PB54 by xiagong0815



                                        Preliminary Physics
your partner for HSC success              the cosmic engine
                               energy emission from the sun
                                 student name . . . . . . . . . . . . . . . . . . . . . . .

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                                              Monday, 20 February 2012
energy emission from the sun               2
the cosmic engine

ad3b4aad-1898-4dab-b139-c1b4d4125686.doc       Prime Coachin
                                                energy emission from the sun
                                                           the cosmic engine

Nuclear radiation
      When small nuclei fuse together to form larger nuclei in stars, energy is released
      because the mass of the large nucleus is less than the sum of the masses of the two
      smaller nuclei from which it formed. This missing mass is called the mass deficit of
      the reaction and it is converted into energy in accordance with Einstein's equation
      E = m2.

      Some nuclei are unstable and emit particles and energy until they become stable.
      Such nuclei are said to be radioactive. Like fusion, radioactivity involves a mass
      deficit with the mass lost being converted into the kinetic energy of the emitted
      particle and the nucleus. The three main types of radioactive particles emitted from
      radioactive nuclei are:

      Alpha (α) particles are identical to a helium nucleus, consisting of two protons and
      two neutrons tightly bound together. Alpha particles have a double positive charge
      and poor penetrating power as they are stopped by a few centimetres of air. Alpha
      particles do not travel far in air because they readily ionise air molecules when they
      collide with them and so lose their kinetic energy.

      Beta (β) particles are very fast moving electrons. They have a single negative charge
      and are more penetrating than alpha particles because they do not ionise air
      molecules as readily.
      Gamma (γ) particles are uncharged highly energetic, very high frequency
      electromagnetic radiation that is very penetrating.

Preliminary Physics                           ad3b4aad-1898-4dab-b139-c1b4d4125686.doc
energy emission from the sun                                                           4
the cosmic engine

Solar wind
     The sun acts as a black body radiator at a temperature of about 6000 K. The most
     intense electromagnetic radiation is emitted in the yellow region of the spectrum,
     which is why the sun appears yellow, but the sun also emits radiation over a wide
     range of frequencies. Most of the electromagnetic frequencies reaching the Earth
     from the sun are absorbed by the Earth's atmosphere and do not reach the surface.
     The atmosphere is only transparent for light in two main bands, one is the visible and
     the other in the radio wave region of the electromagnetic spectrum.

     The fusion reactions that power the sun occur in its core. Energy from the core is
     brought to the surface of the sun through a radiation zone and a convection zone. The
     visible surface of the sun is a layer of gas about 500 km deep called the photosphere.
     Above the photosphere lies the chromosphere, a nearly invisible layer of gas that
     extends over 10000 km from the photosphere. The chromosphere can only be seen
     when the moon blocks the bright light from the photosphere during a total solar
     eclipse. Beyond the chromosphere is the corona, a region of hot gas that extends over
     thirty solar radii from the surface. It is possible to see the corona out to
     approximately ten solar radii during a total solar eclipse. For reasons not well
     understood, the sun's corona is hotter than the chromosphere. The temperature of
     the corona is believed to rise from 500 000 K near the chromosphere to over
     3 000 000 K in the outer corona. The hot gases are blown away from the sun and
     form the solar wind. The solar wind travels past the Earth at about 400 km s-1 and
     consists mostly of ionised hydrogen (i.e. protons and electrons) and some heavier
     elements. The sun is believed to be losing about 107 tonnes of mass per year due to
     the solar wind. This amount, however, represents only one part in 1014 of the sun's
     total mass and is insignificant compared to the amount of mass lost due to the
     conversion of mass to energy in fusion. The fast moving charged particles in the solar
     wind are trapped by the Earth's magnetic field and form two doughnut shaped
     regions around the Earth called the Van Allen belts. The inner Van Allen belt
     extends from an altitude of 2000 to 5000 km and is made up largely of trapped
     protons. The outer belt, made up largely of electrons, is about 16000 km above the
     Earth's surface. The solar wind's effect on the Earth's magnetic field
     and the Van Allen belts.

ad3b4aad-1898-4dab-b139-c1b4d4125686.doc                                                      Prime Coachin
                                               energy emission from the sun
                                                          the cosmic engine

      Sunspots are dark, cool areas on the sun's surface. The average size of a sunspot is
      about two Earth diameters and each lasts about a week. Sunspots often occur near
      one another in groups of up to 100, which may last for months. Galileo used the
      sunspots to determine that the sun rotated with a period of about 27 days.
      Sunspots look dark because they are cooler than the surrounding photosphere.
      Sunspots are about 4200 K, compared to 5800 K for the photosphere.
      Careful observation of the spectra emitted from sunspots showed that the magnetic
      field in sunspots is about 1000 times stronger than the sun's average magnetic field.
      Powerful magnetic fields are believed to reduce the normal circulation of material in
      sunspots causing such regions to cool. The number of sunspots varies with a period
      of 11 years. When the sunspot cycle is at its peak about 100 sunspots are visible, and
      when the cycle is at its minimum only a few sunspots are visible at anyone time. The
      sun spot cycle is related to the magnetic cycle of the sun.

      Prominences and solar flares are associated with the high magnetic fields near
      sunspots. At the peak of the sunspot cycle solar flares can emit huge amounts
      of X-rays, UV and visible radiation as well as bursts of high-speed protons and
      electrons. When the atmosphere absorbs this intense solar radiation, it increases the
      level of ionisation in the upper atmosphere. This increased ionisation affects the
      reflection of short wave radio signals and may even absorb them, thus interfering
      with satellite communications. The high-speed charged particles reach the Earth as
      powerful blasts of solar wind. Such blasts can damage satellites, are dangerous for
      airline passengers and astronauts, and produce huge currents in the ionosphere that
      flow down towards the poles and cause the auroras. The blasts also affect the Earth's
      magnetic field, causing compasses to malfunction and producing large induced
      current surges in electrical power lines that can damage electrical circuits and
      machinery (sometimes causing blackouts).

Preliminary Physics                          ad3b4aad-1898-4dab-b139-c1b4d4125686.doc

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