A2 Physics |
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Thermal Physics
Hot metal glowing red.
Contents:
Laws of thermodynamics
Internal energy
Temperature
Specific heat capacity
Specific latent heat
Laws of Thermodynamics
First law = The change in internal energy, ΔU, of a system is equal to the heat, ΔQ, added to the system minus the work done, ΔW, by the system:
The second and third law (sometimes called the 'zeroth' law) are not required for the A-level syllabus. However, they are placed here for completeness:
Second law = Encompasses theory about the direction of heat transfer and efficiencies of engines. Otherwise known as the law of entropy, S:
Second law = Encompasses theory about the direction of heat transfer and efficiencies of engines. Otherwise known as the law of entropy, S:
Third law = If 2 systems are at the same time in thermal equilibrium with a 3rd system, they are in thermal equilibrium with each other.
Internal Energy
The interal energy of a system changes as a result of heat/energy transfer to or from an object or work done by or on an object. In truth, the internal energy, U, is a sum of all the kinetic energies of the molecules, Ekin, plus the potential energies, Epot, in the chemical bonds holding atoms and molecules together. The more energetic the particles and the more stored energy in the bonds the higher the internal energy.
The arrangements of particles in a substance have everything to do with binding energies of atoms. When heated, a solid vibrates more in its place. When it receives enough energy to break bonds it turns into a liquid (melts). It does this whilst staying at the same temperature. This is part of the 'kinetic theory'; the idea that the internal energy of a system is related to the kinetic and potential energies of particles within that system..
Particles in a solid, lowest internal energy.
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Particles in a liquid.
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Particles in a gas, highest internal energy.
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Temperature
Temperature is an indicator of internal energy. There have been various ways of measuring temperature to make it relevant to us. Here are a few:
Daniel Gabriel Farenheit (1686-1736)
Brine Freezes at 7.5 degrees, body temperature is at 22.5 and water boils at 60 degrees... multiply by 4 = Fahrenheit scale! These temperatures were relevant and thus used in the Fahrenheit temperature scale!
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Anders Celcius (1701-1744)
By marking the freezing and boiling points of water on his thermometer and dividing by 100 the Celcius scale was made.
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William Thomson, 1st Baron Kelvin (1824-1907)
Creator of the absolute temperature scale in Kelvin.
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A hot object will have a higher internal energy. Heat always 'travels' from hot to cold. When at constant temperature, and no heat transfer occurs, the object is said to be in 'thermal equilibrium'.
In the Celcius scale 0C is the freezing point of water and 100C is the boiling point (at standard pressure). In the absolute temperature scale 0K is the lowest possible temperature and 273K is the freezing point of water. At absolute zero particles have lost their kinetic energy and are even no longer vibrating. There can be no temperature below this temperature because temperature a measure of how energetic particles are. If they are not moving, the temperature cannot be less.
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At absolute zero an object has minimum internal energy. The pressure, of a fixed mass of an ideal gas in a sealed container of fixed volume, decreases as the gas temperature is reduced. This line always passes through -273C regardless of which gas is used. |
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Specific Heat Capacity, c
Specific heat capacity (Jkg^-1K^-1) = The energy needed to raise the temperature of a unit mass of the substance by 1K without change of state.
For an object of mass, m, with a change in temperature from T1 to T2 of specific heat capacity, c, and energy change, ΔQ, the following relation applies.
The specific heat capacity is constant for that material. Here are some values for the specific heat capacity of different substances.
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The specific heat capacity can be measured by first measuring the energy input, Q, into the block: 
Using the current, I, the voltage, V, and the time, t, the heater is on for, you can measure the specific heat capacity. In practice the specific heat capacity is normally slightly off. This is because if the block heats up too much then heat will leave the block rather than increase its temperature i.e. temperature will be lost to the system. |
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Changes of State and Latent Heat
The state (solid, liquid, gas) of a substance depends on the energy the atoms hold. If the energy is too high for bonds to be held then the bonds will break.
Latent heat of fusion = the total energy required to melt a solid at its melting point.
Latent heat of vaporisation = the energy required to vaporise a liquid at its boiling point.
Specific Latent Heat
Specific latent heat of fusion, lf = energy required to change the state of unit mass of the substance from solid to liquid without change of temperature (Jkg-1).
Specific latent heat of vaporisation, lv = energy required to change the state of unit mass of the substance from liquid to vapour without change of temperature (Jkg-1).
Q = heat energy (J), m = mass, (kg), l = specific latent heat (Jkg^-1).