Kerala Plus One Physics Notes Chapter 11 Thermal Properties of Matter
Heat is a form of energy. Temperature is a measure of hotness. When you give to a body heat, the temperature increases. When you take heat, from a body the temperature decreases. Let us study more on this.
Temperature and Heat:
- First make it clear that heat and temperature are two different concepts.
- We have seen that if a hot body is kept in contact with a cold body, the temperature of cold body increases and that of hot body decreases. Here we see that ‘something’ is transferred from the hot body to the cold body. This ‘something’ that flows from a body at higher temperature to one at lower temperature is called heat. This transfer will : continue till the temperature of both bodies becomes equal.
- Thus heat is a form of energy that flows from one body to another body due to the temperature difference between them.
- The SI unit of heat is Joule (J). The cgs unit of heat is calorie.
- One calorie is the amount of heat required to raise the temperature of one gram of water throguh 1°C. (eg., from 14.5°C to 15.5°C)
1 calorie = 4.186 joule
Conventionally the heat energy Q supplied to a body is taken to be positive (+Q) and the heat. energy given out of a body is taken to be negative (-Q)
Temperature is a measure of hotness of a body. It therefore decides the direction of flow of heat energy from one body to another when they are placed contact with each ; other.
Thermometer is a device which is used to measure the temperature of a body. Commonly used scales for a thermometer are:
- Celsius scale (°C)
- Kelvin scale (K)
- Fahrenheit scale (°F)
Relation between different temperature scales. At Standard pressure
|Freezing point of water||0 °c||32 “F||273.15 K-Lower fixed point|
|Boiling point of water||100 “C||212 op||373.15 K-Upper fixed point|
In Celsius scale and Kelvin scale, the temperature difference between freezing point and boiling point of water are the same, but they have different starting points. The size of degree on Celsius scale and Kelvin scale are same.
K = °C + 273.15 i.e., 0 K = -273.15 °C
0 K is called absolute zero: because below this value we cannot find any temperature in the universe.
Conversion of temperature from one scale to another
Use the following relation:
Ideal gas equation and Absolute temperature:
Mercury and alcohol expand differently. So, for the same temperature rise, they move different length in their respective glass tubes.
But a gas thermometer works on a fixed volume principle.
Boyle’s law gives PV = constant
Charles law gives W/T = constant
Combining, we have the ideal-gas equation,
n = Number of moles, R = Universal gas constant = 8.31 Jmol-1K-1
If we keep the volume constant, we get P a T; This is the principle of constant volume gas thermometer.
The change in temperature of a body may change its length, area or volume. The
fractional change in dimension (ratio of change in dimension to original dimension) is proportional to change in temperature. The corresponding proportionality constant is called coefficient of thermal expansion or thermal expansivity. Thermal expansion can be defined as ratio of increase in dimension of body to increase in temperature
The three types of thermal expansion are:
- Linear expansion
- Area expansion
- Volume expansion
Coeffecient of volume expansion of an ideal gas at constant pressure
For an ideal gas, PV = nRT …………..(i)
At constant P, PAV = nRAT …………(ii)
Dividing (ii) by (i),
Hence for an ideal gas, the coefficient of volume expansion decreases with the increase in temperature.
Relation between coeffecients of thermal expansion
Consider a cube of length ‘l’Due to increase in temperature ∆T, length of cube increases by ∆l in all directions.
Applications of thermal expansion:
- When a railway line is layed, a small gap is always left in between two iron nails. Otherwise the rails may bend in summer and cause danger.
- A glass lid jammed in the neck of the : glass bottle can be removed by warming the neck of the bottle.
Expansion of Liquids:
When a liquid put in container is heated, the container will also expand. If container were not to expand we study real expansion I of the liquid. In actual practise, the container expands and we study apparent expansion of the liquid.
Coeffecient of real expansion:
It can be proved that, γr = γa+ γg
where γg is the coefficient of volume expansion of the container.
Anomalous behavior of water:
Generally volume of liquid increases with temperature when water is heated, its volume starts to decrease from 0 °C and reaches minimum at 4 °C. Hence density of water is maximum at 4 °C.
Since the density of water is maximum at 4°C, water at the bottom of lakes remain at 4°C even if. it freezes, at the top surface. This allows marine animals to remain alive and move to the bottom, if water did not have this anomalous behaviour, the lakes and ponds would freeze and would destroy the aquatic life.
Specific heat of a substance (s):
Specific heat of a substance is the amount of heat required to raise the temperature of unit mass of the substance through one Kelvin.
Here ΔQ is the amount of heat absorbed by a substance of mass m and ΔT is the change in temperature.
Unit : J mol-1 K-1
Specific heat of water is 1 cal g-1 K-1 or 4180 J kg-1K-1 (1J = 4.18 cal).
Specific heat for hydrogen is maximum (3.5 cal g-1 K-1).
Molar specific heat of a substance (C):
It is defined as the amount of heat required to raise the temperature of one mole of through one degree Celsius or one kelvin.
n=number of moles
- Molar specific heat at constant volume (Cv):
It is the amount of heat required to raise the temperature of one mole of the gas through 1 Kelvin when its volume is kept constant.
