Transcript potential energy

Chapter Two
Solar Radiation and the Seasons
Energy is defined as “the ability to do work.”
The standard unit of energy in the International System (SI)
used in scientific applications is the joule (J).
Power is the rate at which energy is released,
transferred, or received. The unit of power is the watt (W),
which corresponds to 1 joule per second
(1 joule = 0.239 calories).
All forms of energy fall into the general
categories of kinetic energy and potential energy.
Kinetic energy can be viewed as energy in use
and is often described as the energy of motion.
Potential energy is energy that has not yet been used,
such as a cloud droplet that occupies some position
above Earth’s surface. Like all other objects, the
droplet is subject to the effect of gravity. The higher
the droplet’s elevation, the greater its potential energy.
Energy can be transferred from one place to another
by three processes: conduction, convection, and radiation.
Conduction is the movement of heat through a substance
without the movement of molecules in the direction of
heat transfer. Conduction is most effective in solid materials,
but it also is an important process in a very thin layer of air
near Earth’s surface.
The transfer of heat by the mixing of a fluid is called
convection. Unlike conduction, convection is accomplished
by displacement (movement) of the medium.
During the daytime, heating of Earth’s surface warms a
very thin layer of air in contact with the surface. Above this
thin laminar layer, air heated from below expands and rises
upward because of the inherent buoyancy of warm air
(the tendency for a light fluid to float upward
when surrounded by a heavier fluid).
Of the three energy transfer mechanisms,
radiation is the only one that can be propagated
without a transfer medium. Unlike conduction or
convection, the transfer of energy by radiation
can occur through empty space.
Virtually all the energy available on Earth originates
from the Sun. However, radiation is emitted by all matter.
In the case of radiation, quantity is associated with the
height of the wave, or its amplitude. Everything else
being equal, the amount of energy carried is
directly proportional to wave amplitude.
The quality, or “type,” of radiation is related to another
property of the wave, the distance between wave crests
or wavelength, which is the distance between any
two corresponding points along the wave.
Electromagnetic radiation consists of an
electric wave (E) and a magnetic wave
(M). As radiation travels, the waves
migrate in the direction shown by the pink
arrow. The waves in (a) and (b) have the
same amplitude, so the radiation intensity
is the same. However, (a) has a
shorter wavelength, so it is qualitatively
different than (b). Depending on the
exact wavelengths involved, the radiation
in (a) might pass through the
atmosphere, whereas that in (b)
might be absorbed.
It is convenient to specify wavelengths using
small units called micrometers (or microns).
1 micrometer equals one-millionth of a meter.
Perfect emitters of radiation, so-called blackbodies
are purely hypothetical bodies that emit the
maximum possible radiation at every wavelength.
Earth and the Sun are almost blackbodies.
The single factor that determines how much energy a
blackbody radiates is its temperature. Hotter bodies
emit more energy than do cooler ones.
The intensity of energy radiated by a blackbody
increases according to the fourth power
of its absolute temperature.
This relationship is represented by the
Stefan-Boltzmann law, expressed as
I = σT4
where I is the intensity of radiation in
watts per square meter,
σ is a constant (5.67 x 10-8 watts per square meter)
and T is the temperature of the body in kelvins.
Celsius Temperature = (oF - 32) / 1.8
Fahrenheit Temperature = (1.8 x oC) + 32
Kelvin Temperature = oC + 273
For any radiating body, the wavelength of peak emission
(in micrometers) is given by Wien’s law:
max = constant (2900)/T
where max refers to the wavelength of energy
radiated with greatest intensity.
Wien’s law tells us that hotter objects radiate energy
at shorter wavelengths than do cooler bodies.
Solar radiation is most intense in the visible portion of
the spectrum. Most of the radiation has wavelengths less
than 4 micrometers which we generically refer to as
shortwave radiation. Radiation emanating from
Earth’s surface and atmosphere consists mainly of that
having wavelengths longer than 4 micrometers. This type
of electromagnetic energy is called longwave radiation.
Energy radiated by substances occurs
over a wide range of wavelengths.
Because of its higher temperature,
emission from a unit of area of the
Sun (a) is 160,000 times more intense
than that of the same area on Earth (b).
Solar radiation is also composed of
shorter wavelengths than
that emitted by Earth.
As the distance from the Sun
increases, the intensity of the
radiation diminishes in proportion
to the distance squared
(inverse square law).
The solar constant is the amount of
solar energy received by a surface
perpendicular to the incoming rays
at the mean Earth–Sun distance
and is equal to 1367 W/m2.
Earth orbits the Sun once every 365 1/4 days as if it were
riding along a flat plane. We refer to this imaginary surface
as the ecliptic plane and to Earth’s annual trip about the
plane as its revolution. Earth is nearest the Sun (perihelion)
on or about January 3 (147,000,000 km). Earth is farthest
from the Sun (aphelion) on or about July 3 (152,000,000 km).
Earth also undergoes a spinning motion called rotation.
Rotation occurs every 24 hours around an imaginary line
called Earth’s axis, connecting the North and South Poles.
The axis is not perpendicular to the plane of the orbit of
Earth around the Sun but is tilted 23.5° from it.
The axis is always tilted in the same direction and always
points to a distant star called Polaris (the North Star).
The Northern Hemisphere has its maximum tilt toward
the Sun on or about June 21, (June solstice).
Six months later (on or about December 21),
the Northern Hemisphere has its minimum availability
of solar radiation on the December solstice.
Intermediate between the two solstices are the
March equinox on or about March 21, and
the September equinox on or about September 21.
On the equinoxes, every place on Earth
has 12 hours of day and night and both
hemispheres receive equal amounts of energy.
The 23.5° tilt of the Northern Hemisphere toward the Sun
on the June solstice causes the subsolar point
(where the Sun’s rays meet the surface at a
right angle and the Sun appears directly overhead)
to be located at 23.5° N. This is the most northward latitude
at which the subsolar point is located (Tropic of Cancer).
On the December solstice, the sun is
directly overhead at 23.5° S (Tropic of Capricorn).
On the two equinoxes, the subsolar point is on the equator.
The latitudinal position of the subsolar point is
the solar declination, which can be visualized
as the latitude at which the noontime Sun
appears directly overhead.
Beam spreading is the increase
in the surface area over which
radiation is distributed in
response to a decrease of solar
angle. The greater the spreading,
the less intense is the radiation.
In (a), the incoming light is
received at a 90° angle. In (b),
the rays hit the surface more
obliquely and the energy is
distributed over a greater area.
A beam of light is more effective if
it has a high angle of incidence.
The next chapter examines
energy balance and temperature.