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Thermal Physics - Group 2, Melting, Vaporisation, Condensation,…
Thermal Physics - Group 2
Change of State
SOLIDS
Molecules vibrate about their fixed positions
Molecules are tightly packed
LIQUIDS
Molecules vibrate and slide past each other
Molecules are close together with no regular arrangement.
GASES
Molecules are widely separated with no regular arrangement.
Molecules vibrate and move freely at high speeds.
Evaporation
Evaporation takes place when a liquid changes to a gas without boiling.
Since molecules can move about to a certain extent in liquids, the more energetic ones break free and turn into vapour.
It has a cooling effect since the molecules absorb heat to break away.
It accounts for 90% of the water vapour in our atmosphere, thus forming an integral part of our water cycle.
Phase Change
A to B - Molecules gain energy. Vibrational kinetic energy increases. Little change in mean separation of molecules. Little change in electrical potential energy.
B to C - Molecules become more disordered. Electrical potential energy increases.
The temperature remains constant during a change of state. Energy is used to break intermolecular bonds and is termed latent heat. Since there is no change in kinetic energy, there is no change in temperature.
C to D - The solid turns into a liquid. Molecules move increasingly rapidly. KE increases. Little change in electrical potential energy, little change in molecular separation.
D to E - Liquid starts boiling. Molecules completely separate. Large increase in electrical potential energy. Movement becomes disorderly.
E to F - The vapour is heated above boiling point. KE continues to increase. Molecular movement is even faster. Electrical potential energy reaches its maximum value (0).
Electrical Potential Energy
The PE of two atoms very close together is large and negative
As the separation between them increases, their PE decreases.
When they are completely separated, their PE is maximum and has a value of 0.
Difference between Temperature and Heat
Heat
: It is the total energy of all the molecular motion inside an object text
Temperature:
It is the measure of thermal energy or average heat of the molecules in a substance.
Thermal energy
There is no transfer of thermal energy between two objects at the same temperature :
The direction of the flow of energy is told by temperature. It flows from hotter areas to cooler areas
Thermal equilibrium: when two objects in contact with each other are at the same temperature
The thermodynamic (kelvin)scale
A temperature scale in which the temperature, T , is a function of the energy possessed by matter.
Zero of the scale is absolute zero. For any matter at absolute zero, it is impossible to remove any more energy from it.
The temperature of the ice point (0 °C) is 273.15 kelvin.
Thermodynamic temperature can be converted to Celsius temperature by subtracting 273.15 from the thermodynamic temperature.
Average kinetic energy is the same for all substances at a particular thermodynamic temperature.
Thermometers
Properties used as a basis of temperature measurement
The resistance of an electrical resistor or thermistor
The voltage produced by a thermocouple
The colour of an electrically heated wire
The volume of a fixed mass of gas at a constant temperature
Must be calibrated at two or more known temperatures, and the scale between divided into equal divisions.
Two types of electrical thermometers
Resistance thermometer
Very robust
thermistor has a narrow range while resistance wire has a wide range
It is larger than a thermocouple and has a greater thermal capacity therefore is slower acting
Thermistor has a high sensitivity over narrow range while the resistance wire is less sensitive
Thermistor is fairly linear over narrow range while the resistance wire has good linearity
Long conducting wires allow the operator to be at a distance from the thermometer
Thermocouple thermometer
It is robust
Can have a very wide range
It is smaller in size than resistance thermometers and has a smaller thermal capacity so is quicker acting and can measure temperature at any point
If the appropriate metal is chosen it can be sensitive
It is non-linear and therefore requires calibration
It has long conducting wires that allow the operator to be at a distance from the thermometer
Internal Energy and Heat transfer
Internal energy is the sum of the random distribution of kinetic energy and potential energy
The internal energy of a system can be increased by introduction of matter or by heat, or reduced when thermodynamic work is done by the system
a change in internal energy can come about due to the heating of a gas or if work is done on a gas
When you heat a gas, the walls of the container become hot and the molecules will gain more energy and thus move vigorously , The molecules will bounce of the walls faster and therefore the kinetic energy would be greater. As a result there will a rise in the internal energy of the gas
When you push the walls of a container inwards, you are doing work . The molecules of the gas strike a moving wall and bounce off faster . They gain kinetic energy and as a result temperature has risen. This is the reason why a gas gets hotter when its compressed. In this case, once again there will be a rise in the internal energy of the gas
internal energy of a system can also be increased by passing an electric current through it
SI unit: J
SI base units: m2*kg/s2
increase in internal energy =
energy supplied by heating +energy supplied by doing work
U= Ekin+Epot
U is a sum of all the kinetic energies of the molecules, Ekin, plus the potential energies, Epot is equal to U. The more energetic the particles and the more stored energy in the bonds the higher the internal energy.
Heat is transfered via solid material (conduction), liquids and gases (convection), and electromagnetical waves (radiation). Heat is usually transfered in a combination of these three types and seldomly occurs on its own.
When particles of matter are in direct contact, heat transfers by means of conduction. The adjacent atoms of higher energy vibrate against one another, which transfers the higher energy to the lower energy, or higher temperature to lower temperature. That is, atoms of higher intensity and higher heat will vibrate, thereby moving the electrons to areas of lower intensity and lower heat
Convection describes heat transfer between a surface and a liquid or gas in motion. As the fluid or gas travels faster, the convective heat transfer increases.Examples of this include the rising clouds of cigarette smoke, or heat from the hood of a car that rises upwards
Radiation (not to be confused with thermal radiation) refers to the transfer of heat through empty space. This form of heat transfer occurs without an intervening medium; radiation works even in and through a perfect vacuum. For instance, energy from the sun travels through the vacuum of space before the transfer of heat warms the Earth.
Melting
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Vaporisation
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Condensation
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Solidification
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Temperature
