---
title: "A First Electric Circuit"
book: "Primary & Middle School Physics"
subject: physics
language: en
chapter: 16
exercises: 11
source: https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit
---

# Chapter 16 — A First Electric Circuit

Click — the flashlight throws its beam across the dark room. Inside that plastic tube live a [battery](#def-g3-first-electric-circuit-battery), a little [bulb](#def-g3-first-electric-circuit-bulb) and two strips of metal, playing a game with strict rules. Today we take the game apart: with a [battery](#def-g3-first-electric-circuit-battery), a [bulb](#def-g3-first-electric-circuit-bulb) and two wires, we will make [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) with our own hands.

## 16.1 A bulb, a battery, two wires

**Definition 16.1 (The battery).**

A *battery* is a small store of electricity. It has two different ends, called its *terminals*: one marked $+$ (plus), the other marked $-$ (minus). Both ends matter — electricity plays only when the two of them join the game.

**Definition 16.2 (The bulb).**

A *bulb* is a little lamp with a thin thread inside its glass that glows white-hot when electricity flows through it. Look closely at its metal foot: the bulb also has *two* contact places — the tip at the very bottom, and the metal side of its foot. Two [terminals](#def-g3-first-electric-circuit-battery) on the [battery](#def-g3-first-electric-circuit-battery), two contacts on the bulb: that is no accident.

**Method 16.3 (Lighting the bulb).**

With one [battery](#def-g3-first-electric-circuit-battery), one [bulb](#def-g3-first-electric-circuit-bulb) and two wires:

1. attach one wire from the [battery](#def-g3-first-electric-circuit-battery) ’s $+$ [terminal](#def-g3-first-electric-circuit-battery) to the tip of the [bulb](#def-g3-first-electric-circuit-bulb) ’s foot;
2. attach the other wire from the [battery](#def-g3-first-electric-circuit-battery) ’s $-$ [terminal](#def-g3-first-electric-circuit-battery) to the metal side of the foot;
3. the [bulb](#def-g3-first-electric-circuit-bulb) [lights](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) — you have built a circuit!

If it stays dark, check the usual suspects: a wire touching the glass instead of the metal contact, a loose connection, or a worn-out [battery](#def-g3-first-electric-circuit-battery).

![The players: a battery with its two terminals, a bulb with its two contacts, and two wires joining them.](https://one-course.com/images/onecourse/chapters/physics-1/g3-first-electric-circuit/fig-3c3d887d0a17.svg)

*The players: a [battery](#def-g3-first-electric-circuit-battery) with its two [terminals](#def-g3-first-electric-circuit-battery), a [bulb](#def-g3-first-electric-circuit-bulb) with its two contacts, and two wires joining them.*

![The moment the loop closes: battery, two wires, and the little bulb comes alight.](https://one-course.com/images/onecourse/chapters/physics-1/g3-first-electric-circuit/img-cfb8747d4511.jpg)

*The moment the loop closes: [battery](#def-g3-first-electric-circuit-battery), two wires, and the little [bulb](#def-g3-first-electric-circuit-bulb) comes alight.*

## 16.2 The circuit must close

**Definition 16.4 (Electric circuit).**

An *electric circuit* is a complete loop for electricity: out of one [terminal](#def-g3-first-electric-circuit-battery) of the [battery](#def-g3-first-electric-circuit-battery), through the wires and the [bulb](#def-g3-first-electric-circuit-bulb), and back into the other [terminal](#def-g3-first-electric-circuit-battery). When the loop is complete we say the circuit is *closed*; if it has a gap anywhere, it is *open*.

**Proposition 16.5 (The loop law).**

The [bulb](#def-g3-first-electric-circuit-bulb) [lights](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) only when the circuit is closed — one unbroken loop from one [terminal](#def-g3-first-electric-circuit-battery), through the [bulb](#def-g3-first-electric-circuit-bulb), back to the other [terminal](#def-g3-first-electric-circuit-battery). One single gap, anywhere in the loop, and the [bulb](#def-g3-first-electric-circuit-bulb) goes dark: electricity does not jump gaps, and it does not flow into dead ends.

**Example 16.6 (Why both ends?).**

Connect *two* wires from the [battery](#def-g3-first-electric-circuit-battery)’s $+$ [terminal](#def-g3-first-electric-circuit-battery) to the [bulb](#def-g3-first-electric-circuit-bulb)’s two contacts, leaving $-$ alone: nothing. One wire from $+$ and one from $-$, but both to the [bulb](#def-g3-first-electric-circuit-bulb)’s tip: nothing again. The [bulb](#def-g3-first-electric-circuit-bulb) [lights](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) only when the loop visits the [battery](#def-g3-first-electric-circuit-battery)’s two different [terminals](#def-g3-first-electric-circuit-battery) *and* the [bulb](#def-g3-first-electric-circuit-bulb)’s two different contacts — a full round trip.

