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Latent heat
The heat energy that is taken in or given out by a substance when it changes state is called latent heat. When a substance changes from solid to liquid, the latent heat involved is called the latent heat of fusion. When the substance changes from a liquid to a vapour, latent heat of vaporisation is involved.
The internal energy of a substance is the sum of the kinetic and potential energy of the particles. Temperature is a measure of the average kinetic energy of the particles. Watch the short video clip below and think about whether the average kinetic energy of the particles changes when the state changes or whether the potential energy of the particles changes.
changes_of_state.mp4Specific latent heat of vaporisation of water
Specific latent heat of vaporisation of water
Specific latent heat
Specific latent heat L is the thermal energy Q per unit mass m required to change the state of a substance at constant temperature.
The aim of this experiment is to measure the specific latent heat of vaporisation of water by measuring the mass of water that boils off in a certain time.
so:
The units of specific latent heat are
[J kg-1]
Apparatus
electronic balance, 300 W immersion heater, 1 litre beaker, stopclock, thermometer, LabQuest2
Instructions
Set up the apparatus as shown above.
Record the power rating of the immersion heater.
Switch the immersion heater on and allow the water to boil - when the temperature does not change any more
At this point note the balance reading and start the stopclock.
Stop the clock after a certain time and record the new balance reading.
Record the mass of water that has boiled off.
Calculate the amount of thermal energy required to evaporate this mass of water using E = Pt.
Calculate the specific latent heat of vaporisation of water using Q = mL.
Specific latent heat of fusion of water
Specific latent heat of fusion of water
The aim of this experiment is to measure the specific latent heat of fusion of water by measuring the mass of ice that melts in a certain time.
Apparatus
electronic balance, electric heater, two funnels, two small beakers, stopclock, power supply, wattmeter
Instructions
Set up the apparatus as shown above.
In addition set up another funnel and beaker but without the heater.
Try to ensure that the heater is surrounded by the ice - crushing the ice into smaller pieces may help.
Switch the heater on, start the stop-clock and allow the ice to melt.
At the same time put a cube of ice into the other funnel - this will melt due to room temperature.
Determine the power usage based on the Voltage and Current readings per trial
Stop the clock after a certain time and measure the mass of water collected in the beaker.
Compare this with the mass of water that melted due to room temperature.
Record the mass of water that melted due to the heater only.
Calculate the amount of thermal energy required to melt this mass of water using E = Pt.
Calculate the specific latent heat of fusion of water using Q = mL.
The rating of the heater was 300 W, determine the average efficiency of the heater in this scenario.
Run the simulation several times.
Latent Heat is often considered a statistical science, how does the simulation support this idea?
Practice Problems and Notes:
Practice Problems and Notes:
2016 Topic 3 Notes 3.1.pdf2016_Topic_3_Problems_3.1.pdf2016_Topic_3_Answers_3.1.pdfSample Problem:
Sample Problem:
Melting an Iceberg:
Melting an Iceberg:
In 1986, a gargantuan iceberg broke away from the Ross Ice Shelf in Antarctica. It was approximately a rectangle 160 km long, 40.0 km wide, and 250 m thick.
(a) Calculate the mass of this iceberg, given that the density of ice is 917 kg/m^3? 1.47 x 10^15 kg
(b) How much heat transfer (in joules) is needed to melt it? 4.90 x 10^20 J
(c) Determine how many years would it take sunlight alone to melt ice this thick, if the ice absorbs an average of 100 W/m^2, 12.00 h per day? 48.5 y
Extinguishing an Oil Fire
Extinguishing an Oil Fire
(a) It is difficult to extinguish a fire on a crude oil tanker, because each liter of crude oil releases 2.80×10^7J of energy when burned. To illustrate this difficulty, calculate the number of liters of water that must be expended to absorb the energy released by burning 1.00 L of crude oil, if the water has its temperature raised from 20.0ºC to 100ºC, it boils, and the resulting steam is raised to 300ºC. (b) Discuss additional complications caused by the fact that crude oil has a smaller density than water. a) 9.67 L b) Crude oil is less dense than water, so it floats on top of the water, thereby exposing it to the oxygen in the air, which it uses to burn. Also, if the water is under the oil, it is less able to absorb the heat generated by the oil
Melting Ice:
Melting Ice:
The formation of condensation on a glass of ice water causes the ice to melt faster than it would otherwise. If 8.00 g of condensation forms on a glass containing both water and 200 g of ice, how many grams of the ice will melt as a result? Assume no other heat transfer occurs. 54.1 g
Rubbing Your Hands to Stay Warm:
Rubbing Your Hands to Stay Warm:
Rubbing your hands together warms them by converting work into thermal energy. If a woman rubs her hands back and forth for a total of 20 rubs, at a distance of 7.50 cm per rub, and with an average frictional force of 40.0 N, what is the temperature increase? The mass of tissues warmed is only 0.100 kg, mostly in the palms and fingers, the human body has a specific heat similar to that to water. 0.143˚C
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