P7-Observing the Universe

P7 21st Century Science, OCR

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Nuclear Fusion

-This is when two nuclei combine to create a larger nucleus

-In the sun's core, hydrogen nuclei fuse into helium nuclei

-Energy is released

-In stars, they fuse other elements once the hydrogen runs out

-All nuclei are positively charged- they only contain protons and neutrons

-There is therefore a repulsive electrical force between the two nuclei

-Nuclei can only fuse if they overcome this electrical force and get close enough from the attractive strong force to hold them together

-This requires a lot of energy, from a high temperature

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A star's different zones

-The core is the furthest inside layer where most of the fusion takes place

-The core is made hot and dense by the pressure caused by the weight of the rest of the star

-The nuclei in the core are therefore close enough & have enough energy to fuse together

-The layer second from the inside of a star is the radiative zone

-The energy released from fusion in the core is transported by photons of radiation towards the surface of the star

-The convection zone is the second layer from the outside of a star

-Here energy is transferred to the surface of the star by convection currents

-The photosphere is the surface of the star where energy is radiated into space

-This is seen from Earth and constantly churning mostly because of the rising and falling of material in the convection zone below it

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The discovery of fusion

-Used to believe that the sun burned its own material

-Realised in 19th century it would need an impossible amount of fuel to burn for so long

-Einstein realised in the 20th century that mass could be converted into energy

-It was then suggested that hydrogen turned into helium inside the sun

-When this happens, so mass gets 'lost' (comparing masses of atoms)

-Perhaps the missing mass was being changed into energy, powering the sun

-Hans Bethe explained how fusion must be powering the sun, and got a Nobel Prize

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Strong force

  • Electrons kept in orbit around nucleus by attractive electrical force between the positive nucleus and negative electrons (opposite charges = attraction)
  • Ionisation happens when the electrons are given enought energy to overcome the attractive force and be removed from the atom
  • The nucleus, however, must be held together by an attractive force much greater than the repulsive force between the positive protons in the nucleus
    • This force is called the strong force
    • It has a very short range and so can only hold the protons and neutrons together when separated by tiny distances
      • At larger separations, the strong force disappears
      • At the small separations, the strong interaction must be repulsive, otherwise there'd be nothing to prevent it from crushing a nucleus to a point
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brill thank you!!

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