Physics Unit 5
Key points to remember:
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pV=NKT - where N=number of atoms
pV=nRT - where n=number of moles
Gamma radiation is very penetrating because it is ‘Weakly’ ionising.
Beta radiation is quite penetrating as it is moderately ionising
MAY 15 QUESTION 18 b(ii). Why this formula? Why not energy conservation?
The temperature of a body is a measure of the mean, random, kinetic energy of its
vibrating atoms.
In an ideal gas we assume that the intermolecular forces are negligible. So no PE.
Trigonometric parallax is used to measure nearby stars, standard candles are used to
measure distant stars.
The smaller the parallax angle, the further the star.
Variable stars are stars whose brightness fluctuates.
Alpha
highly ionising
short range in air (3 - 5 cm)
stopped by paper
deflected slightly in magnetic field
Beta
fairly ionising
medium range in air (~15 cm)
stopped by lead or thick aluminium
deflected in magnetic field
Gamma radiation is the most penetrating. Even small levels can penetrate air, paper or thin
metal. Higher levels can only be stopped by many centimetres of lead, or many metres of
concrete
What is meant by ‘Random’?
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We cannot predict which nucleus will decay next.
We cannot know when an individual nucleus will decay.
What is meant by ‘Spontaneous’?
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Spontaneous means that the decay cannot be influenced by any external factors.
State what is meant by ‘half life’.
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The average time taken for the activity of the source to be halved.
State what is meant by ionising radiation.
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Ionising radiation removes electrons from atoms/molecules
Explain what is meant by a radioactive atom.
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A radioactive atom has an unstable nucleus which emits alpha, beta or gamma radiation.
State what is meant by the activity of a radioactive source.
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It is the rate of decay of unstable radioactive nuclei.
What are the sources of background radiation?
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Cosmic rays from outer space
Rocks from Earth’s crust (uranium and such)
Humans
Food and drink
Radioactive gases
How would you use carbon dating to determine age of a substance?
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All living organisms contain certain amount of carbon-14 when the matter is alive. Hence
the activity from living sample remains constant.
After death, the activity decreases. Comparing yhe activity of an archeological sample or
animal remains with its initial activity when it died, the age can be calculated using half
life or decay constant.
Outline how parallax measurements are used to determine the
distance to nearby stars.
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The star is viewed from two positions at 6 month intervals
The change in angular position of the star relative to fixed distant stars is measured
The diameter of the Earth’s orbit about the Sun must be known and trigonometry is used
to calculate the distance to the star.
Describe the process of nuclear fusion.
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Small mass nuclei come very close together
Nuclei join to form a more massive nucleus
Explain why the fusion of hydrogen nuclei should release energy.
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Mass of fused nucleus is less than sum of masses of fusing nuclei
Mass difference/deficit releases energy according to E = mc
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Explain why most of the energy released in the fusion of one
deuterium nucleus with one tritium nucleus is transferred to kinetic
energy of the neutron.
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Momentum is conserved
Mass of neutron is smaller, so speed is greater
E = ½ mv , so E is larger.
k
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k
Conditions for fusion to take place and why is the interior of star ideal
for this?
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For fusion to occur there must be a very high temperature so that protons/nuclei have
sufficient kinetic energy to overcome the electrostatic repulsion
So that the protons/nuclei come close enough for fusion to take place Or so that the
protons/nuclei come close enough for the strong nuclear force to act
There must be a very high density so that a sufficient collision rate is maintained as this
will make the fusion reactor sustainable.
The enormous gravitational forces acting in stars create a very high temperature and
density.
Outline the technical difficulties of producing a practical nuclear
fusion reactor.
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There is little possibility of a runaway fusion reaction unlike fission
There would not be any radioactive waste produced in the fusion process
A very/extremely high temperature plasma is required
Plasma must not touch reactor walls, so strong magnetic fields are required
If plasma touches the walls of the reactor its temperature falls and fusion stops
Discuss the potential advantages of nuclear fusion, compared with
nuclear fission, as a means of supplying our power demands.
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Hydrogen fuel for fusion is virtually unlimited
Fission relies upon uranium a relatively limited resource
Fusion results in few/no radioactive products
Radioactive products produced in fission present significant storage, disposal,
transportation problems
For a given mass of fuel, the energy released by fusion is greater than the energy
released by fission
Define Binding Energy.
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Binding energy is the energy released when a nucleus is formed from its component
nucleons
Explain how standard candles are used in astronomy.
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Standard candles are (stellar) objects of known luminosity
Standard candle’s brightness on earth is measured
Use inverse square law [F=L/4πd ]
Hence distance to standard candle is calculated
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How would a layer of dust around a Cepheid variable star affect the
conclusions drawn by astronomers?
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Dust layer will reduce brightness of Cepheid
Cepheid will appear to be further away than it is
Explain how the light emitted from a star enables us to determine the
temperature of the star and hence its luminosity.
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Determine
from spectrum.
Idea that Wien’s law is used to calculate the temperature.
Use H-R diagram to determine the luminosity.
max
Explain why standard candles are important to astronomers.
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Distances to stars too far for parallax to be measured.
Explain what is meant by redshift.
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Redshift is the fractional increase in wavelength received by an observer
Due to source and observer receding from each other
Conclusion:
• Galaxies are moving away from us and from each other
• The greater the distance the greater the velocity
• The universe is expanding
Discuss how astronomers were led to propose the existence of dark
matter and the consequences of its existence for the ultimate fate of
the universe.
