Thermal Physics

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Solids, Liquids, Gases


  • Particles in a giant lattice vibrate about a fixed point
  • Particles are held in place by strong forces of attraction


  • Particles are constantly moving around
  • They are free to move past each other but are still slightly attracted to each other


  • Particles are moving around in constant random motion
  • In an ideal gas, there are no forces of attraction between molecules
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Specific Heat Capacity

The specific heat capacity of a substance is the energy needed to raise one kilogram of a substance by 1 degree (K/celsius).

  • Energy = mass x specific heat capacity x change in temperature (E=m x c x delta theta)

Measuring Specific Heat Capacity

Solids and Liquids

  • Heat the substance with an electrical heater (start 10 degrees below room temp and raise to 10 degree above)
  • With an ammeter and voltmeter attached to the electrical heater you can work out energy supplied (E=VIt)
  • With the calculated value for E (energy supplied by the heater) rearrange E=mc(change in temp)
  • c (specific heat capacity) = E/m(temperature change)
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Specific Latent Heat

Specific Latent Heat of Fusion

  • This is the energy needed to change one kilogram of a substance from solid state to liquid state
  • To melt a solid, bonds between atoms need to break
  • In this process, the potential energy of the substance becomes less negative
  • This increases the internal energy of the substance

Specific Latent Heat of Vaporisation

  • This is the energy required to change one kilogram of a substance from liquid state to gasous state
  • The bonds between atoms need to break further (no intermolecular forces in an ideal gas)
  • Again, the internal energy increases as potential energy becomes even less negative

NOTE: When a substance is at the point of changing state, the kinetic energy remains the same. It is ONLY potential energy that changes while a substance changes state

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Ideal Gases

ALL equations in thermal physics measure temperature on the Kelvin scale.

Boyles Law - At a constant temperature, pressure is inversely proportional to the volume of an ideal gas

Charles' Law - At a constant pressure, the volume of a gas is proportional to the absolute temperature of an ideal gas

Pressure Law - At a constant volume, pressure is proportional to the temperature of an ideal gas

Combining these ideas - pV=nRT (Ideal Gas Equation)

R = molar gas constant (8.31)

  • The number of particles of a gas, N, equals the number of moles multiplied by Avogadro's number
  • N = n x Na
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Kinetic Theory and Brownian Motion


  • Forces of attraction between particles are negligible except during a collision
  • Particles are moving around in constant random motion
  • The volume of the particles is negligible compared to the volume of the container
  • The motion of the particles follows Newton's Laws
  • Collisions between the particles themselves and with the walls of the container are perfectly elastic
  • The gas contains a large number of particles

Brownian Motion

  • Observe a smoke cell using a microscope
  • The particles appear as bright specks moving in constant random motion
  • This is caused by the 'invisible' air particles colliding randomly with the smoke particles in the smoke cell
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Internal Energy and Temperature

Thermal energy is ALWAYS transferred from a region of higher temperature to an area of lower temperature.

  • Particles in a gas do not all travel at the same speed
  • The distribution looks like a Maxwell-Boltzmann distribution graph
  • As the temperature of a gas increases:
    • The average particle speed increases
    • The maximum particle speed increases
    • The distribution curve becomes more spread out
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Energy Changes and Internal Energy

  • As a result of collisions between particles, energy will be transferred between particles
  • Some particles will gain speed, others will slow down
  • Between collisions, particles will travel at a constant speed
  • Although individual energy changes occur, the total energy of the sytem remains constant
  • Therefore, the average speed of the particles will stay the same (providing temperature remains constant and no other changes to the system)

Internal Energy

This is the amount of energy contained in a system. It is the sum of the random distribution of potential and kinetic energies within the system.

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Kinetic Energy and Temperature

  • Average kinetic energy is proportional to the absolute temperature
  • In an ideal gas there are no intermolecular forces between particles, therefore its potential energy will equal zero.
  • The only internal energy that an ideal gas will contain is its kinetic energy
  • We can say that the internal energy of an ideal gas is proportional to its kinetic energy
  • This means that an increase in temperature will be accompanied with an increase in kinetic energy
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