Ch#8 : Thermal Properties of Matter Flashcards
Learn about specific heat and thermal expansion.
Explain specific heat.
Generally when a body is heated , the temperature increases. Increase in temperature is found to be proportional to the amount of heat absorbed. It has also been observed that the Quantity of heat ∆Q required to raise temperature ∆T of the body is proportional to the mass m.
∆Q < m∆T
∆Q = mc∆T
∆Q is the amount of heat absorbed by the bodg , c is the constant of proportionality called specific heat.
Specific heat : The specific heat of a substance is the amount of heat required to raise the temperature of 1kg mass of that substance through 1K.
c = ∆Q/m ∆T
Si Unit : J/KgK
Why is the temperature variation smaller at places near the sea?
Specific heat of water is 4200J/KgK and that is dry soil is 810J/KgK . As a result , the temperature of soil would increase 5 times more than the same mass of water. So the temperature of land rises and falls quickly more rapidly than that of the sea. Hence the temperature variations from summer to winter are much smaller at places near the sea than those far away from the sea.
Uses of the large specific heat of water in automobiles and central heating systems. (Draw diagrams)
Water has a large specific heat capacity. For this reason it is very useful for storing and carrying unwanted thermal energy due to its high specific heat.
The cooling system of automobiles uses water to carry away unwanted thermal energy. In an automobile , a large amount of heat is produced by the engine due to which its temperature increases. The engine would cease if it isnt cooled down. Water circulating around the engine maintains its temperature. The water absorbs the unwanted thermal energy of the engine and dissipates it through the radiator.
In central heating systems , hot water is used to carry thermal energy through pipes from boilers to radiators. These radiators are fixed inside the building at suitable places.
Describe heat capacity of a body.
Heat Capacity : It is the quantity of thermal energy absorbed by a body for a one kelvin increase in its temperature.
If the temperature increases through ∆T by adding ∆Q amount of heat , the heat capacity will be ∆Q/∆T , putting the value of ∆Q , we get.
Heat capacity = mc∆T/∆T
Heat capacity = mc
SI unit : J/K
Describe thermal expansion.
Most substances , solids , liquids and gases expand on heating and contract on cooling. Their thermal expansions are usually small and not noticable. However , these expansions and contractions are not important in our daily life.
The kinetic energy of molecules depends on its temperature. Its molecules vibrate with larger amplitude at high temperature rather than low temperature. Thus , on heating the amplitude of the vibration of molecules increases. They push one another farther away as the amplitude of vibration increases. This results in an increase in length , breadth and thickness.
State and explain linear thermal explantion. State value of a for Brass , Copper and Steel.
It has been observed that solids expand on heating and that their expansion is almost uniform over a wide range of temperature. Consider a metal rod of length Lo at a certain temperature To. Let its length on heating to temperature T become L
Increase in length = ∆L = L - Lo
Increase in temperature = ∆T = T - To
Its found that a change in length ∆L is directly proportional to the original length and the change in temperature ∆T.
∆L > Lo ∆T
∆L = a Lo ∆T
L - Lo = a Lo ∆T
L = Lo(1 + a ∆T)
Where a is the cofficient of linear thermal expansion of the substance.
a = ∆L/Lo ∆T
We can define coefficient of lineat thermal expansion as a fraction increase in its length per kelvin rise in temperature. Si Unit : per kelvin.
Brass = 1.9 x 10^5 Copper = 1.7 x 10^5 Steel = 1.2 x 10^5
What are the consequences of thermal expansion? Also draw diagrams.
Gaps are left in railway tracks? The expansion of soilds may damage bridges , railways and roads as they are constantly subjected to temperature changes. So provision is made during contruction for the expansion and contraction with temperature. For example , railway tracks will be buckled on a hot summer day due to thermal expansion if gaps arent left between section.
Bridges made of steel girders also expand during the day and contract during the night. They will bend if the their ends are fixed. To allow thermal expansion. One end is fixed while the other rests on rollers in the gao left for expansion. Overhead transmission lines are given a certain amount of sag to prevent them from snapping in winter due to contraction.
What are the different applications of thermal expansion in our daily life?
Thermal expansion is used in our daily lives. In thermometers , thermal expansion is used in temperature measurements. To open the cap of a bottle that is tightly sealed , immerse it in hot water for a minute or so. The netal cap expands and becomes loose so it will be easy to open.
To join steel plates tightly together , red hot rivets are forced through the holes in the plates. The end of the hot rivet is hammered. On cooling , the rivets contract and bring the plates together with tight grip.
Iron rims are fixed on the wheels of wooden carts. The iron rims are first heated. The thermal expansion allows them to slip over the wooden wheel. Water is poured on them to cool them. The rims contract and become tight over the wheel.
Bimetal Strip: A bimetal strip consists of two thin strips of different metals such as brass and iron joined together. On heating , brass expands more than iron. This unequal expansion causes the strip to bend.
Bimetals strips are used for various purposes. Bimetal thermometers are used to measure temperature especially in furnaces and ovens. Bimetal strips are used to control of the temperature of the heater coil in an electric irob
State and explain volume thermal explantion. State relationshio between a and b.
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State and explain volume thermal explantion. State relationshio between a and b.
It has been observed that solids expand on heating and that their expansion is almost uniform over a wide range of temperature. Consider a solid of volume Vo at a certain temperature To. Let its volume on heating to temperature T become V
Increase in volume = ∆V = V - Vo
Increase in temperature = ∆T = T - To
Its found that a change in volume ∆L is directly proportional to the original volume and the change in temperature ∆T.
∆V > Vo ∆T
∆V = b Vo ∆T
v - Vo = b Vo ∆T
v = Vo(1 + b ∆T)
Where b is the cofficient of volume thermal expansion of the substance.
b = ∆V/Vo ∆T
Relation between a and b:
b = 3a
We can define coefficient of volume thermal expansion as a fraction increase in its volume per kelvin rise in temperature. Si Unit : per kelvin.
