HEAT PART -2
Real and apparent expansion of liquids:
A liquid is heated in a container. Heat flows through the container to the liquid. This means
that the container expands first, due to which the level of the liquid falls.
When the liquid gets heated, it expands more and
beyond its original level. We cannot observe the
intermediate state. We can only observe the initial and
the final levels. This observed expansion of the liquid is
known as the apparent expansion of the liquid and is
less than its actual expansion.
On heating, the expansion of liquid appears to be from
L1to L3. This is called the apparent expansion of liquid.
But actually the liquid on heating has expanded from L2
to L3. This is called the real expansion of the liquid.
The real expansion of the liquid is always more than the
apparent expansion of the liquid.
The fall in the level of the liquid from L1 to L2 is
actually equal to the increase in the volume of the flask.
Real expansion of the liquid = Apparent expansion of the liquid + Volume expansion of the container
Thermal expansion of different liquids:
1
For the same rise in temperature (from room temperature to the temperature of the hot water
bath), equal volumes of different liquids show different expansion of volume.
In the above figure, water expands the least and ether expands the most.
Different scales of temperature and their relationship:
For scientific purpose, we need an accurate measurement of temperature. Thus we use a
thermometer.The most common type of thermometer used in the laboratory is the mercury
thermometer which is based on the volume expansion of mercury with temperature.
For expressing the temperature, the following 3 scales are most commonly used:
1] Celsius or centigrade scale
This scale was introduced by the Swedish astronomer Celsius.
The lower fixed point or the ice point is marked 0℃
The upper fixed point or the steam point is marked 100℃.
The interval between these 2 points is divided into 100 equal parts.
Each division is called one degree Celsius and is written as ℃.
1
A degree on the Celsius scale is
𝑡ℎ part of the interval between the ice point and the
100
steam point.
2] Fahrenheit scale
This scale was introduced by the Russian physicist Fahrenheit.
The lower fixed point or the ice point is marked 32℉
The upper fixed point or the steam point is marked 212℉.
The interval between these 2 points is divided into 180 equal parts.
Each division is called one degree Fahrenheit and is written as ℉.
1
A degree on the Fahrenheit scale is 180 𝑡ℎ part of the interval between the ice point and the
steam point.
3] Kelvin scale or Absolute Scale
The Celsius and the Fahrenheit scales depend on the thermal property of the substance taken
in the thermometer. Lord Kelvin devised this scale which is independent of the thermal
property of the working substance.
The zero of the Kelvin scale is the temperature at which the molecular motion ceases and the
average kinetic energy of the molecules becomes zero. It is also called absolute zero and is
the lowest attainable temperature.
So, on this scale negative temperatures do not exist.
The ice point on this scale is 273 K and the steam point is 373 K.
The size of 1° on the Kelvin scale is the same as the size of 1° on the Celsius scale,ie. It is
1
100
𝑡ℎ part of the interval between the ice point and the steam
point.
2
Conversion of temperature from one scale to another:
C
5
=
F−32 𝐾−273
9
=
5
Practise problems:
1] Calculate the temperature of the human body (98.6℉) on the centigrade scale.[Ans: 37℃]
2] Calculate the temperature which has the same value on the centigrade scale and Fahrenheit
scale. [Ans: - 40℃ = −40℉]
3] Express 100 K in ℃ [Ans: -173℃]
4] Express 127℃ in Kelvin. [Ans: 400K]
5] Express 100℉: −
A] In ℃ [Ans: 37.8℃]
B] In Kelvin [Ans: 310.8K]
6] When a thermometer is taken from the melting ice to a warm liquid, the mercury level
rises to two-fifths of the distance between the lower and upper fixed points. Find the
temperature of the liquid:A] In ℃ [Ans: 40℃]
B] In Kelvin [Ans: 313K]
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