Energy Transferred by Working
Energy and Thermal Physics

The energy costs of living

Physics Narrative for 11-14 Supporting Physics Teaching

Keeping a human alive for a day

Let's start with thinking about the energy costs of keeping a human alive and functioning for one whole day. The average daily energy food intake (carbohydrate, protein, fat and alcohol) in the UK is equivalent to 10,500 kilojoule for men and 8400 kilojoule for women. The different components of our diet are not equally good at providing this energy. The following table shows how much energy is provided by one gram of each component:

componentenergy/kilojoule
carbohydrate (starch or sugar)16
protein17
fat37
alcohol29

Over the course of a typical day, if we are not to retain this energy in a chemical store as fat, we need to be active. Different activities help with this energy balance to greater or lesser extents. Just to maintain the basic bodily functions of an adult costs about 4.6 kilojoule a minute.

This Basal Metabolic Rate (BMR) varies among individuals, and with age and population group. It is always measured at rest and usually accounts for 75 % of human energy requirements.

The following figures showing how further activity increases the energy cost:

energy for each minute of activityenergy cost/kilojoule
sitting6
standing7
washing or dressing15
walking slowly13
walking moderately quickly21
walking up and down stairs38
light work (most domestic work, lorry driving, carpentry, bricklaying)10-20
moderate work (gardening, tennis, dancing, jogging, cycling up to 20 km/hour, digging)21-30
strenuous work (coal mining, cross-country running, football, swimming the crawl)>30

Feeding up the UK population

Where do people in the UK get their energy from?

This is how people in the UK satisfy their energy needs:

food grouppercentage
milk and milk products11
meat and meat products16
eggs and egg dishes2
fish and fish dishes2
fat spreads6
sugar, confectionery and preserves6
vegetables12
fruit and nuts2
cereals (including bread, cakes, pastries)30
other foods4
beverages9

These days foods are marked with the energy that they provide:

foodenergy provided per 100 gram / kilojoule
white rice1450
whole milk258
lettuce49
margarine3036
lamb chop1468
white flour1456
plain chocolate2126
white bread965

Energy balance in humans

Comparing the amount of energy coming in with the amount going out, over a day or a longer period, allows us to make simple predictions about whether or not particular activities will be viable. Food input limits possibilities.

For example, you might calculate whether or not your energy intake is sufficient to sustain a week long back-packing holiday in the Scottish Highlands.

If the energy balance falls short on the input side you will soon start to feel lethargic and weary, It's just the same as with cash-flow problems, if expenditure outstrips income, the enterprise soon grinds to a halt. There are choices, but these are restricted by the ultimate limiting factor: energy.

Energy Transferred by Working
appears in the relation dU=dQ+dW
is used in analyses relating to Working Engines Thermionic Emission
is a special case of Work
has the special case Potential Energy Kinetic Energy
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