Evaporation
Difference between boiling and evaporation
Evaporation
- Occurs at any temperature
- Slow process
- Takes place only at liquid surface
- No bubbles formed in liquid
- Temperature may change
- Thermal energy supplied by surroundings
boiling
- Occurs at fixed temperature.
- Quick process
- Takes place throughout liquid
- Bubbles formed in liquid
- Temperature remains constant
- Thermal energy supplied by energy source
Evaporation causes cooling
Evaporation requires thermal energy from the surrounding. Thermal energy from our body helps the water on the skin to evaporate. When it evaporates, it takes away the thermal energy from the surface of the skin, thus causing the body to feel cold.
How does evaporation occur?
Molecules of liquid are always moving randomly at different speeds. At the liquid surface, the more energetic molecules are able to overcome the downward attractive forces of other molecules and escape into the atmosphere. The liquid left behind are the less energetic molecules. The liquid with slower moving molecules is cooler.
Applications of evaporation
1) Experiencing cooling effect on skin when perspire evaporates, or when you apply perfume.
2) Wet clothes or puddles of water dry up after awhile.
3) A person with fever, sponged with water. When water evaporates, the temperature of the person will go down.
4) Refrigerator uses principle of condensation and evaporation, to keep the contents cold.
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
Tuesday, September 2, 2008
Continue from chp 9
Melting and Solidification
How does solid melt?
Molecules in a solid are held by strong intermolecular bonds. For solid to melt, these bonds have to be broken. The thermal energy supplied will break the intermolecular bonds between the molecules of the solid. When the intermolecular bonds are broken, the molecules can move out of their fixed positions.
Solidification and freezing point:
It is the reverse of melting. Pure substance will solidify/ freeze at temperature which is equal to its melting point, 0°c (Freezing point of water). Melting point is also the freezing point.
Boiling and Condensation
When a liquid is heated and changes into vapour at a fixed or constant temperature, its change of state is boiling. Reverse of boiling is condensation. It is the change in state from vapour to liquid when it is cooled at constant temperature, thermal energy is given out during condensation.
How does a liquid boil?
At boiling point, thermal energy is supplied to the liquid, but there is no rise in the temperature. Thermal energy is used to separate the molecules as well as to push back the surrounding atmosphere.The molecules will then be further apart with little intermolecular forces of attraction between them, causing a change of state.
How does solid melt?
Molecules in a solid are held by strong intermolecular bonds. For solid to melt, these bonds have to be broken. The thermal energy supplied will break the intermolecular bonds between the molecules of the solid. When the intermolecular bonds are broken, the molecules can move out of their fixed positions.
Solidification and freezing point:
It is the reverse of melting. Pure substance will solidify/ freeze at temperature which is equal to its melting point, 0°c (Freezing point of water). Melting point is also the freezing point.
Boiling and Condensation
When a liquid is heated and changes into vapour at a fixed or constant temperature, its change of state is boiling. Reverse of boiling is condensation. It is the change in state from vapour to liquid when it is cooled at constant temperature, thermal energy is given out during condensation.
How does a liquid boil?
At boiling point, thermal energy is supplied to the liquid, but there is no rise in the temperature. Thermal energy is used to separate the molecules as well as to push back the surrounding atmosphere.The molecules will then be further apart with little intermolecular forces of attraction between them, causing a change of state.
Chapter 9
This chapter is about the thermal properties of matter.
What is internal energy?
The total energy form when a solid vibrate about fixed positions, held by strong intermolecular bonds. It comprises of two components, kinetic energy and potential energy.
Kinetic energy:
Cause by the vibration of the particles. The higher the temperature the more vigorous the vibrations of the particles.
Potential energy:
It is due to the stretching and compressing of the intermolecular bonds when the particles vibrate.
What is internal energy?
The total energy form when a solid vibrate about fixed positions, held by strong intermolecular bonds. It comprises of two components, kinetic energy and potential energy.
Kinetic energy:
Cause by the vibration of the particles. The higher the temperature the more vigorous the vibrations of the particles.
Potential energy:
It is due to the stretching and compressing of the intermolecular bonds when the particles vibrate.
Continue from chp 8
Radiation
What is radiation?
Definition:
Radiation is the continual emission of infrared waves from the surface of all bodies, transmitted without the aid of a medium.
Absorption of infrared radiation:
Infrared radiation is absorbed by all objects and surfaces, especially things with dull, black surfaces, and causing a rise in temperature.
Emission of infrared radiation:
It is emitted by all objects and surfaces, causing temperature to fall.