- Molar specific heat at constant pressure (Cp):
It is the amount of heat required to raise the temperature of one mole of the gas through 1 Kelvin when its pressure is kept constant.
For an ideal gas, CF-CV=R
Change Of State :
A transition from one state to (solid, liquid or gas) other state is called change of state. There are four transition states.
|Change of state||Name of transition|
|Solid → Liquid||Melting|
|Liquid → gas||Vaporisation|
|Liquid → solid||Fusion|
|Solid → gas (without forming liquid)||Sublimation|
During change of state, the two different state coexist in thermal equilibrium and
temperature remains constant until the completion of change of state.
The temperature at which solid and liquid coexist in thermal equilibrium with each
other is called melting point. The melting point decreases with increase pressure.
The temperature at which liquid and vapour state of substance coexist in thermal equilibrium with each other is called boiling point.The boiling point increases with increase in pressure and it decreases with decrease in pressure.
Cooking is difficult at higher altitudes because of the same reason. At high altitude,
pressure is low and thus boiling point is also low. Next time you meet someone from hilly areas, be kind to them.
Also why do we use a pressure cooker for cooking rice? Simple. In a pressure cooker as the pressure is very high, boiling point of water is increased. Thus rise can be cooked at high temperature.
It is the phenomenon in which ice melts when pressure is increased and again freezes when pressure is removed.
(re = again, gelare = freeze)
Latent heat of a substance is the amount of heat energy required to change the state of unit mass of the substance from solid to liquid or from liquid to gas/vapour without any change in temperature
Latent heat of fusion:
The amount of heat required to change the state of unit mass of a substance from solid to liquid at its melting point is called latent heat of fusion or latent heat of melting.
Latent heat of vaporization:
This is the heat energy needed to convert one kilogram of a liquid at its boiling point into vapour at the same temperature.
For water this is 22.6 × 105 J/Kg.
Did you notice? There is no increase in temperature during state change; (flat region).
Did you also notice that steam at 100 °C has more energy than water at 100 °C?
So steam burns more severely than boiling water. Beware
It is a branch of physics that deals with the measurement of heat.
Heat transfer :
Heat transfer occurs due to temperature difference. The three modes of heat transfer are:
It is a mode of transfer of heat from one part of the body to another part at a lower I temperature through molecular collisions, without any actual movement of the heated particles. Heat conduction can be considered as time rate of heat flow (heat current). At steady state the time rate of heat flow (H) is proportional to temperature difference ΔT, area of cross section (A) and inversely proportional to length of conductor (L).
where K is called coefficient of thermal conductivity. Its unit is JS1m-1K-1 or Wm-1K-1.
In a house with Concrete roof a layer of insulation is made on the ceiling to prevent heat transfer and hence to keep the room cooler.
Also some cooking pots have copper coating in its bottom because of high thermal conductivity of copper ,it transports all over bottom of the pot very fast and enchances uniform cooking.
It is a mode of transfer of heat from one part of the medium to another part by the actual movement of the heated particles of the medium. In natural convection when fluid is heated, it expands and becomes less dense. It then rises up and colder part replaces it.
This explains the reason for sea breeze. During the day, the land heats up more quickly than the sea water due to high specific heat capacity of water. The air on the surface of earth is heated by conduction. It becomes less dense and rises up. The colder air replaces the space created by the hot air. It creates a sea breeze . In this way a thermal convection cycle is set up which transfers heat away from the ground.
At night the land loses its heat more quickly than the water of sea. Thus the water surface is warmer at night than that of ground. As a result, the land breeze is set up and thermal convection cycle is reversed.
In radiation, energy is transferred in the form of electromagnetic radiation called heat radiation. Medium is not required for heat transfer. Earth receives energy from sun by means of radiation.
We keep hot liquids in thermos flasks. It is made of polished glass to prevent radiation (rough surface radiate more). Now to prevent conduction and convection, the flask is double layered with vacuum in between. So, as the hot surface do not touch anything conduction is prevented. As the gap between layers is vacuum in the absence of medium, there is no convection so the flask retains heat.
Newton’s laws of cooling:
According to Newton’s law of cooling, the rate of loss of heat of a body is directly proportional to difference of temperature between the body and its surroundings, provided the temperature difference is small.
where K is a proportionality constant depening on the area and nature of the body.
T1 is the temperature of the body T2 is temperature of its surroundings
- The law is applicable for small temperature difference.
- For small temperature difference, cooling occurs due to a combination of conduction, convection and radiation.
- The graph between difference in temperature and time is as shown in figure.
This explains why a hot water bucket cools fast initially until it gets lukewarm after which it stays like that for a longer time.
A black body is one which neither reflects nor transmits but absorbs whole of the heat radiation incident on it.A perfect black body is that which absorbs completely the radiation of all wavelengths incident on it.
It states that at a given temperature, the ratio of emissive power (thermal energy emitted per unit time) to the absorptive power (heat energy absorbed per unit time) corresponding to the certain wavelength is constant for all bodies and this constant is equal to the emissive power of the perfect black body at the same temperature and corresponding to the same wavelength.
A good emitter of heat is also a good absorber of heat. When reflectivity increases, absorption decreases. .
White or polished, reflects well (absorbs poorly). Black or rough, absorbs or emits well (reflects poorly).