall [switch](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#ex-g3-first-electric-circuit-switch) *and* the whole [branch](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel)’s breaker in the fuse box. Using series thinking, explain what each cut achieves — and why the careful man makes both.

**Solution of Exercise 41.11.**

The wall [switch](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#ex-g3-first-electric-circuit-switch) opens the fixture’s own loop — the lamp’s road is cut. The breaker opens the whole [branch](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel) farther upstream — even if the fixture’s wiring is faulty or the wall [switch](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#ex-g3-first-electric-circuit-switch) mislabeled, the [branch](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel) is dead. Two gaps [in series](https://one-course.com/books/physics/1/en/chapter/23-series-circuits#def-g4-series-circuits-series): either alone stops the current, both together forgive a mistake about either.

**Exercise 41.12 ★★★.**

Design the wiring of a puppet theater: one master [switch](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#ex-g3-first-electric-circuit-switch) that darkens everything; two stage lamps that must fail independently; a curtain [motor](#ex-g6-circuits-and-safety-motor) and a showtime [buzzer](#ex-g6-circuits-and-safety-motor), each with its own control [switch](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#ex-g3-first-electric-circuit-switch). Describe (or draw) the diagram — which devices ride common road, which ride [branches](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel), where each [switch](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#ex-g3-first-electric-circuit-switch) sits — and check your design against every rule of series and parallel you own.

**Solution of Exercise 41.12.**

[Battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery), then the master switch on the common road, then four parallel [branches](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel): stage lamp 1; stage lamp 2; curtain [motor](#ex-g6-circuits-and-safety-motor) with its own series [switch](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#ex-g3-first-electric-circuit-switch); [buzzer](#ex-g6-circuits-and-safety-motor) with its own series [switch](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#ex-g3-first-electric-circuit-switch). Checks: master commands all (common road); lamps fail independently (separate [branches](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel)); [motor](#ex-g6-circuits-and-safety-motor) and [buzzer](#ex-g6-circuits-and-safety-motor) each obey their own [switch](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#ex-g3-first-electric-circuit-switch) (series within their [branch](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel)) and ignore each other (parallel between [branches](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel)).

## 41.5 Problem: Wiring the Garden Shed

**Problem 41.1.**

Weekend problem — a family wires its garden shed; diagrams before screwdrivers; the inspector’s checklist

The family plans the shed’s little battery-powered network on paper first — as professionals do.

**Part I — The plan on paper.** The wish list: one ceiling lamp switched at the door; a workbench lamp with its own [switch](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#ex-g3-first-electric-circuit-switch); a small ventilation fan ([motor](#ex-g6-circuits-and-safety-motor)) with its own [switch](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#ex-g3-first-electric-circuit-switch).

1. Why must the three devices ride parallel [branches](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel) rather than one series loop? Give two comforts that would break [in series](https://one-course.com/books/physics/1/en/chapter/23-series-circuits#def-g4-series-circuits-series) .
2. Each [branch](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel) carries its own [switch](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#ex-g3-first-electric-circuit-switch) . What does each [switch](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#ex-g3-first-electric-circuit-switch) command, and what does none of them command?
3. Where must a master [switch](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#ex-g3-first-electric-circuit-switch) sit to darken and silence the whole shed at once?
4. Draw (or describe precisely) the full diagram: [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery) , master [switch](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#ex-g3-first-electric-circuit-switch) , three [branches](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel) , four [switches](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#ex-g3-first-electric-circuit-switch) in all.

**Part II — The build, inspected.**

5. Built and switched on, the ceiling lamp shines — but dimly, and the workbench lamp shines only when the *fan’s* [switch](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#ex-g3-first-electric-circuit-switch) is closed. The finger-walk finds the bench lamp and fan sharing one [branch](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel) , one behind the other. Name their arrangement, and explain both symptoms with the old rules.
6. Give the one-wire repair that restores the plan.
7. During tidying, a screwdriver briefly lies across the [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery) ’s two bare [terminals](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery) and grows warm. Name the event, and state what the family should check about the [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery) afterward.
8. The inspector-parent adds a rule: “every joint wrapped, no bare copper anywhere.” Which two dangers is that one rule guarding against?

