John McCarthy: He who refuses to do arithmetic is doomed to talk nonsense. (Progress and its Sustainability)
I'd forgotten a lot of my basic physics, so here is a refresher.
Power is the rate of energy conversion, or, the rate of doing work. This applies to both mechanical power, measured in joules / sec and electrical power, measured in watts
Work is done when a force is moved through a distance (W=F*D)
Mechanical force = mass * acceleration, the UNITS are kg.metre/sec/sec or Newtons
Mechanical work = force * distance, the UNITS are newton.metres or N.m or Joules
Mechanical power is work divided by time, or, Work / time, the UNITS are N.m /s or Joules / sec
Electrical power is measured in Watts. 1 Watt = 1 Joule / sec. A watt is also a volt.amp. Watts= Volts * amps
To obtain electrical energy then multiply the power * time. The UNITS here could be joules or watt.seconds. The normal units for larger amounts of electrical energy are KW.hrs. To convert watts to kilowatts divide by 1000, to convert seconds to hours divide by 3600 or 60*60.
We need to have an internalised way of having a feel for these values. A joule is the energy or work required to lift a small apple one meter straight up. If you perform that action in one second then that is equivalent to a watt or joule/sec of electrical power.
The small apple weighs 0.1 kg
The acceleration caused by the earth's gravity is roughly 10 m/sec/sec (9.8 m/s/s more accurately)
The distance we move the apple through is 1 metre
Work = force * distance
Work = mass * acceleration * distance
Work = 0.1 * 10 * 1
Work = 1 joule or 1 Newton.metre or 1 kg. metre ^2 / sec ^2, where ^2 means squared
Some other joule practical examples at the wikipedia joule page
A 40 watt incandescent light bulb produces forty joules of energy per second or the equivalent of lifting 4kg of apples 1 metre in one second. Fluorescent or LED lights achieve a similar effect with much less energy.
How much energy does a 40 watt incandescent light bulb use in one day?
40 * 24 / 1000 = 1 KW.hr energy used in one day
How much energy does the average person in an industrialised country use? The figures here are for the UK
Answer: The equivalent of 125 of these light bulbs running all the time
How many wind turbines would it require to meet the energy needs of everyone in the UK?
Answer = 600,000, which would cover half of the UK. The UK currently has 2408 wind turbines
How many nuclear power stations would it require to meet the energy needs of everyone in the UK?
Answer = 300. The UK currently has 10 nuclear power stations.
Here is a video from David MacKay illustrating the need to do the arithmetic. Irrespective of whether you are an alarmist, denier or somewhere in between you still need to do the arithmetic and basic physics to intelligently discuss these issues:
David MacKay is the author of Sustainable energy - without the hot air. He has recently been appointed the UK's Department of Energy and Climate Change Chief Scientific Advisor.
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