**Example 16.7 (The switch).**

A *switch* is a door in the circuit: a little metal bridge that the click lays down (closed — the loop is complete, [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source)!) or lifts open (a gap — dark). The flashlight’s button, the [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) switch on the wall, the button of a doorbell: all of them are doors in a loop. You can build one from a clothes-peg and a strip of kitchen foil folded over its jaws.

![The same circuit drawn the tidy way, with symbols. Left: switch closed, the loop is complete, the lamp lights. Right: one gap at the switch — dark.](https://one-course.com/images/onecourse/chapters/physics-1/g3-first-electric-circuit/fig-dec46b96f009.svg)

*The same circuit drawn the tidy way, with symbols. Left: [switch](#ex-g3-first-electric-circuit-switch) closed, the loop is complete, the lamp [lights](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source). Right: one gap at the [switch](#ex-g3-first-electric-circuit-switch) — dark.*

**Remark 16.8 (Tidy drawings).**

Instead of drawing every [battery](#def-g3-first-electric-circuit-battery) bump and [bulb](#def-g3-first-electric-circuit-bulb) screw, electricians use *symbols*: two unequal bars for the [battery](#def-g3-first-electric-circuit-battery) (the long one is $+$), a circle with a cross for the lamp, a little drawbridge for the [switch](#ex-g3-first-electric-circuit-switch), and straight lines for the wires. A symbol drawing of a circuit is quicker to draw, and anyone in the world can read it — we will use these drawings for many years.

**Remark 16.9 (Battery yes, wall socket never).**

Everything in this chapter is played with small round or flat batteries: their push is tiny and safe for experiments. The *wall socket* is another world — its push is hundreds of times mightier, enough to hurt or kill. Never put wires, fingers or anything else into a wall socket, and never take apart anything that plugs into one. [Battery](#def-g3-first-electric-circuit-battery) games: yes. Socket: never.

## 16.3 Exercises

**Exercise 16.1 ★.**

Name the two [terminals](#def-g3-first-electric-circuit-battery) of a [battery](#def-g3-first-electric-circuit-battery) and the two contact places of a [bulb](#def-g3-first-electric-circuit-bulb).

**Solution of Exercise 16.1.**

The [battery](#def-g3-first-electric-circuit-battery)’s [terminals](#def-g3-first-electric-circuit-battery): $+$ (plus) and $-$ (minus). The [bulb](#def-g3-first-electric-circuit-bulb)’s contacts: the tip at the bottom of the foot, and the metal side of the foot.

**Exercise 16.2 ★.**

What is a [closed circuit](#def-g3-first-electric-circuit-circuit)? What is an [open circuit](#def-g3-first-electric-circuit-circuit)? In which one does the [bulb](#def-g3-first-electric-circuit-bulb) [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source)?

**Solution of Exercise 16.2.**

Closed: one unbroken loop from [terminal](#def-g3-first-electric-circuit-battery) to [terminal](#def-g3-first-electric-circuit-battery). Open: a loop with a gap somewhere. The [bulb](#def-g3-first-electric-circuit-bulb) [lights](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) only in the closed one.

**Exercise 16.3 ★.**

Lea connects the [battery](#def-g3-first-electric-circuit-battery)’s $+$ [terminal](#def-g3-first-electric-circuit-battery) to the [bulb](#def-g3-first-electric-circuit-bulb)’s tip with one wire, and stops there. The [bulb](#def-g3-first-electric-circuit-bulb) stays dark. What is missing, in the words of the loop law?

**Solution of Exercise 16.3.**

The return trip. One wire is only half a loop: the electricity must go out of one [terminal](#def-g3-first-electric-circuit-battery), through the [bulb](#def-g3-first-electric-circuit-bulb), and *back to the other [terminal](#def-g3-first-electric-circuit-battery)* — Lea needs a second wire from the [bulb](#def-g3-first-electric-circuit-bulb)’s side contact to the $-$ [terminal](#def-g3-first-electric-circuit-battery).

**Exercise 16.4 ★.**

What job does a [switch](#ex-g3-first-electric-circuit-switch) do in the loop? What lies inside a [switch](#ex-g3-first-electric-circuit-switch) when it is closed?

**Solution of Exercise 16.4.**

The [switch](#ex-g3-first-electric-circuit-switch) is a door in the loop: closed, it completes the circuit; open, it makes a gap. Inside a closed [switch](#ex-g3-first-electric-circuit-switch) a little metal bridge joins the two wires.

