Important Concepts
SL/HL 3.1 Thermal Physics
- Molecular theory of solids, liquids and gases
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Temperature and absolute temperature
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Internal energy
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Specific heat capacity
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Phase change
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Specific latent heat
Application of Skills
- Describing temperature change in terms of internal energy
- Using Kelvin and Celsius temperature scales and converting between them
- Applying the calorimetric techniques of specific heat capacity or specific heat experimentally
- Describing phase change in terms of molecular behaviour
- Sketching and interpreting phase change graphs
- Calculating energy changes involving specific heat capacity and specific latent of fusion and vaporization
Temperature
- Temperature is the average kinetic energy of molecules that can be measured with a thermometer. It is directly proportional to the average kinetic energy of the molecules.
- A thermal interaction occurs when objects contact each other, transferring energy until they are at the same temperature, reaching a thermal equilibrium. There are three mechanics to transfer energy
- Conduction: energy transfer through particle-particle collisions and movement.
- Convection: energy transfer through the distribution and movement of a fluid (liquid or gas).
- Radiation: energy transfer through electromagnetic radiation that each body emits.
The proportionality between temperature and random kinetic energy is in Kelvin or absolute temperature scale. The lowest kinetic energy is zero (at rest) at zero Kelvin, known as the absolute zero degree.
Heating and Internal energy
- Heat is the energy transfer from one body to another as a result of difference in temperature.
A thermal interaction occurs when objects with different temperature contact each other, transferring energy until they reach a thermal equilibrium. Energy transferred form a hot body to a cold body by heating increases the internal energy of the cold object and decreases the internal energy of the hot object.
- Internal energy is the total random kinetic energy of the particles of a substance, plus the total inter-particle potential energy of the particles. The intermolecular potential energy is highly dependent on what state the object is in (solid, liquid or gas).
- The internal energy of a system can change as a result of heat added or taken out and as a result of work performed.
Notice: Do not confuse heat with internal energy. Heat is the process of energy transfer and internal energy is a quantity that is being transferred.
- Melting: from solid to liquid, energy is absorbed by the substance.
- Freezing: from liquid to solid, energy is released by the substance.
- Vaporization or evaporation: from liquid to a gas (vapor), energy is absorbed by the substance.
- Condensation: phase change from vapor (gas), to liquid, energy is released by the substance.
Specific Heat Capacity
The amount of energy transferred in changing the temperature of an object depends the change in temperature required, the mass of an object, and its specific heat capacity. The larger the change in temperature and mass, the more energy is transferred.
The amount of energy transferred in changing the temperature of an object is calculated by the formula below.
Q=mcΔT
Q = the amount of energy transferring
m = mass
c = specific heat capacity
ΔT = change in temperature
The specific heat capacity is the energy required to change the temperature of a unit mass of a substance by one kelvin.
- The above figure shows a heating curve from a solid state of a gaseous state. In flat regions of the graph, kinetic energy remains constant since temperature is unchanged, and energy provides by heat increases internal energy increases by overcoming intermolecular forces using energy by heating. This can also be understand from the conservation of energy, where total energy of a system must be conserved. Since the heat provides energy and kinetic energy is unchanged, potential energy must increases.