This is my reflections on chapter 7, 8 and 9 of physics:)
chapter 7 (kinetic model of matter):
Matter is made of tiny particles either atoms or molecules in a continous, random motion. Matter exists in three states: solid, liquid or gas, in which Brownian motion is observed in liquids and gases.
Solids have closely packed and orderly arranged atoms or molecules that vibrate about in their fixed positions and they are held together by strong intermolecular bonds. THis arrangement and movement of particles causes solids to have high densities and fixed shapes and volumes.
Liquids have closely packed(but slightly further apart than the particles of solids) atoms and molecules that are not arranged in an orderly pattern. They are free to move about between clusters and have attractive forces between particles to particles. As a result of their arrangement and movement, liquids have high densities, fixed volumes and does not have fix shapes.
Gases contains atoms and molecules which are far apart from each other. The particles have weak attractive forces between one another thus, they move about in random at high speeds. Their arrangement and movement causes gases to have low densities, no fixed shapes and volumes and can be easily compressed.
The random and irregular motion of particles in gases and liquids is known as Brownian motion. Brownian motion can only happen in fluids that has the ability to flow or has particles that move freely. As the temperature increases, the molecular motion increases as well. This is because air molecules have greater speeds at higher temperatures. As the temperature increases, more thermal energy is converted to kinetic energy of these air molecules. Therefore, the molecules move faster.
Chapter 8(transfer of thermal energy) :
Thermal energy is transferred only when there is a difference in temperature. Thermal energy always flows from a region of higher temperature to lower temperature. When two bodies reaches thermal equilibrum, there is no net gain or loss of thermal energy between them. Thermal energy is transferred by either of the three processes: conduction, convection and radiation.
Conduction is the process of thermal energy transfer without any flow of the material medium. For example, when one end of the rod is heated, the other end will also feel hot after some time. When thermal energy is given to one side of the rod, the particles starts vibrating vigorously and hit onto neighbouring particles, making them vibrate as well. Thus, the kinetic energy of the particles are transferred. Metals contain free electrons which moves about freely while non-metals do not have. The free electron diffusion is a much faster mechanism of thermal energy transfer as the free electrons in metals gain kinetic energy and and move faster. These fast moving electrons then diffuse or spread to cooler parts of the metal, they also transfer kinetic energy when they hit other atoms. The process of conduction in liquids and gases is not efficient. The paricles in liquids and gases are further away from each other thus, the collision of atoms are less frequent. The transfer of kinetic energy thus becomes slower which is why liquid and gas are poor conducters of heat.
Convection is the tranfer of thermal energy by means of currents in a fluid(liquid or gases.Convection involves the bulk movements of the fluids which carry thermal energy with them. For example, when the water at the bottom of a flask is heated, it expands and becomes less dense than the other water and starts to rise. The cooler regions of the water sinks due to its higher density. This movement of the liquid due to a difference in densities sets up a convection current. This applies to gases as well.
Radiation is the continual emmision of infared waves from the surface of all bodies, transmitted without the aid of a medium. Radiation does not require a medium for energy transfer, it takes place in a vacuum. For example, the sun reaches the Earth through radiation. Thermal energy from infared waves is called radiant heat. The darker a surface is, the better it absorbs infared radiation. The higher the temperature of the surface of the object relative to the surrounding temperature, the higher the rate of infared radiation. Objects with larger surface area will emit infared radiation at a higher rate as well.
Common applications of conduction are:
1. cooking utensils
2.soldering iron rods
3.heat exchangers
common applications of convection are:
1. electric kettles
2. houshold hot water systems
3. air conditioners
4. refrigerators
common applications of radiation are:
1. teapots
2. greenhouse
3. vacuum flasks
chapter 9(thermal properties of matter):
Heat is the amount of thermal energy that flows from a hotter region to a cooler region. The SI unit of thermal energy is joule(J). When a beaker of water is heated with a Bunsen Burner, the flame provides thermal energy to the water and the thermal energy leads to a gain in the internal energy of the water molecules. Internal energy is made up of kinetic energy and potential energy.
The kinetic component of internal energy is due to the vibration of the particles. It is directly related to temperature. The higher the temperature, the more vigorously the particles vibrate. The potential component of internal energy is due to the stretching and compressing of the intermoleculer bonds as the particles vibrate. The amount of potential energy stored in the bonds depends on the force between the particles and how far apart the particles are.
When a solid changes to a liquid upon heating, the state is called melting. Melting occurs at a definite and constant temperature. The molecules in a solid are held by strong intermolecular bonds. For a solid to melt, these bonds must be broken. The thermal energy given is used to break these bonds. When the intermolecular bonds are broken, the molecules are able to move freely. The solid id now melted and change its state form solid to liquid. There is also no change in temperature during this process.
The reverse process of melting is solidification, where liquid is changed to solid. When liquid molecules come together and solidify, the intermolecular bonds formed and thermal energy is released. There is no temperature change in this process also. Temperature changes only when the substance is cooled within the same state.
When a liquid is heated and changed to vapour at a fixed temperature, this change is called boling. The reverse of boiling is condensation. It changes the state of vapour to liquid when a substance is cooled at the same constant temperature as boiling. Thermal energy is given out during condensation. The thermal energy is used to seperate the molecules as well as to push back on the surrounding atmosphere. The molecules are now further apart with weak attractive forces and the liquid changes its state to gaseous.
Evaporation also changes liquid to gaseous state. However, evaporation is different from boiling as evaporation can occur at any temperature. Evaporation also requires thermal energy from the surrounding unlike boiling where it gains thermal energy from an energy source. The molecules of liquid are always moving randomly at different speeds. Thermal energy are absorbed by the liquid from the surroundings. At liquid surface, the more energetic molecules are able to overcome the downward attractive forces of other molecules and escape into the atmosphere, leaving the less energetic molecules behind. A liquid with slower moving molecules is cooler as the temperature of the liquid is directly proportional to the average kinetic energy of molecules.
The rate of evaporation depends on the temperature- the higher the temperature, the faster evaporation occurs, the humidity in the surrounding air- the lower the humidity, the faster the rate of evaporation, the surface area of the liquid- the larger the surface area, the more molecules escape from the liquid, the movement of air- the rate of evaporation increases when the surrounding air is moving, pressure- the lower the atmospheric pressure, the faster the rate of evaporation and the boiling point of liquid- liquids with lower boiling point will evaporate faster.
physics 