**Part III — The night of the storm.**

9. A rainy night: the shed roof leaks onto the workbench. Why must the family not touch the wiring with wet hands, even in a battery-powered shed — and why would the same touch be far graver in the mains-powered house?
10. The house’s own [lights](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) flicker and one breaker trips. What has the breaker detected, and what is the wise order of actions?
11. Morning: the family debates adding a doorbell — [buzzer](#ex-g6-circuits-and-safety-motor) at the shed, [push-switch](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#ex-g3-first-electric-circuit-switch) at the house, long twin wire between. Series or parallel with the shed’s network? Where does its current come from? Sketch the loop.
12. Write the family’s three-line safety charter for the shed, one line each: sockets, water, bare wires.

**Solution of Problem 41.1.**

**1.** [In series](https://one-course.com/books/physics/1/en/chapter/23-series-circuits#def-g4-series-circuits-series), one dead or switched-off device would darken everything, and three devices would share the push, all dim and feeble. Parallel keeps each device independent and at full strength. **2.** Each [switch](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#ex-g3-first-electric-circuit-switch) commands exactly its own [branch](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel)’s device; none of them commands the other [branches](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel). **3.** On the common road, between the [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery) and the first fork — upstream of every [branch](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel). **4.** [Battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery) $\to$ master [switch](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#ex-g3-first-electric-circuit-switch) $\to$ [junction](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel) forking into three [branches](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel) — (ceiling lamp $+$ its [switch](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#ex-g3-first-electric-circuit-switch)), (bench lamp $+$ its [switch](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#ex-g3-first-electric-circuit-switch)), (fan [motor](#ex-g6-circuits-and-safety-motor) $+$ its [switch](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#ex-g3-first-electric-circuit-switch)) — rejoining and returning to the [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery). **5.** They are [in series](https://one-course.com/books/physics/1/en/chapter/23-series-circuits#def-g4-series-circuits-series) on one [branch](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel): sharing the push (both feeble — the dim ceiling lamp was really the bench pair’s symptom), and the fan’s [switch](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#ex-g3-first-electric-circuit-switch), sitting on their shared road, commands them both. **6.** Split them: run one new wire so the bench lamp gets its own [branch](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel) across the supply — each then full-strength and independent. **7.** A [short circuit](#def-g6-circuits-and-safety-short) across the [terminals](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery). Check whether the [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery) still delivers — shorts drain and can ruin cells (a warm [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery) deserves retirement). **8.** [Short circuits](#def-g6-circuits-and-safety-short) (bare wires meeting each other) and shocks or burns (fingers meeting bare copper). **9.** Damp skin conducts far better than dry; even the [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery) build deserves dry-hands discipline — and in the house the same touch meets the mains’ push, hundreds of times mightier: possibly deadly. Habits must be trained on the safe circuit. **10.** The breaker detected that too much current was rushing in its [branch](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel) — a fault, likely rain in a fitting. Wise order: leave the breaker off, unplug and inspect the [branch](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel), fix or dry the fault, only then reset. **11.** Its own loop, separate from the shed’s network (or as one more parallel [branch](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel) on the shed [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery)): [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery), [push-switch](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#ex-g3-first-electric-circuit-switch) at the house, long twin wire, [buzzer](#ex-g6-circuits-and-safety-motor) at the shed, back again — a doorbell is a series loop of its own however far its wires stretch. **12.** For example: “Sockets: our experiments end at the [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery) — the socket is not ours. Water: wet hands touch no wiring, indoors or out. Bare wires: every joint wrapped; a bare wire is a fault, not a shortcut.”