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Physics chapter 10(thermal physics) - Coggle Diagram
Physics chapter 10(thermal physics)
internal energy
It is an energy store that is made up of the total kinetic energy associated with the random motion of the particles and the total potential energy between the particles in the system
Heat capacity and specific heat capacity
Different bodies or objects have different capacities to store internal energy
Number of particles:When there are more particles,more energy can be stored.Thus,larger objects tend to have larger heat capacities
Nature and strength of intermolecular forces:
A stiffer spring(which represents a stronger force between particles) allows more energy to be stored for the same extension or compression.
Intermolecular forces:They are the attractions between the molecules which may determine the many physical properties of a substance
Heat capacity of an object is the change in the amount of its internal energy per unti change in its temperature
Formula for heat capacity:C=Q/triangle0
Q=Amount of energy transferred(joules) to or from the internal store of the object by heating
SI unit of heat capacity is joule per kelvin or joule per degree celcius
Eg:An energy transfer of 460j by heating is needed to raise the temperature of the 500g iron cube by 2k or 2 degree celcius
This means its heat capacity is 230J/K(460J divide by 2 degree celcius
Iron specific heat capacity(The heat capacity per unit mass is called specific heat capacity)For the iron cube-> c=230J/K divide by 500g=0.46J/(gK).Specific heat capacity is more useful than heat capacity
Specific heat mass of a material per gram such as iron can be used to calculate the heat capacity of any mass of iron.For eg:Heat capacity of a 600g iron cube=0.46J/(K g)x600g=276J/K
Specific heat capacity of a material is the change in amount of its internal energy per unit mass for each unit change in its temperature
Heat capacity is seen as a property of an object that can consist of parts made of different materials.Whereas, specific heat capacity is seen as the property of a uniform material
The SI unit of specific heat capacity is joule per kilogram per kelvin
Some examples of heat capacity applied in daily life
Even though dry sand and seawater receive the same amount of energy(joules) everyday, sand always feel hotter than seawater
The heat capacity of sand is lower than seawater although the amount of energy and change in temperature is the same, the specific heat capacity of sand is lower than seawater so its temperature will rise faster than seawater
Why does the land's temperature drop faster than seawater during night time to develop land breeze
At night, without the sun's radiation, land and seawater transfer energy to the surroundings by conduction,convection and radiation
The land cools faster than seawater because it has a lower specific heat capacity.Additionally, land has a higher temperature difference with surroundings as compared to seawater at sunset
Hence, energy is transferred from the land to the surroundings faster than from the seawater to surroundings
Calculation involving heat capacity and specific heat capacity:
The formula can be rewritten for heat capacity and specific heat capacity.Thus the energy transferred by heating to an object of heat capacity so that the object has a temperature change triangle0
What are the processes that involve a change of state
Atmospheric pressure is lower at higher altitudes.This reults in a lower boiling point of water. An increased pressure above the hot water surface will make it more difficult for steam to form and escape
This will raise the boiling point of water so cooking food in a pressure cooker is much faster
Freeze drying is a method offered in modern technology
The method works by freezing the food and creating a low-pressure partial vacuum.Th frozen water in the food will directly turn into vapour and be removed from the food.Nutrional values of the food remain unchanged
Boiling and evaporation
Involves vaporisation(change from a liquid to a gas).During boiling, bubbles are seen throughout the liquid.The boiling point remains constant assuming that there is no change in atmospheric pressure
Particles in liquid move at different speeds
Evaporation occurs when the faster particles at the surface of the liquid have enough energy to break away from the other liquid particles to escape into the air(assuming that particles dont collide with fast moving air particles and move back into liquid)
After the faster particles have escaped into the air, the particles left behind have a slightly lower average speed.As temperature decreases with average kinetic energy of the particles in a body, the liquid is slightly cooler than the surroundings
Factors affecting rate of evaporation
Atmospheric pressure
