PHYSICS · Stage 7 · Chapter 1: Energy
Physics 1.1 What is energy? — Joules, Kilojoules and Energy in Food and Fuels
5 min read 6 parts Lesson 1 of 8
Lesson summary
1.1 What is energy? — Joules, Kilojoules and Energy in Food and Fuels — Alhussan International Academy — Physics (Stages 7–9) notes: Describe where we get energy from, the unit of energy, and compare the energy content of foods and fuels.Uses — energy from food is needed to walk,…
- What the idea of energy does
- Energy is not a substance
- Kilojoule (kJ)
- Kilocalorie (kcal)
- Conversions
- Stored energy
Part 1 of 6Why we need energy
- Uses — energy from food is needed to walk, run, ride a bike, grow and stay alive.
- Fuels — energy from fuels is needed for transport and to produce electricity.
- What the idea of energy does — it helps explain what can happen but not why things happen.
- Fuel allows a car to move, but does not tell you where it will go, and without fuel the car cannot move.
Part 2 of 6The unit of energy
- Energy is not a substance — it does not move from one object to another as a substance, but is a way of keeping track of a very important quantity, a bit like money.
- Joule (J) = the unit of energy, and one joule is a very small amount.
- A small apple lifted by one metre needs approximately 1 J.
- Kilojoule (kJ) = 1000 J, used for larger amounts.
- Kilocalorie (kcal) — it is an older unit for the energy in food, where 1 kcal = 4.2 kJ.
- People call it a "calorie" for short, and kilocalories still appear on food labels.
- Conversions — 3.5 kJ = 3500 J and 450 J = 0.45 kJ.
Part 3 of 6Energy in food
- Stored energy — different foods store different amounts of energy, which the body releases by respiration.
| Food (per 100 g) | Energy (kJ) |
|---|---|
| Banana | 340 |
| Beans | 400 |
| Rice | 500 |
| Cooked chicken | 800 |
| Chocolate | 1500 |
- Nutrientsfats store about 37 kJ per gram, whereas carbohydrates and proteins store about 17 kJ per gram.
- Comparingcompare foods using the same mass, such as per 100 g, so foods of different package sizes can be judged fairly.
- Food labelsthe energy in a serving = energy per 100 g × serving mass ÷ 100.
Part 4 of 6How the body uses energy
1Energy cost1 point
— all activities have an energy cost, including keeping warm, breathing, moving and talking.
- Children need energy to grow bigger bones, muscles and brains, and the brain uses energy to learn.
2Resting1 point
— about three-quarters of the energy needed each day is for processes in the body like breathing.
- Sleeping needs about 300 kJ per hour.
3Activities
— running or playing can use 3500 kJ per hour.
| Activity | Energy (kJ) per minute |
|---|---|
| Sitting | 6 |
| Standing | 7 |
| Walking slowly | 13 |
| Washing, dressing | 15 |
| Cycling | 25 |
| Playing football | 59 |
| Swimming | 73 |
- More active — the more active you are, the more energy you need, and polar walkers need even more because they carry food and need energy to keep warm.
Part 5 of 6Energy balance
- Balancean adult should take in only as much energy as they need for their activities.
- Too muchif more energy is taken in than needed, the body stores it as fat for future use.
- Too littleif less is eaten than needed, the body uses energy from its store of fat and the person loses weight.
- Daily needit depends on age, sex and activity, and the energy in the food someone eats should equal the energy they need.
Part 6 of 6Energy in fuels
- Fuel a store of chemical energy that can be released by burning, such as coal, oil, wood or gas.
- Useswood or coal can heat a room or cook food, and a kettle's energy is transferred by electricity that may have come from a fuel such as coal or oil.
- Fossil fuelscoal, oil and natural gas formed over millions of years from the remains of plants and animals.
- Renewablewood and biofuels can be replaced because new supplies can be grown.
- Energy per kilogramhydrogen gives about 140 000 kJ/kg, petrol about 46 000 kJ/kg and wood about 16 000 kJ/kg.
- Carbon dioxideburning fossil fuels produces it, which contributes to global warming.
- Original sourcethe Sun is the original source of energy for most food and fossil fuels, because plants store light energy as chemical energy by photosynthesis.
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