---
title: "Energy in Everyday Life"
book: "Primary & Middle School Physics"
subject: physics
language: en
chapter: 29
exercises: 11
source: https://one-course.com/books/physics/1/en/chapter/29-energy-in-everyday-life
---

# Chapter 29 — Energy in Everyday Life

A wind-up toy scurries across the floor, slows, and stops — until you [wind](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-wind) it again. A flashlight dims as its batteries tire. You yourself run out of steam before lunch. Everything that moves, shines, [sounds](https://one-course.com/books/physics/1/en/chapter/19-sound-around-us#def-g3-sound-around-us-sound) or warms seems to be spending something. Physics has a name for that something, and it may be the most important word in this whole book: [energy](#def-g5-everyday-energy-energy).

## 29.1 The something that gets things done

**Definition 29.1 (Energy).**

*Energy* is what it takes to make things happen: to set things moving, to lift them, to [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) them up, to warm them, to make them [sound](https://one-course.com/books/physics/1/en/chapter/19-sound-around-us#def-g3-sound-around-us-sound). Nothing goes, glows, or grows without energy being spent on it — and whatever is doing the spending must first have energy to spend.

**Definition 29.2 (Energy stores).**

An *energy store* is anything holding [energy](#def-g5-everyday-energy-energy) ready for use. The great everyday stores: *food* (your body’s fuel), *fuels* like wood and petrol, charged *batteries*, a wound-up *spring*, water held *high* behind a dam — and the hot *Sun*, pouring [energy](#def-g5-everyday-energy-energy) down on everything, all day, for free.

**Example 29.3 (Spot the store).**

Every going thing has a store behind it. The scurrying toy: its wound spring. The flashlight: its batteries. The car: the fuel in its tank. You: your breakfast. The sailboat: the moving [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) of the [wind](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-wind). When the store runs empty — spring unwound, batteries flat, tank dry, lunchtime near — the going stops.

**Remark 29.4 (No feeding, no going).**

For centuries, inventors chased machines that would run forever on nothing — wheels that turn themselves, clocks that never need winding. Every single one failed. Their failure is one of physics’ deepest lessons: [energy](#def-g5-everyday-energy-energy) is never conjured from thin [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air). Whatever runs is being fed — find the store, and you understand the machine.

## 29.2 Energy changes form

**Example 29.5 (Forms of energy).**

[Energy](#def-g5-everyday-energy-energy) shows itself in different forms: *motion* [energy](#def-g5-everyday-energy-energy) in the rolling ball and the blowing [wind](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-wind); *[light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source)* [energy](#def-g5-everyday-energy-energy) pouring from lamp and Sun; *heat* [energy](#def-g5-everyday-energy-energy) in everything warm; *electrical* [energy](#def-g5-everyday-energy-energy) traveling through wires; *stored* [energy](#def-g5-everyday-energy-energy) waiting quietly in food, fuel, springs and lifted things. Same something, many forms.

**Proposition 29.6 (The energy chain).**

Whenever something happens, [energy](#def-g5-everyday-energy-energy) is drawn from a store and passed along, usually changing form on the way. It is never created from nothing — and never truly destroyed either: what a machine “uses up” has only been passed on, most often ending, sooner or later, as gentle warmth spread into the surroundings.

**Method 29.7 (Reading an energy chain).**

To understand any device, trace its chain:

1. find the *store* the [energy](#def-g5-everyday-energy-energy) comes from;
2. find the *converter* — the part where the [energy](#def-g5-everyday-energy-energy) changes form;
3. name the forms coming out — the useful one, and the warmth that leaks alongside.

The flashlight, traced: [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery) (store) $\to$ [bulb](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-bulb) (converter) $\to$ [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) (useful) $+$ a little warmth (the leak — feel the [bulb](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-bulb)).