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Dark matter has mass but does not emit e-m radiation
Dark matter proposed when observations of galaxies indicated that they must contain
more matter than could be seen.
The existence of dark matter will increase the average density of the universe
This may make it more likely that the universe is closed
Describe how you would expect the average density of matter in the
universe to affect its ultimate fate.
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If density less than critical value, expansion would continue for ever
If density greater than critical, expansion would stop and universe would contract again
If density equals critical value, expansion rate would decrease to zero but universe
would not contract again
Explain why it is difficult for scientists to estimate the average density
of the universe reliably.
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The mass of the universe is uncertain because of the amount of dark matter is uncertain
The value of the Hubble constant is uncertain
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The amount of dark matter (in the universe) is uncertain
Since dark matter doesn’t interact via the electromagnetic interaction
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The value of the Hubble constant is uncertain
Since measurements of distances to distant galaxies are uncertain
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Explain why the observable universe has a finite size
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The universe started from a small initial point
Idea that universe has a finite age
Idea that observable universe is finite because we can only see as far as (speed of light)
× (age of universe)
Light reaching us must have travelled a finite distance since the Big Bang
Some parts of the universe are so distant, light has not had time to reach us yet
Describe how astronomers would have determined the velocity of
each galaxy.
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Measure wavelength of light from the galaxy
Compare it to the wavelength for a source on the Earth
Reference to spectral line or line spectrum
Reference to Doppler effect or redshift v is found from: fractional change in wavelength
equals ratio of speed of source to speed of light
Reference to v/c = Δλ/λ with terms defined
Describe the lifecycle of the Sun.
Area 1:
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The Sun is fusing hydrogen into helium in its core
When hydrogen fusion ceases the core of the Sun cools
The core collapses under gravitational forces.
Area 2:
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The Sun expands and becomes a red giant
The core becomes hot enough for helium fusion to begin in the core
Helium begins to run out and the core collapses again under gravitational forces
Area 3:
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Idea that outer layers of Sun are ejected into space
The temperature doesn’t rise enough for further fusion to begin
The core/Sun becomes a white dwarf star
Explain why scientists are uncertain about the ultimate fate of the
universe.
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Due to the difficulty in making accurate measurements of distances to galaxies
Hubble constant has a large uncertainty
Because of the existence of dark matter
Values of the average density of the universe have a large uncertainty
Hence measurements of the critical density of the Universe have a large uncertainty
Describe the similarities and differences between electric and
gravitational fields.
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Gravitational fields are regions in which a mass experiences a force due to its mass
Electric fields are regions in which a charge experiences a force due to its charge
Both types of field have an infinite range
In each type of field the force varies as an inverse square
The force between masses is always attractive whereas the force between charges can
be attractive or repulsive
The force between unit charges at a given separation is much stronger than the force
between unit masses at the same separation
Explain why it is essential for communications satellites to be in an
orbit with time period of 24 hrs.
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Communications satellites must be in the same position in sky at all times
So communications satellites must rotate at the same rate as the Earth
State how the orbit of a GPS satellite differs from that of a
communications satellite.
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The radius of the GPS satellite orbit is smaller
The orbit of the communications satellite must be in an equatorial plane
State what is meant by ‘mean square speed’.
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The ‘mean square speed’ is the average of the squares of the speeds of all the individual
molecules of the gas and is given by the symbol <c >
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Explain what is meant by internal energy of a liquid.
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Internal energy is the sum of total kinetic energy and potential energy of molecules
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Explain why the internal energy of the air can be taken as the total
kinetic energy of the molecules of the air.
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Internal energy is the sum of kinetic energy and potential energy.
Potential energy of molecules in an ideal gas is zero.
Why do we assume an ideal gas to have no PE?
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In an ideal gas the intermolecular forces are negligible.
Collisions with each other and with the container walls are elastic.
As collisions between gas molecules take place randomly, there is a continuous
interchange of KE and PE in this way
Duration of collisions is negligible compared with the time spent in between collisions
and so we consider the internal energy to be entirely kinetic
Describe, in terms of temperature and energy, what happens when ice
melts.
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Ice will heat up as energy is transferred to the ice from the room to the ice because of
temperature difference.
The molecules of the ice will gain internal energy, both kinetic and potential.
Because the molecules gain kinetic energy, the temperature of the ice will increase.
This continues until the temperature reaches 0 degrees and begins to melt.
At this point, all the energy it receives is used to do work, increasing the potential energy
and overcoming the bonds that keep the ice as solid.
The solid changes into a liquid.
During this process there is no increase in the kinetic energy of the molecules and so the
ice remains at 0 degrees until it has all melted. Although the ice is still receiving heat
from the warmer surroundings, effect of this heat is not observed as a rise in
temperature of the icea. It is called latent heat.
Explain why the pressure increases when the air is heated in a tyre of
fixed volume.
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Average kinetic energy of molecules is greater
Collision rate with walls of container is greater
The rate of change of momentum is greater
Therefore a greater force on the container walls
State the conditions required for the oscillations to be simple
harmonic.
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Force (or acceleration) proportional to displacement from equilibrium position
Always in the opposite direction to the displacement
Acting towards the equilibrium
Explain how the principle of conservation of energy applies to
oscillation.
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Conservation of energy is followed as total energy remains constant
Some energy is dissipated as thermal energy
Work is done against resistive forces such as friction and air resistance/drag.