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It is a flask with two silvered walls enclosing a vacuum. It is used for keeping contents at a fairly constant temperature.
-A thermos flask also called vacuum flask keeps hot liquids hot and cold liquids cold.
-This is because heat losses are minimized. There are three ways by which heat can be lost namely: Conduction, convection and radiation.
Heat losses by the above ways are minimized by the vacuum flask as follows:
-Conduction and convection are minimized by the vacuum since for heat to be transferred by these ways, a material medium is required.
-Convection from the hot liquid upward to the outside is reduced by the cork which also reduces heat loses by conduction because it is a poor conductor of heat.
-Radiation is also minimized by the two silvered surfaces since they are bad emitters.
However when a hot liquid is kept in the vacuum flask for a long time, it cools because at a small rate, heat is lost by conduction, convection and radiation.
Choice of dress
The choice of dress one puts on depends on conditions of the environment. On hot days, a white dress is preferable because it reflects most of the heat radiations falling on it.
On cold days a dull black woolen dress is preferred because it absorbs most of the heat incident on it and can retain for a longer time.
Thermal explanation of matter
This is the increase in size of matter in all directions whenever matter is heated.
- Expansion of solids.
Expansion of solids can be illustrated using a metal ball with a ring as shown below.
-The metal ball passes through the ring when it is cold, but when heated, the ball doesn’t pass through the ring any more, showing that it has expanded.
-It passes through the hole again when it cools, meaning that the metal contracts when it loses heat.
Bi- metallic strip
Different metals expand at different rates when equally heated;
This can be shown using a metal strip made of two metals such as copper and iron bounded tightly together (bi- metallic strip) when the bi metallic strip is heated, the copper expands more than iron and the strip bends as shown.
-When the bimetallic strip of iron and Brass is heated, it bends with brass on the outside of the curve.
-This is because Brass expands more than iron.
Uses of a metallic strip (application of expansion of solids)
Bimetallic strips are useful in the following devices by completing the metallic circuit.
i) Ringing alarm bells
ii) Thermostats
a) Fire alarm
Heat from the source makes the bi metallic strip bend and completes the electric circuit and the bell rings.
b) Thermostat
This is a device that makes temperature of appliances or room constant. The thermostat shown below uses a bi metallic strip in the heating circuit of a flat iron.
How an electric Iron works
-Setting the temperature: The control knob is set to the required temperature.
-Bimetallic strip heating: On reaching the required temperature, the bimetallic strip bends away breaking the circuit at contact C. This switches off the heater.
-Cooling bimetallic strip: On cooling just below the required temperature, the bimetallic strip makes contact and switches on the heater again. So a nearly steady temperature results.
-Knob: If the control knob is screwed more, the bimetallic strip has to bend more in order to break the heating circuit thus giving a high temperature
Disadvantage of expansion of expansion in our every day life
Steel bridges
Bridges are constructed with one end fixed and the other side is placed on rollers in order for the structure to expand or contract freely with changing temperature without damaging the bridge.
Railways:
Railway lines are constructed with gaps between the consecutive rails in order to allow free expansion of the rails is the temperature increases.
If no gaps are left between rails, the rails buckle during a hot day.
If no gap is left in the rails, they bend on hot days.
Cold day Hot day
Electricity Transmission cables
The wires which are used for the transmission of electricity or telephone wires are usually left sagging in order to allow them free expansion and contraction.
Linear expansivity
Linear expansivity of a material is the fraction of its original length by which it expands per Kelvin rise in temperature.
Examples:
- In an experiment to measure linear expansivity of a metal, a rod of this metal is 800mm long is found to expand 1.36mm when the temperature rise from 15oc to 100oc.
- Expansion of liquids
Liquids expand when they are heated. Different liquids expand by different amount when equally heated.
Liquids expand much more than solids because according to the kinetic theory, liquid molecules are far apart compared to the solids and the intermolecular forces are weaker in liquids
Experiment to demonstrate expansion of water.
-Fill the flask completely with coloured water. Pass the narrow tube through the hole of the cork and fix the cork tightly to the flask.