![The flashlight’s energy chain: stored energy becomes light — and a little warmth always slips out alongside.](https://one-course.com/images/onecourse/chapters/physics-1/g5-everyday-energy/fig-2463947d03f7.svg)

*The flashlight’s [energy](#def-g5-everyday-energy-energy) chain: stored [energy](#def-g5-everyday-energy-energy) becomes [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) — and a little warmth always slips out alongside.*

**Example 29.8 (Chains everywhere).**

Your morning ride, traced: breakfast (store) $\to$ your muscles (converter) $\to$ motion of the bicycle $+$ warmth (you feel it — you glow). The [wind](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-wind) farm: moving [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) $\to$ turning blades $\to$ electrical [energy](#def-g5-everyday-energy-energy) in the wires $\to$ [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) in your lamp tonight. The dam: high water $\to$ falling water spins the turbines $\to$ electrical [energy](#def-g5-everyday-energy-energy). Long chains are just short ones linked: form after form after form.

![An energy chain you can see from the road: moving air turns the blades, and the turbines send electrical energy into the wires.](https://one-course.com/images/onecourse/chapters/physics-1/g5-everyday-energy/img-e95e6c835af8.jpg)

*An [energy](#def-g5-everyday-energy-energy) chain you can see from the road: moving [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) turns the blades, and the turbines send electrical [energy](#def-g5-everyday-energy-energy) into the wires.*

## 29.3 The Sun at the head of the table

**Example 29.9 (Almost every chain starts at the Sun).**

Follow most chains backward and you arrive at the Sun. Breakfast? Plants grew it by catching sunlight; your cereal is tinned sunshine. [Wind](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-wind)? The Sun warms the lands unevenly, and the stirred [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) is [wind](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-wind) — so even sailboats run on sunlight. Wood? Sunlight stored by trees. The dam’s high water? The Sun lifted it: it evaporated the sea, the vapor rained on the mountains. Solar panels just skip the middlemen.

![Two chains traced back to their source: breakfast and wind alike begin at the Sun.](https://one-course.com/images/onecourse/chapters/physics-1/g5-everyday-energy/fig-417721f9af2f.svg)

*Two chains traced back to their source: breakfast and [wind](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-wind) alike begin at the Sun.*

**Remark 29.10 (A word we will sharpen).**

For now, [energy](#def-g5-everyday-energy-energy) is a story well told: stores, forms, chains. Can it be *measured*, like length in [metres](https://one-course.com/books/physics/1/en/chapter/7-measuring-length#def-g2-measuring-length-units) and [mass](https://one-course.com/books/physics/1/en/chapter/20-measuring-mass#def-g3-measuring-mass-mass) in [grams](https://one-course.com/books/physics/1/en/chapter/20-measuring-mass#def-g3-measuring-mass-units)? It can — [energy](#def-g5-everyday-energy-energy) has its own [unit](https://one-course.com/books/physics/1/en/chapter/7-measuring-length#def-g2-measuring-length-measure) and its own exact bookkeeping, and they arrive later in this book, once you have met speed and [force](https://one-course.com/books/physics/1/en/chapter/12-pushes-and-pulls-forces#def-g2-pushes-and-pulls-force) properly. The chains you trace this year are the map; the numbers will come to live on it.

## 29.4 Exercises

**Exercise 29.1 ★.**

What is [energy](#def-g5-everyday-energy-energy), in the words of this chapter? Name four things that cannot happen without it.

**Solution of Exercise 29.1.**

[Energy](#def-g5-everyday-energy-energy) is what it takes to make things happen. Without it, nothing can move, be lifted, shine, warm up, or [sound](https://one-course.com/books/physics/1/en/chapter/19-sound-around-us#def-g3-sound-around-us-sound) — any four.

**Exercise 29.2 ★.**

Find the store: a scurrying wind-up mouse; a cyclist; a campfire; a sailboat; a flashlight.

**Solution of Exercise 29.2.**

The wound spring; the cyclist’s food; the burning wood; the moving [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) of the [wind](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-wind); the batteries.

**Exercise 29.3 ★.**

Name five forms of [energy](#def-g5-everyday-energy-energy), with one example of each from your own day.

**Solution of Exercise 29.3.**

Motion (the rolling ball), [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) (the lamp), heat (the warm bath), electrical (the wires to the toaster), stored (breakfast, a [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery), a wound spring).

