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HEAT, EXAMPLES, Superficial expansion in solids, cubical expansion,…
HEAT
earlier thought as a fluid
definition- energy that is transferred from a body at higher temperature to a body at a lower temperature.
transfer can be done by:
1- conduction
2- convection
3- radiation
causes-
1- change in temperature
2- expansion
3- change in state
NOTE-
matter does not contain heat but it contains internal energy in the form of molecular kinetic energy and PE.
increasing internal energy is called heating
decreasing internal energy is called cooling
Internal energy- KE + PE
change in avg. KE- change in temp.
change in PE- change in state/ phase
TEMP- measure of avg. KE of all molecules (total energy divided by no. of molecules)
since avg. measurement- hence does not depend on size
hot object- rapid movement
cool object- slow movement
heat flows from hot to cool
hot- slow down
cool- speed up
until same temp.- thermal equilibrium
no change in avg. KE but change in PE when it is at a same temp.
EXAMPLES
lids of jam jars get loosened in water, it expands and becomes easy to remove
bridges, roads and railway lines- made in sections and separated by rubber strips
to allow expansion and contraction in summer and winter.
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pendulum of a wall clock and wristwatch made of invar - very low coefficient of expansion
expansion- loosing time
contraction- gaining time
mercury in thermometers- large coefficient of expansion and hence rises up
laboratory thermometer- ethanol
temp. can be measured when it rises through the capillary
Bi-metallic strip- made of brass and iron/ steel and copper, brass and copper
used in thermostat switches to control heat in refrigerators and ovens
when heated- it bends and thus breaks the connection.
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incandescent bulb- wire should not have the same rate of expansion so that vacuum of the bulb is preserved
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EVAPORATION
change of state from liquids to gas that takes place at the surface of the liquid.
occurs at temp. below BP
like- wet ground
wet clothes etc.
reduces volume of liquid or water
energy is used to break the bonds that hold liquid molecules together.
variety of speeds and hence no same energy
collide with each other and therefore some gain energy and some lose energy.
surface of the liquid gain enough KINETIC energy to escape.
evaporation occurs with molecules of liquid that have greater KE.
Decreases avg. temp. of molecules left behind. and hence cools the liquid.
all about energy of individual molecules and not avg. energy of the system.
FACTORS AFFECTING:
1- area of exposed surface- more
more- evaporation
hence spread the clothes for drying
2- Temp. of surroundings-
more
more- evaporation
hence- fast drying in summer.
3- Temp. of the liquid- more the initial temp.
more- evaporation
hence feel cold after a hot bath
4- presence of water vapor in the air- more
less- evaporation
hence slow drying in rainy season.
5- wind- more
more- evaporation
hence fast drying on a windy day than on normal day.
6- nature of liquid- certain liquid evaporate more rapidly compared to other liquids at the same temp.
eg- petrol evaporates more rapidly than water.
energy needed for water molecule to turn into vapor is the same in case of boiling and evaporation.
high temp.- less energy needed as they already have high KE
and vice versa energy is taken continuously from the surroundings and hence it becomes cool.
APPLICATIONS-
Cold water on floor, switch the fan on and hence the water evaporates since it uses heat from the surroundings.
perspirations- sweat cools the skin a
nd the blood underneath and helps maintain a cool temp.
fever- cloth dipped in water or vinegar can help as when is evaporates it takes heat from the body. and helps to reduce the body temp.
perfume on hand- cooling effect because of evaporation.
Refrigerator- combination of 2 principles- evaporation and condensation.
special liquid- refrigerant i
s pumped through a series of tubes
passed around and hence absorbs heat from inside and becomes a gas
in condenser- gas to liquid and hence releases stored energy to the outside and the cycle continues.
THERMAL EXPANSION
tendency of matter to change in shape, area and volume in response to a change in temp.
rise in temp. increases energy of atoms and avg. distance between atoms increases.
hence matter as a whole expands
least in case of solids
most- gases
because of intermolecular force
solids-
1 dimension
linear expansion
2 dimensions
superficial expansion
3 dimensions
cubical expansion
liquids and gases- volume expansion
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BOILING
definition- rapid change of state from liquid to vapor at a particular temp.
accompanied by formation of bubbles
takes place throughout the liquid.
heat breaks the bonds between liquid molecules and hence start to break
increases PE and not KE of the liquid.
as a result temp. of the liquid remains the same and break away to the surrounding space.
BP is unique for each pure substance
BP- the temp. at which a liquid boils
factors affecting-
atmospheric pressure and impurities
water boils at a lower temp. at a higher altitude where atmospheric pressure is less
we cannot eat boiled egg at mt. Everest
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