When the pressure is higher, the rate of evaporation is lower.At a higher pressure liquid molecules escape into the air less quickly, it helps to retain the heat in a volume of liquid
Humidity of air
Humidity is a measure of the amount of water vapoour in the air.When the humidity is high,there is a lot of water vapour and the rate of evaporation is lowered
Surface area of liquid
When the exposed surface area of the liquid is higher, the rate of evaporation is higher.This is because more molecules can escape from the surface of the liquid
Temperature
When the temperature is higher,rate of evaporation is higher.At a higher temperature,the average kinetic energy of the liquid molecules is higher.More liquid particles can escape into the air
Wind speed
Wind removes the molecules that have just escaped into the air.Thus the air surrounding the liquid is drier.Hence when the wind speed is higher,the rate of evaporation is higher
Boiling point of liquid
A liquid with a lower boiling point evaporates more quickly than 1 with a higher boiling point under similar conditions.This is because the attractive forces between the particles of the liquid with a lower boiling point are weaker
Latent heat
Latent heat is the energy released or absorbed to change the state of a substance,at constant temperature
There are 2 types of latent heat
For melting and solidification,there is the latent heat of fusion.For boiling and condensation,there is the latent heat of vaporisation
SI unit for latent heat is joules
Latent heat of fusion
When a substance is melting, energy is transferred to the substance through work done against the attractive forces between particles
There is an increase in the potential energy of the particles but the kinetic energy of the particles remain unchanged
When solidifying,energy is transferred out of the substance which leads to a decrease in the potential energy of the particles but kinetic energy of the particles remains unchanged
Hence, there is no temperature change when a substance is solidifying or melting
Latent heat of fusion:It is the amount of energy transferred to change a substance between the solid and liquid states,at constant temperature
Specific latent heat of fusion
The mass of a substance affects the latent heat.A large cup of ice cream has a greater mass and takes a longer time to melt than a small cup of ice cream.
It is said that the large cup of ice cream has a higher latent heat of fusion than the small cup of ice cream
Specific latent heat of fusion:It is the amount of energy transferred per unit mass of a substance to change between the solif and liquid states,at constant temperatures
Formula for latent heat of fusion:Lf=lf x m
Lf=Latent heat of fusion
lf=specific latent heat of fusion
m=mass of substance
Latent heat of vaporisation
When a substance is boiling,energy is transferred to the substance through work done against the attractive forces between particles.
There is an increase in the potential energy of particles but the kinetic energy of particles remain unchanged
It is the amount of energy transferred to change a substance between the liquid and gaseous states,at constant temperature
Specific latent heat of vaporisation is the amount of transferred per unti mass of a substance to change it between liquid and gaseous states,at constant temperatures
Example of specific latent heat of fusion
When the ice cubes start to melt, the heater is turned on and the reading on the electronic balance is 30g.After 2min,the reading on the weighing scale is 68g,The heater supplies energy at a rate of 100W
Why must the ice be melting before starting the measurement
When the ice cubes are just taken out of the freezer, their temperature is below 0 and thus has not reached the melting point(the kinetic energy of water molecules still remain the same even though its losing its kinetic potential energy so it must be melting first in order for the kinetic energy to have a constant decrease in energy or temperature)
Find the specific latent heat of fusion of ice
Given that the mass of ice cubes m=30g, let the specific latent heat of fusion of water be lf.
Assuming that the energy is transferred to melt the ice only comes from the heater
Energy from heater=m x lf
Power x time=(0.068kg-0.030kg) x lf
100W x (2x60)s = 0.038 x lf
lf=316kJ/kg
Other than the heater, state another source that transfers energy to ice cubes
The surrounding warm air
Explain how measurements can be made more accurate
The ice cubes can be crushed so that there is a greater surface area in contact with the heater.This will reduce the amount of energy transferred to the air instead of ice cubes
In addition,the top of the funnel can be covered with a plastic sheet to reduce the energy transfer from the heater to the air
The sides of the funnel can be wrapped with an insulating material to reduce energy transfer from surroundings to ice cubes by conduction