Factors affecting rate of infrared radiation:
1) Colour and texture of the surface
2) Surface temperature
3) Surface area
Common applications of radiation
1) Teapot
2) Greenhouse
3) Vacuum flasks (Use conduction, convection, radiation and evaporation)
What is radiation?
Definition:
Radiation is the continual emission of infrared waves from the surface of all bodies, transmitted without the aid of a medium.
Absorption of infrared radiation:
Infrared radiation is absorbed by all objects and surfaces, especially things with dull, black surfaces, and causing a rise in temperature.
Emission of infrared radiation:
It is emitted by all objects and surfaces, causing temperature to fall.
Factors affecting rate of infrared radiation:
1) Colour and texture of the surface
2) Surface temperature
3) Surface area
Common applications of radiation
1) Teapot
2) Greenhouse
3) Vacuum flasks (Use conduction, convection, radiation and evaporation)
Continue from chp 8
Convection
What is convection?
The transfer of thermal energy by means of currents in a fluid, liquids or gases.
How does convection work?
Definition:
Convection currents occur only in fluids such as liquids and gases as it involves the bulk movement of the fluids which carry thermal energy with them.
When the flask containing water is heated, it expands. The expanded water which is the hot part is less dense than the surrounding water, therefore it will rise up. The cooler part of the water at the upper part of the flask will sink as they are denser. The same principle applies for air.
Common applications of convection
1) Electric kettles
2) Household hot water system
3) Air conditioners
4)Refrigerators
What is convection?
The transfer of thermal energy by means of currents in a fluid, liquids or gases.
How does convection work?
Definition:
Convection currents occur only in fluids such as liquids and gases as it involves the bulk movement of the fluids which carry thermal energy with them.
When the flask containing water is heated, it expands. The expanded water which is the hot part is less dense than the surrounding water, therefore it will rise up. The cooler part of the water at the upper part of the flask will sink as they are denser. The same principle applies for air.
Common applications of convection
1) Electric kettles
2) Household hot water system
3) Air conditioners
4)Refrigerators
Monday, September 1, 2008
Chapter 8
This chapter is about the transfer of thermal energy. Thermal energy always flows from a region of higher temperature to a region of lower temperature.
Transfer of Thermal energy:
- conduction
- convection
- radiation
Conduction
I learnt 2 important ponts of it :
1) Thermal energy flows through the medium itself. The transfer of thermal energy without any movement of the material medium is called conduction.
2)Different materials conduct heat at different rate.
How does conduction work in a solid?
Metals contain many free electrons which move randomly between the atoms. When thermal energy is supplied to one end of it, the particles at the hot end will vibrate vigorously. It will then collide with the neighbouring particles, making them vibrate as well.
The free electrons in it gain kinetic energy and thus move faster. These fast moving electrons will then spread to the cooler part of the metal, colliding with the atoms and transfering the kinetic energy to them.
Conduction in liquids and gases?
The particles in liquids and gases are spaced further apart than in solid, causing collisions between particles to be less frequent. Thus, gas is a poor conductor of heat and solid is a good conductor of heat, liquid being in between.
Common applications of conduction
Uses of good conductors of heat
1) Cooking utensils
2) Soldering iron rods
3) Heat exchangers
Uses of bad conductors of heat
1) Handles of appliances and utensils
2) Table mats
3) Sawdust
4) Wooden ladles
5) Woollen clothes
6) Fibreglass, felt and expanded polystyrene foam
Transfer of Thermal energy:
- conduction
- convection
- radiation
Conduction
I learnt 2 important ponts of it :
1) Thermal energy flows through the medium itself. The transfer of thermal energy without any movement of the material medium is called conduction.
2)Different materials conduct heat at different rate.
How does conduction work in a solid?
Metals contain many free electrons which move randomly between the atoms. When thermal energy is supplied to one end of it, the particles at the hot end will vibrate vigorously. It will then collide with the neighbouring particles, making them vibrate as well.
The free electrons in it gain kinetic energy and thus move faster. These fast moving electrons will then spread to the cooler part of the metal, colliding with the atoms and transfering the kinetic energy to them.
Conduction in liquids and gases?
The particles in liquids and gases are spaced further apart than in solid, causing collisions between particles to be less frequent. Thus, gas is a poor conductor of heat and solid is a good conductor of heat, liquid being in between.
Common applications of conduction
Uses of good conductors of heat
1) Cooking utensils
2) Soldering iron rods
3) Heat exchangers
Uses of bad conductors of heat
1) Handles of appliances and utensils
2) Table mats
3) Sawdust
4) Wooden ladles
5) Woollen clothes
6) Fibreglass, felt and expanded polystyrene foam
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