Energy and Power

Machines need fuel – electricity, oil, coal etc... Humans too are machines. Biological machines, and need a source of fuel too. Food is fuel for the human body. Food is digested by a process of slow combustion – it is oxidised – to produce energy to power all the bodies processes.

Some processes occur all the time at various rates – breathing, circulation of the blood, sweating, . Power must be constantly consumed so that cells can function – cells that consume no power are dead. Heavier people tend to produce more power – they have more 'body' to maintain. Others occur when we exercise or exert ourselves. The rate at which we burn up energy is called the metabolic rate, and metabolic rates for various people in various activities is shown in the following table.


Metabolic Rate

System

Watts, W

kcal/hour

kcal/day

100 lb person at rest

80

70

1600

150 lb person at rest

100

90

2100

200 lb person at rest

120

110

2600

Person walking at 3 mph

280

240

5800

Moderate exercise

470

400

10000

Heavy Exercise

700

600

14000

Compare the energy requirements of a human with the energy consumption of some typical machines.

A typical low energy light bulb might use 20W.

A car engine might produce a power of 60000W.

A heater might consume 1000W of electrical energy.

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