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Temperature and Internal Energy
What is internal energy?
The total energy of these particles is called the internal energy. Internal energy comprises two components- kinetic and potential energy.
When a body is heated, its temperature increases.
(a) For a solid:
1. The potential energy of the atom increases because the separation between atoms increases.
2. The kinetic energy of the atoms increases because the atoms vibrate faster and with larger amplitudes.
(b) For a liquid:
1. The potential energy of the atoms increases because the separation between atoms increases.
2. The kinetic energy of the liquid molecules increases because the molecules move faster.
(c) For a gas:
1. The potential energy of atoms is zero and is unaffected by change in temperature.
2. The kinetic energy of the atoms increases as they move faster at higher temperatures. Hence the internal energy of a body increase as the temperature increases.
Melting and solidification
When a solid melts, it changes from the solid to the liquid state at a certain temperature known as its melting point.
During solidification, a liquid changes into a solid. Latent heat of fusion is released but the temperature remains unchanged.
For a pure substance, both melting point and solidification point are the same.
Factors Affecting the Melting Point of Water
- Addition of impurities, such as salts, lowers the melting point of water to below 0 degree Celsius.
Hence, antifreeze is added to the water in the cooling system of a car to prevent it from freezing in the winter and salt is added to ice as freezing mixture to obtain temperatures below 0 degree Celsius.
- Increase in pressure reduces the melting point of ice.
Boiling and condensation
Boiling is the process in which a liquid changes to gas at a constant temperature which is known as the boiling point of the liquid.
When a liquid boils thermal energy is transferred to the liquid without a change in temperature.
The heat required by a liquid to change to vapour at its boiling point is known as the latent heat of vapourisation.
Condensation is the process when a vapour changes into liquid. During condensation heat is lost by the vapour at constant temperature.
Evaporation
Evaporation is the process in which a liquid changes to vapour at temperatures below the boiling point of the liquid.
During evaporation, liquid molecules that have high kinetic energy escape from the liquid surface.
The molecules that remain in the liquid have less kinetic energy. Hence the temperature of the liquid drops.
Factors affecting the rate of evaporation:
1. Temperature
2. Humidity of the surrounding air
3. Surface area of the liquid
4. Movement of air
5. Pressure
6. Boiling point of the liquid
Transfer of Thermal Energy
Thermal energy always flows from a region of higher temperature to a region of lower temperature.
How is thermal energy transferred?
Thermal energy is transferred by any of these three processes: conduction, convection and radiation.
Conduction
Conduction is the process of thermal energy transfer without any flow of the material medium.
It occurs mainly in solids.
Conduction occurs when the vibration energy of the molecules is transferred from one molecule to the next. The free electrons in metals make metals a very good conductor of heat.
In conduction, heat is transferred from one atom to another by the vibration of atoms.
Heat is also transferred by the movement of free electrons from the hot end to the cool end, thus metals are good conductors of heat.
Most liquids are poor conductor of heat, so gases such as air are among the poorest conductors of heat.
The link below is a video on conduction. It shows a kind of conduction.
http://www.youtube.com/watch?v=V7QceNEIR-w
Convection
Convection is the transfer of heat energy involving movement of the material itself.
The heated portions of the gases or liquids becomes less dense and rise up, creating convection currents.
Similarly, convection currents are created when the colder portions of liquids or gases becomes denser and sink.
Convection is the transfer of thermal energy by means of currents in a fluid.
How does convection works?
Convection currents occurs only in fluids such as liquids and gases but not in solids.
The link below is a video on convection. It shows a kind of convection.
http://www.youtube.com/watch?v=5pG-tkbQgMo
Radiation
Radiation is the transfer of energy in the form of electromagnetic waves.
Radiation can pass through vacuum.
Radiation is the continual emission of infrared waves from the surface of all bodies, transmitted without the aid of a medium.
Radiation does not require a medium for energy transfer.
Factors affecting rate of infrared radiation:
1. Colour and texture of the surface
2. Surface temperature
3. Surface area.
Applications of Thermal Energy Transfer
Common application of conduction:
Uses of good conductors of heat
1. Cooking utensils- usually made of aluminum or stainless steel
2. Soldering iron rods- made up of iron
3. Heat exchangers
Uses of bad conductors of heat
1. Handles of appliances and utensils- made up of woods or plastics
2. Table mats- usually made up of cork
3. Sawdust- used to cover ice blocks to reduce melting
4. Wooden ladles
5. Woolen clothes
6. Fibreglass, felt and expanded polystyrene foam
Common applications of convection
1. Electric kettles
2. Household hot water systems
3. Air conditioners
4. Refrigerators
Common applications of radiation
1. Teapots
2. The green house
3. Vacuum flasks
Chapter 7- Kinetic Model of Matter
The State of Matter
The state of water depends on its temperature and atmospheric pressure.
Example, water can be exist as ice when the temperature is below zero degree Celsius.
Therefore, substances like water can exist in three states of matter- solids, liquids and gases.
Solid has a fixed shape and volume, normally hard and rigid: a large force is needed to change its shape, high desity and incompressible.
Liquid has a fixed volume but does not have a fixed shape, high density and incompressible.
While gas has no fixed shape or volume, low density and compressible.
Kinectic model of matter
LIQUID - Particles are randomly arranged with the particles slightly further apart as compared to that of solid. Therefore, liquids have relatively high densities. Particles are free to move about but confined within the vessel containing it. It have attractive forces between particles. This explains why liquids have fixed volumes but will take the shape of vessels containing them.
GAS - Particles are very far apart.Particles are randomly arranged and will occupy any available space. Therefore, gases have very low densities. Particles have very little attraction between them and move about randomly at very high speeds. This explains why gases have no fixed volume and shape, and why they are highly compressible.
Random or irregular motion of smoke particles in air is called Brownian motion. Brownian motion only occurs in fluids. Fluid is any substance that has the ability to flow or has particles that can move freely.
Kinetic Model of Matter
From: EdiSon, 2 years ago
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