Complete set of detailed notes for OCR Physics A (2015) A Level - Newtonian World and Astrophysics by a student that achieved a high A at AS and a high A* at A level.
A LEVEL PHYSICS – MODULE 5: NEWTONIAN WORLD
AND ASTROPHYSICS
5.1 Thermal physics
5.1.1 Temperature
Temperature and thermal equilibrium
• Temperature – measure of hotness on an arbitrary scale
• Temperature measures only the KE component of an object’s total internal
energy → greater the KE of the particles (atoms or molecules), higher the temp
• Thermal energy is transferred from regions of higher temperature to regions of
lower temperature
• If one object is hotter than another → net flow of thermal energy from the hotter
object into the colder one → increases temp of colder, decreases temp of hotter
• Thermal equilibrium – when there is no net heat flow between objects in contact
with each other at the same temperature
Measuring temperature
• Celsius scale of temperature – uses the freezing and boiling points of pure
water as the two fixed points, with the scale between 0 °C and 100 °C
• Absolute (or thermodynamic) scale of temperature – independent of the
properties of any particular substance → uses absolute zero and the triple point
of pure water as the two fixed points
• Absolute zero – the temperature at which the substance has minimum internal
energy → the lowest limit for temperature
• where T = temperature in K, θ = temperature in °C
• Absolute zero (0 K) = – 273 °C
5.1.2 Solid, liquid and gas
Kinetic model of matter
• Kinetic model of matter – describes how all substances are made up of
particles which are in constant motion, and are arranged differently depending on
the phase of the substance
• In solids: particles are regularly arranged and packed close together with strong
electrostatic forces of attraction between them holding them in fixed positions →
they can vibrate about their individual fixed positions → high density and a
definite shape
• In liquids: particles cannot move much but can slide/flow past each other →
mean separation > solids ∴ attractive forces are weaker → lower densities than
solids and can be poured
• In gases: particles are free to move around very quickly in random directions →
further apart so weaker attractive forces → much lower densities than solids and
liquids and can be compressed relatively easily
1|T. Chaudhary
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• Heating a solid causes it to melt and become a liquid
• Heating a liquid causes it to evaporate and become a gas
• This is because the particles gain energy resulting in them moving further and
further apart until they have enough energy to overcome the forces of attraction
to adjacent particles
Brownian motion
• Brownian motion – random movement of small visible particles suspended in a
fluid due to collisions with much smaller, randomly moving particles of the fluid
Can observe Brownian motion using a smoke cell:
• The microscope should be focused on the smoke,
which you see as tiny dots of light
• The smoke particles move due to collisions with
the randomly moving molecules of air around
them → air molecules do not hit the smoke
particles equally from all directions or with equal
speeds ∴ net impulse gained by a smoke particle is not zero
• In accordance with the kinetic model of matter, all particles above absolute zero
are in constant motion → however, cannot accurately predict the motion of any
single smoke particle since there are so many air molecules moving around,
bombarding the smoke particles as well as themselves and the walls of the
container
• Heating a solid → rise in temp, KE of particles increases → however, position of
particles does not change due to intermolecular forces → increase in KE results
in greater vibration of particles around their equilibrium positions
• Heating a liquid → also small amount of translational KE added to vibrational KE
as the molecules can now move past one another
• Heating a gas → almost all of the KE is translational in the form of linear motion
Internal energy
• Internal energy – sum of the randomly distributed kinetic and potential energies
of atoms or molecules within a system
• The internal energy of a substance is minimum at absolute zero → mean
translational KE ∝ absolute temp ∴ KE is zero at absolute zero but there is some
PE due to bonds between molecules or atoms
• Increasing the temp of a body increases the KE of the particles inside the body
as they vibrate with greater amplitude or move at a greater speed → PE also
2|T. Chaudhary
, 3
increases slightly as mean separation between atoms increases slightly →
therefore the hotter the substance, the greater the internal energy
• When a substance changes phase (e.g. solid to liquid), the temp does not
change so the KE does not change → however, PE does
• Energy is supplied to overcome the attraction of the intermolecular forces, so the
large increase in mean separation of the molecules means a large change of PE
• Gas: PE is zero due to negligible electrical forces between atoms or molecules
• Liquid: PE has a negative value → negative as energy must be supplied to break
the intermolecular bonds
• Solid: electrostatic forces are very large so the PE has a large negative value
• Electrostatic PE is lowest in solids, higher in liquids and the highest in gases
5.1.3 Thermal properties of materials
Specific heat capacity
• Specific heat capacity – the energy required to raise the temperature of 1kg of
the substance by 1K
• where E = change in thermal energy, m = mass, c = specific heat
capacity (Jkg-1K-1); Δθ = change in temp (K or °C)
• Higher specific heat capacity → takes more energy and time to change its temp
Determining specific heat capacity:
• Insulate the material being heated & use
a lid for liquids → to minimise energy
loss to the surroundings
• Temp rise should be as high as possible
to reduce the % uncertainty in the temp
change
• For a liquid, it must be stirred to ensure
uniform temperature
• Use an ammeter and voltmeter to obtain values for I and V, E = VIt
• Assuming all energy transferred electrically is used to heat the material → using
principle of conservation of energy: VIt = mcΔθ → rearrange for c
• Method of mixtures: known masses of 2 substances at different temps are
mixed together → recording their final temp at thermal equilibrium allows the s.h.c
of one to be measured if the s.h.c of the other is known → m1c1Δθ1 = m2c2Δθ2
Specific latent heat
• Specific latent heat of fusion (Lf) – energy required to change the phase of 1kg
of a substance at constant temperature from a solid to a liquid
• Specific latent heat of vaporisation (Lv) – energy required to change the phase
of 1kg of a substance at constant temperature from a liquid to a gas
• E = mL where L = specific latent heat (Jkg-1)
3|T. Chaudhary
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