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1.1Chapter 1: Energy

What is energy? — Alhussan Physics - Stage 7 Revision Notes & Topical Questions

5 min read 8 parts Lesson 1 of 8 1 quiz

📚 Alhussan Physics - Stage 7 Hub / Topical Revision
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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

Section 1 of 8Why we need energy

Why we need energy
Uses
  • Uses energy from food is needed to walk, run, ride a bike, grow and stay alive.
Fuels
  • Fuels energy from fuels is needed for transport and to produce electricity.
What the idea of energy does
  • 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.

Section 2 of 8The unit of energy

The unit of energy
Energy is not a substance
  • 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)
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)
  • 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.

Section 3 of 8Energy in food

  • Stored energy
    • different foods store different amounts of energy, which the body releases by respiration.
Food (per 100 g)Energy (kJ)
Banana340
Beans400
Rice500
Cooked chicken800
Chocolate1500
Nutrients
  • Nutrients fats store about 37 kJ per gram, whereas carbohydrates and proteins store about 17 kJ per gram.
Comparing
  • Comparing compare foods using the same mass, such as per 100 g, so foods of different package sizes can be judged fairly.
Food labels
  • Food labels the energy in a serving = energy per 100 g × serving mass ÷ 100.

Section 4 of 8How the body uses energy

Energy cost
  • Energy cost 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.
Resting
  • Resting about three-quarters of the energy needed each day is for processes in the body like breathing.
  • Sleeping needs about 300 kJ per hour.
Activities
  • Activities running or playing can use 3500 kJ per hour.
ActivityEnergy (kJ) per minute
Sitting6
Standing7
Walking slowly13
Washing, dressing15
Cycling25
Playing football59
Swimming73
  • 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.

Section 5 of 8Energy balance

Balance
  • Balance an adult should take in only as much energy as they need for their activities.
Too much
  • Too much if more energy is taken in than needed, the body stores it as fat for future use.
Too little
  • Too little if less is eaten than needed, the body uses energy from its store of fat and the person loses weight.
Daily need
  • Daily need it depends on age, sex and activity, and the energy in the food someone eats should equal the energy they need.

Section 6 of 8Energy in fuels

Fuel
Fuel a store of chemical energy that can be released by burning, such as coal, oil, wood or gas.
Uses
  • Uses wood 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 fuels
  • Fossil fuels coal, oil and natural gas formed over millions of years from the remains of plants and animals.
Renewable
  • Renewable wood and biofuels can be replaced because new supplies can be grown.
Energy per kilogram
  • Energy per kilogram hydrogen gives about 140 000 kJ/kg, petrol about 46 000 kJ/kg and wood about 16 000 kJ/kg.
Carbon dioxide
  • Carbon dioxide burning fossil fuels produces it, which contributes to global warming.
Original source
  • Original source the Sun is the original source of energy for most food and fossil fuels,
    • because plants store light energy as chemical energy by photosynthesis.

Section 7 of 8Key terms

TermMeaning
Joulethe unit of energy.
Kilojoule1000 joules.
Fuela store of chemical energy that is released when burned.
Renewableable to be replaced as it is used.
Respirationthe reaction that releases energy from food in cells.
Kilocaloriean older energy unit equal to 4.2 kJ.

Section 8 of 8Exam technique

1
Saying energy is a substance — it is a quantity used to keep track of changes.
2
Mixing up J and kJ — 1 kJ = 1000 J.
3
Thinking food has no energy without cooking — raw food stores chemical energy too.
4
Saying fossil fuels are renewable — they take millions of years to form.

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