Geo 1012 : Planet Earth : lecture outlines


Energy from the Sun



  1. All external processes on the earth are driven by energy from the Sun - the spheres affected are: atmosphere, hydrosphere, biosphere and lithosphere.
  2. What is energy? (review p.14 of text)
    To understand the scientific meaning of energy, we need to introduce the familiar concept of work
    Force =mass (Kg) x acceleration (m/s2)
    1 Newton of Force = 1 kg accelarated by 1 m/s2
    Work is done whenever a force is exerted over a distance
    work = force x distance
    1 Joule of work = 1 Newton of force x 1 meter distance
    Energy is the ability to do work. If a system is capable of exerting a force over a distance, it possesses energy. It has the same units as work: Joules (in SI system)
    Power is rate at which work is done or energy is expended.
    Power = work/time, or Power = energy/time
    Energy = Power x time, expressed in Watts
    Watt = 1 joule per second
  3. Types of energy
    Kinetic Energy is associated with moving objects.
    Potential Energy is defined as energy waiting to be released.
    We encounter energy in many ways in the world around us. A list is below.
    Potential Energy
    Gravitational - perched boulder
    Chemical - gasoline, food etc.
    Electric - batteries etc.
    Elastic - tightly coiled spring, flexed muscle
    Electromagnetic
    Kinetic energy
    Moving objects
    Heat
    Sound, water waves
  4. The interchangeability of energy - most fundamental property of the universe
  5. Energy transfer in the physical world - either by particles (matter) or by waves
  6. Properties of waves:
    Wavelength:
    distance between the crests or highest points of two adjacent waves.
    Frequency (f):
    number of wavecrests that go by a given point in one second.
    A wave that sends one crest every second has a frequency of one Hertz (one cycle per second).
    Velocity:
    speed of the wavecrest.
    Water waves have a velocity of a few meters per second, while sound waves travel about 340 meters per second
    Amplitude:
    height of the wavecrest.
  7. Relationship between wavelength, frequency, and velocity
    There is a simple relationship among these fundamental wave quantities. If we know any two of the three, we can calculate the other.
    Velocity = wavelength x frequency
  8. Two kinds of waves:
    Transverse waves:
    motion of the medium is perpendicular to the direction in which the wave moves.
    Longitudinal waves:
    the medium moves in the same direction as the wave.
  9. Important: In both types of waves, the energy always moves in the direction of the wave.
  10. The Sun is at a distance of 150 million km from the earth. How does the Sun produce the energy and how does it reach the earth? Nuclear Energy and electromagnetic radiation
  11. Energy production in stars
    Fusion of hydrogen in small stars like the sun
    Fusion of hydrogen through intermediate reactions in bigger stars
    Gamma rays (Electromagnetic radiation) (98% of total energy) and nearly massless neutrino particles (2% of total energy) travelling at the speed of light. Neutrinos leave the sun quickly, but gamma rays are affected by layers
  12. Structure of the Sun: The radius of the Sun is ~700,000 km
    The core where fusion takes place - nearly 1-2 x107 K, about 170,000 km
    The radiative layer - electrons absorb the gamma rays, the ~400, 000 km thick layer gets opaque and hot
    Convective layer - about 4/5 of the way to the surface, the upper 100,000 km of the Sun, hot gas layer where energy is transported up by convection
    The Photosphere - the layer that emits the light, about 300 km thick, at a pressure of less than 1/100 of our atmosphere. The temperature of the photosphere is about 5800K
    The Chromosphere - layer of low density incandescent gas, ~ 2500 km observed during eclipse of the Sun. This layer is transparent to light and light passes through it.
    The Corona - diffuse ionized gas envelope that grades into space over a million km. The temperature exceeds 1 million K, but because the gas is much less dense than that of the photosphere, it radiates much less energy. The high temperature is caused by the turbulence and motions of the gas in the photosphere
  13. Electromagnetic waves : waves in which energy is transferred not through matter but through electric and magnetic fields. (Review p. 50-51 Closer Look)
    It is a self-propagating wave made up of fluctuating electric and magnetic fields. Its velocity is always equal to the velocity of light but can have a range of wavelengths.
    The entire range is called the Electromagnetic Spectrum
    Wavelength x frequency = c (299,793 km/sec), so if wavelength is known, frequency can be calculated or vice versa.
    The energy of an EM wave is inversely proportional to its wavelength and directly proportional to its frequency. Red light from glowing embers has low energy (700 nm) corresponding to a frequency of 4.3x1014 Hz compared to violet light in hot objects (400 nm) corresponding to a frequency of 7.5x1014 Hz.
  14. The only way to observe EM waves is by its interactions with matter. When a wave hits matter it can do one of three things:
    Transmission: passes through, perhaps a bit slowed, or perhaps with a changed direction.
    Absorption: the energy is absorbed by the matter and converted to some other form, generally heat.
    Scattering and reflection: absorbed and rapidly re-emitted. White materials scatter it in all directions, mirrors reflect it.
  15. Nomenclature of the electromagnetic spectrum:
    Radiowaves
    the longest wavelengths. Wavelength is from a few meters to longer than the size of the earth. Corresponding f is 1 kHz to several hundred mHz. They can travel long distances in the atmosphere.
    Microwaves
    Wavelength from about 1 meter to 1 millimeter. Used in communications and radar and cooking.
    Infrared
    Wavelength from 10-3 to 10-6 meters (micron, µ). Absorbed in matter, this is recognized primarily as heat radiation.
    Visible Light
    Wavelength from 700 nanometers (10-9 meters) at the Red light to Violet light at 400 nanometers. Most of the colors are rather broad range frequencies except Yellow at 570-590 nm. The Sun produces most energy at this range, so our eyes may have evolved to take maximum advantage.
    Ultraviolet
    Wavelength from 400 to 100 nm. The higher frequency carries lot more energy and is dangerous to tissues and cells. Some special materials absorb UV to produce fluorescence - the black light effects.
    X- Rays
    Wavelength from 100 nm to 0.1 nm. High frequency, high energy waves that can penetrate several centimeters into most solid matter.
    Gamma rays
    Wavelength from less 0.1 nm to less than 10-16 meters (the size of a nucleus). These are the highest energy rays in the EM spectrum. These are generally produced in nuclear reactions and not as common in nature as others.
  16. Laws of radiation
    1. All matter above absolute zero (0 Kelvin or -273° C) temperature emits radiant energy as electromagnetic radiation.
    2. The hotter the body, the higher the total energy per unit area.
    3. The higher the temperature, the shorter the wavelength.
  17. The Sun’s total energy budget
    Energy due to direct sunlight on the ground = 750 W/m2
    Energy outside of the earth’s atmosphere = 1370 W/m2
    Distance between the earth and the Sun = 1.5x1011m
    Surface area of sphere centered on the Sun and with a radius equal to the earth-sun distance = 2.8x1023 m2
    Total energy of the Sun (Luminosity) = 1370 x 2.8x1023 m2 =3.8 x 1026W
  18. The energy received by the earth
    The Earth is a disk with a radius of 6.4 x 106m
    Surface area of the earth disc = 1.3 x 1014m2
    Energy captured by the earth = 1.3 x 1014m2 x 1370W = 1.8 x 1017 W

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