**Exercise 16.5 ★.**

Draw, with the tidy symbols of [Remark 16.8](#rem-g3-first-electric-circuit-symbols), a circuit with one [battery](#def-g3-first-electric-circuit-battery), one closed [switch](#ex-g3-first-electric-circuit-switch) and one lamp. Which symbol is which?

**Solution of Exercise 16.5.**

A loop of straight lines joining: two unequal bars (the [battery](#def-g3-first-electric-circuit-battery), the long bar being $+$), the drawbridge symbol lying closed (the [switch](#ex-g3-first-electric-circuit-switch)), and a circle with a cross (the lamp).

**Exercise 16.6 ★.**

The flashlight goes dark in the middle of a camping trip. Give three possible gaps or faults to check, from [Method 16.3](#met-g3-first-electric-circuit-light).

**Solution of Exercise 16.6.**

A worn-out [battery](#def-g3-first-electric-circuit-battery); a loose connection (wire or [bulb](#def-g3-first-electric-circuit-bulb) not screwed tight, spring bent); a broken [bulb](#def-g3-first-electric-circuit-bulb) — its glowing thread snapped, making a gap inside the [bulb](#def-g3-first-electric-circuit-bulb) itself.

**Exercise 16.7 ★.**

In the doorbell circuit, where is the “[switch](#ex-g3-first-electric-circuit-switch)”? Why does the bell ring only while the button is pressed?

**Solution of Exercise 16.7.**

The button *is* the [switch](#ex-g3-first-electric-circuit-switch). Pressing it lays the metal bridge and closes the loop, so the bell rings; releasing it lifts the bridge, the gap opens, and the ringing stops.

**Exercise 16.8 ★.**

Why must you never poke anything into a wall socket, when playing with a [battery](#def-g3-first-electric-circuit-battery) is fine? (See [Remark 16.9](#rem-g3-first-electric-circuit-safety).)

**Solution of Exercise 16.8.**

The [battery](#def-g3-first-electric-circuit-battery)’s push is tiny and safe; the wall socket’s push is hundreds of times mightier and can hurt or kill. Sockets are not for experiments — ever.

**Exercise 16.9 ★.**

A [bulb](#def-g3-first-electric-circuit-bulb) held with its tip on the [battery](#def-g3-first-electric-circuit-battery)’s $+$ [terminal](#def-g3-first-electric-circuit-battery), and one wire from the [battery](#def-g3-first-electric-circuit-battery)’s $-$ [terminal](#def-g3-first-electric-circuit-battery) pressed against the metal side of the [bulb](#def-g3-first-electric-circuit-bulb)’s foot: will it [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source)? Follow the loop with your finger to decide.

**Solution of Exercise 16.9.**

Yes, it [lights](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source). Follow the loop: $+$ [terminal](#def-g3-first-electric-circuit-battery), into the tip, through the glowing thread, out by the side contact, along the wire, back to $-$. A complete round trip visiting both [terminals](#def-g3-first-electric-circuit-battery) and both contacts.

**Exercise 16.10 ★★.**

Sam builds a loop: [battery](#def-g3-first-electric-circuit-battery), wire, [bulb](#def-g3-first-electric-circuit-bulb), wire, back to the [battery](#def-g3-first-electric-circuit-battery) — but he attaches both wires to the same [terminal](#def-g3-first-electric-circuit-battery), $-$, “because it was nearer”. The [bulb](#def-g3-first-electric-circuit-bulb) stays dark although the loop looks complete. Explain what his loop is missing, using [Example 16.6](#ex-g3-first-electric-circuit-bothends).

**Solution of Exercise 16.10.**

His loop never visits the second [terminal](#def-g3-first-electric-circuit-battery). A circuit must go from one [terminal](#def-g3-first-electric-circuit-battery) *through the [bulb](#def-g3-first-electric-circuit-bulb) to the other* [terminal](#def-g3-first-electric-circuit-battery): out of $-$ and back into $-$ is a round trip to nowhere — the [battery](#def-g3-first-electric-circuit-battery)’s push needs both of its different ends in the game.

**Exercise 16.11 ★★.**

A string of party [lights](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) has one [bulb](#def-g3-first-electric-circuit-bulb) missing, and the *whole* string is dark, though all the other [bulbs](#def-g3-first-electric-circuit-bulb) are fine. What does this tell you about how the [bulbs](#def-g3-first-electric-circuit-bulb) are arranged in the loop? (Next year’s circuits chapter will give this arrangement a name.)

**Solution of Exercise 16.11.**

The [bulbs](#def-g3-first-electric-circuit-bulb) must all sit on one single loop, one after the other: the missing [bulb](#def-g3-first-electric-circuit-bulb) leaves a gap, and one gap darkens the whole string — the loop law. (Next year this one-after-the-other arrangement gets its name: a series circuit.)