Unit Conversions - Energy, Power, Flux - Space Math
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Unit Conversions – Energy, Power, Flux
Energy is measured in a number of ways depending on what property is being
represented.
Total Energy
- Joules and ergs - The total amount of energy in various forms
(kinetic, potential, magnetic, thermal, gravitational)
Power
- Watts, Joules/second or ergs/second – the rate at which energy is
produced or consumed in time. Power = Energy/Time
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2
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Flux
- Watts/meter
, Joules/sec/meter
or ergs/sec/meter
– the rate with which
energy flows through a given area in given amount of time: Flux=Power/Area
1 Joule = 10 million ergs
1 kilowatt = 1,000 watts
1 Watt = 1 Joule/1 second
1 megaJoule = 1,000,000 Joules
1 hour = 3600 seconds
3 feet = 1.0 meters
Example:
A 5-watt flashlight is left on for 1 hour: Convert its energy
consumption of 5 watt-hours to Joules.
1 Joule
3,600
sec
5
Watt-hours
x ----------------- x --------------- =
18,000 Joules
1
sec
1
watt
1
hour
Notice how the compound unit ‘watt’ is handled so that the appropriate colored
units cancel. The canceling units are color-coded for convenience
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Problem 1 – The flux of sunlight at Earth’s surface is 1300 Watts/meter
.
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Convert this flux to ergs/sec/cm
.
Problem 2 – A 100-watt bulb shines light over a wall with a surface area of 25
2
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meters
. What is the flux of light energy in Joules/sec/meter
?
Problem 3 – The common energy unit for electricity is the watt-hour (Wh),
which can be written as 1 watt x 1 hour. How many megajoules equal 1 kilowatt-
hour (1 kWh)?
Problem 4 – How many ergs of energy are collected from a solar panel on a
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roof, if the sunlight provides a flux of 300 Joules/sec/meter
, the solar panels
have an area of 27 square feet, and are operating for 8 hours during the day?
Space Math
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