**Exercise 29.4 ★.**

Trace the [energy](#def-g5-everyday-energy-energy) chain of a toaster, as in [Method 29.7](#met-g5-everyday-energy-read): store, converter, forms out.

**Solution of Exercise 29.4.**

Electrical [energy](#def-g5-everyday-energy-energy) from the wall (fed, farther back, by a power station’s store) $\to$ the glowing wires inside the toaster (converter) $\to$ heat toward the bread — with a little [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source), the orange glow, alongside.

**Exercise 29.5 ★.**

Trace the chain from a dam’s lake to your reading lamp tonight.

**Solution of Exercise 29.5.**

High water (store) $\to$ falling water spins the turbine (converter) $\to$ electrical [energy](#def-g5-everyday-energy-energy) in the wires $\to$ the lamp (second converter) $\to$ [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source), plus a little warmth.

**Exercise 29.6 ★.**

A phone “dies”. What has actually run out? Where did that [energy](#def-g5-everyday-energy-energy) go, according to the chain proposition?

**Solution of Exercise 29.6.**

Its battery-store has run empty. The [energy](#def-g5-everyday-energy-energy) was not destroyed: it was spent along the day’s chains — lighting the screen, making [sound](https://one-course.com/books/physics/1/en/chapter/19-sound-around-us#def-g3-sound-around-us-sound) — and has ended spread out as gentle warmth in the phone and the room.

**Exercise 29.7 ★.**

Why does a laptop grow warm underneath while you use it? Which link of the chain is showing itself?

**Solution of Exercise 29.7.**

The leak: every converter passes some [energy](#def-g5-everyday-energy-energy) into warmth alongside its useful work. The laptop’s chips convert electrical [energy](#def-g5-everyday-energy-energy), and the warmth underneath is the chain’s unavoidable side-channel.

**Exercise 29.8 ★.**

Show that a sailboat runs on sunlight, by tracing its chain backward step by step.

**Solution of Exercise 29.8.**

The sails are pushed by [wind](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-wind); [wind](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-wind) is [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) stirred into motion because the Sun warms lands and seas unevenly; so the boat’s motion [energy](#def-g5-everyday-energy-energy) arrives, two links back, from sunlight.

**Exercise 29.9 ★★.**

An inventor announces a lamp that “needs no [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery), no plug, no fuel — it simply shines forever”. Which lesson of [Remark 29.4](#rem-g5-everyday-energy-nofree) does the announcement defy? What question should you ask the inventor first?

**Solution of Exercise 29.9.**

That [energy](#def-g5-everyday-energy-energy) is never conjured from thin [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) — whatever runs is being fed. First question: “What is the store, and where does the lamp draw from it?” If the inventor has no answer, the lamp belongs with the forever-wheels: in the drawer of famous failures.

**Exercise 29.10 ★★.**

A rubber ball dropped from shoulder height bounces lower and lower and finally lies still. “Its [energy](#def-g5-everyday-energy-energy) is destroyed,” says Leo. Use [Proposition 29.6](#prop-g5-everyday-energy-chain) to defend a better ending to the story — into what final form did the bouncing [energy](#def-g5-everyday-energy-energy) slip?

**Solution of Exercise 29.10.**

The [energy](#def-g5-everyday-energy-energy) was passed on, not destroyed: each bounce spent some of the ball’s motion [energy](#def-g5-everyday-energy-energy) squashing the ball and rubbing the [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) and floor, and all those payments end as a whisper of warmth in ball, floor and [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air). Too spread out to gather, but not gone.

**Exercise 29.11 ★★.**

Some houses heat water in black rooftop panels; some cook with sunlight and curved mirrors; solar cells make electricity from daylight. Explain the phrase “solar panels just skip the middlemen” from [Example 29.9](#ex-g5-everyday-energy-sun): which middlemen stand between the Sun and a wood fire’s warmth?

**Solution of Exercise 29.11.**

Between Sun and wood fire stand the middlemen: trees, which spent years catching sunlight and storing it as wood, and the fire that finally unpacks the store. The rooftop panel takes the sunlight’s [energy](#def-g5-everyday-energy-energy) directly — no tree, no years, no fire.