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Lesson8
Linear thermal expansion
coefficient thermal expansion
Different materials have different coefficients of linear thermal expansion.
The unit for the coefficient of linear thermal expansion () is [per K] and can be written as K-1.
Strain being a ratio of lengths, it has no units.
Coefficient of linear thermal expansion is normally defined at a particular temperature or averaged over a temperature range.
Example:
If a material has a coefficient of linear thermal expansion of 23.1 x 10-6 K-1, it means that for every 1 K increase in temperature, the strain experienced by the material is 23.1 x 10-6
Thermal strain
Thermal strains are typically linear in nature.
A uniform change in strain will occur for every degree change in temperature no matter where on the temperature scale it starts from.
If temperature approaches the material’s melting point, the change in strain will no longer be uniform (i.e. not proportional with temperature change).
Heat transfer
Heat transfer is the physical act of thermal energy being exchanged between two systems by dissipating heat.
Temperature and the flow of heat are the basic principles of heat transfer.
The amount of thermal energy available is determined by the temperature, and the heat flow represents movement of thermal energy.
On a microscopic scale, the kinetic energy of molecules is the direct relation to thermal energy.
As temperature rises, the molecules increase in thermal agitation manifested in linear motion and vibration.
Thermal conduction
The different sensations we feel is explained by the fact that different materials transfer heat at different rates.
Tile and stone conduct heat more rapidly than carpet and fabrics, so tile and stone feel colder in winter since they transfer heat out of your foot faster than the carpet does.
In general, good conductors of electricity (metals like copper, aluminium, gold, and silver) are also good heat conductors, whereas insulators of electricity (wood, plastic, and rubber) are poor heat conductors.
Thermal resistance
In summary, thermal conductivity describes a material’s ability to conduct heat. Thermal resistance describes its ability to resist heat transfer.
Thermal resistance is a measure of opposition to the flow of heat.
More the thermal resistance it becomes difficult for the heat energy to flow from one place to another place.
Thermal resistance is similar to electrical resistance.
Thermal expansion
Thermal expansion is the tendency of matter to change in shape, area and volume in response to a change in temperature.
When a solid is heated, the kinetic energy of its molecules increases.
Thus, the molecules begin vibrating/moving more and move further apart from one another.
The solid expands in this way.
Most substances expand when heated and contract when cooled.
Lesson 9
Boyle’s law
Gases have various properties which we can observe with our senses, including the gas pressure, temperature, mass, and the volume which contains the gas.
Careful, scientific observation has determined that these variables are related to one another, and the values of these properties determine the state of the gas.
In the mid 1600’s, Robert Boyle studied the relationship between the pressure p and the volume V of a confined gas held at a constant temperature
Charles’ law
The relationship between temperature and volume, at a constant number of moles and pressure, is called Charles’ law in honour of the French scientist who first investigated this relationship.
Charles observed that if the pressure is held constant, the volume V is equal to a constant times the absolute temperature T:
V = constant × T
For example, suppose we have a theoretical gas confined in a jar with a piston at the top.
The initial state of the gas has a volume equal to 4 m3, and the temperature is 300 K.
Avogadro’s law
The volume (V) of a gas is determined not only by the pressure and volume but also by the quantity of gas.
When the quantity is given in number of moles (n), the mathematical relation is
𝑉/𝑛 = constant
This relationship is known as Avogadro’s law because, in 1811, Amedeo Avogadro of Italy proposed that equal volumes of all gases contain the same number of molecules.
Avogadro’s law holds for all gases.
The number of molecules in 1 mole of a gas is known to be 6.023 × 1023, a value called Avogadro’s number.
Gay-Lussac’s law
When the temperature of a sample of gas in a rigid container is increased, the pressure of the gas increases as well.
The increase in kinetic energy results in the molecules of gas striking the walls of the container with more force, resulting in a greater pressure.
The French chemist Joseph Gay-Lussac discovered the relationship between the pressure of a gas and its absolute temperature.
Gay-Lussac’s law states that the pressure of a given mass of gas varies directly with the absolute temperature of the gas, when the volume is kept constant.
Ideal gases
Ideal gases are gases which are not influenced by real world factors like intermolecular forces.
They are a concept that developed over hundreds of years and follow a law known as the ideal gas law, which is a combination of other gas laws which were all independently discovered.
These gas laws are Boyle’s law, Charles’ law, Gay-Lussac’s law and Avogadro’s law.
When combined, these form the ideal gas law, which was compiled by Dmitri Mendeelev.