-Note the first level of water on a narrow tube
-Heat the bottom of the flask and observe the new level of water on the capillary tube. Initially there will be a momentary drop of the water level in the tube then after wards the level rises.
-Therefore liquids expand when heated since there was a rise in the levels of water in the capillary tube.
Explanation
-When the flask is heated, the flask first receives heat before the water in it so the flask expands and its volume increases causing the slight fall in level. -However, when heat reaches the water, the volume of water expands more than the increase in volume of the flask.
Comparing expansion of different liquids
Three identical flasks A, B and C are filled with alcohol, kerosene and water respectively.
-Fit a narrow capillary tube in each flask through the cork, cool flasks to the same temperature, adjust the levels such that they are equal and mark the original levels.
-Place the flasks in a trough of hot water
-After some time, the liquid levels rise to different levels. This shows that different liquids expand differently when heated through the same temperature range.
Liquid C expands more than B and more than A in that order.
Application of expansion property of liquids
This property is used in thermometer; the liquids used include alcohol and mercury.
Anomalous expansion of water
(Unusual expansion of water)
For all solids expect ice, when heated, they melt to form liquids. They expand just after melting but ice which melts at 0oc to form water contracts until 4oc. Water is thus exceptional or anomalous in the range 0oc to 4oc.
Sketch of volume against temperature.
From the sketch, it is noted that water has its minimum volume at 4oc
AB: As the temperature rises ice expands.
CB: The ice is melting to form water at 0oc
CD: As the temperature rises, the formed water at 0oc contracts until 4oc.
DE: At 4oc the water expands just like other liquids do.
Sketch of density against temperature.
-Since density is mass/volume but mass is unaltered by warming.
-It is only volume which decreases between 0oc to 4oc.
-It follows that water has its maximum density at 4oc. From the sketch it is noted that ice is less dense than water.
-This is because for any given mass at 0oc the volume of ice is greater than the volume of water. This is why ice is less dense than water.
-When ice is mixed with water, ice floats on water because when ice melts to form water, density increases as volume decreases until 4oc.Therefore, ice is less dense than water.
Note: During very cold weather, pipes of water burst because at 0oc when water freezes, considerable expansion occurs resulting in increase in volume.
Biological importance of abnormal expansion
The unusual expansion of water has some biological importance in the preserving of aquatic life during cold weather.
a) Water at the top cooling
During cool weather, water at the top of the sea cools first, contracts and being denser to the bottom. The warmer and less dense water rises to the surface to be cooled.
b) At 4oc
When all the water is at 4oc the circulation stops.
c) Temperature below 4oc
When the temperature of water surface falls below 4oc, it becomes less dense and remains at the top, eventually forming a layer of ice at 0oc.
The lower layer of water at 40c can only lose heat by conduction. So in deep water there will be always water beneath the ice in which fish and other aquatic life can thrive.
Explanation of unusual expansion of water by kinetic theory
-The expansion of water between 4oc and 0oc is due to the breaking up of groups of water molecule below 4oc and formation of groups of water molecules above 4oc which require a large volume. -So the anomalous expansion of water at 4oc is because water molecules bond together differently above and below 4oc.
Expansion of gases
A gas expands when heated almost 10,000 times more than solids.
The greater expansion of gasses is due to very weak intermolecular forces which can be broken easily.
Experiment to demonstrate expansion in gases
In the above set up the flask is slightly heated.
Air bubbles will be seen coming out from the other end of the tube
This shows that air expand when heated.
In the second set up, when the source of heat is removed and the flask is allowed to cool by pouring cold water, the level of water will rise. This shows that air contracts when cooled.
Alternatively
When hands are rubbed together thoroughly and held around the flask as shown above, bubbles of air start coming out of water. This is because the heat produced by the hands was enough to cause the air in the flask to expand. When the hands are removed and flask left to cool, the water rises in the tube. This is because cooling the air contracts and pressure of the inside are becomes less than the atmospheric pressure.
Application of expansion of air.
Hot air balloon
Expansion of air is used in hot air balloon. When air in the balloon is heated, it expands and becomes less dense and as a result the balloon rises up.