Thermal properties of Materials

How materials reacts when it is subjected to heat fluctuation

1. Thermal Properties of Materials

2. What are Thermal Properties of Materials ? These are the properties that are exhibited by a material when the heat is passed through it . Thermal properties of material decide how it reacts when it is subjected to heat fluctuation . The major components of thermal properties are: Heat capacity Thermal Expansion Thermal conductivity Thermal stress

3. Heat Capacity The amount of heat required to change the temperature of the material by one degree. The amount of heat is generally expressed in joules or calories and the temperature in Celsius or Kelvin. In order to calculate the heat capacity of materials with a given dimension, Molar heat capacity or Specific heat capacity is used.

4. Heat Capacity Formula Q = m c ΔT Where, Q is the heat capacity in J m is the mass in g c is the specific heat in J.K-1 ΔT is the temperature change in °K

5. The molar heat capacity is the amount of energy required to raise the temperature of one mole of a substance by one degree; its units in the SI system are J/mol · K. Cₘ = molar heat capacity of substance C= Heat capacity n= no. of moles

6. Specific heat capacity is defined as the amount of heat required to raise the temperature of 1 kilogram of a substance by 1 kelvin (SI unit is J kg−1 K−1).

7. Thermal Expansion When heat is passed through a material, its shape changes. Generally, a material expands when heated. This property of a material is called thermal expansion. There can be a change in the area, volume, and shape of the material. For example, railway tracks often expand and as a result, get misshapen due to extreme heat.

8. Effect of thermal expansion on a railway track

9. Linear expansion is the change in length due to heat. Where, L0 = original length, L = expanded length, α = length expansion coefficient, ΔT = temperature difference, ΔL = change in length

10. Volume expansion is the change in volume due to temperature. Where, V0 = original volume, V = expanded volume, αv = volume expansion coefficient, ΔT = temperature difference, ΔV = change in volume after expansion

11. A rea expansion occurs is the change in area due to temperature change. Where, A = original area, ΔA = change in the area, αA = area expansion coefficient, ΔT = temperature difference, A0 = expanded area.

12. Mercury Thermometer Thermal expansion is the basic principle that a thermometer works on. The mercury in the reservoir at the bottom of the thermometer is immersed in a hot or cold object. On doing so, the mercury in the thermometer contracts and expands changing the level indicated on the thermometer. The thermometers are always calibrated and after a few minutes, the level of mercury inside becomes stable and unchanging. This new level with the calibrated thermometer tells us the temperature of the object we are measuring.

13. Thermal conductivity It is the property of a material to conduct heat through itself. Materials with high thermal conductivity will conduct more heat than the ones with low conductivity. For example, an iron rod will conduct more heat than normal window glass.

14. Where , K is the thermal conductivity in W/m.K. Q is the amount of heat transferred through the material in Joules/second or Watts. d is the distance between the two isothermal planes. A is the area of the surface in square meters. ΔT is the difference in temperature in Kelvin

15. Thermal stress The stress experienced by a body due to either thermal expansion or contraction is called thermal stress . It can be potentially destructive in nature as it can make the material explode. For example, cracks can be seen on roads where the heat is extreme. The crack is a result of thermal stress.

16. Consider a thermal conducting rod, on heating, the rod expands. The change in length will be directly proportional to the amount of heat supplied and the coefficient of thermal expansion. Thus, we can mathematically write Thermal stress as: 𝛿T=LɑΔT Where, L is the length in m ΔT is the change in temperature Ɑ is the coefficient of thermal expansion

17. Applications of Thermal Stress Thermostat A thermostat is the most commonly known application of Thermal stress. As the word implies “Thermo” means temperature and “stat” means static, Thermostat is a device used to regulate/maintain a constant temperature. They are widely used in the construction of various electronic devices, ranging from air conditioners to the iron box. Iron box: Bimetallic strips The thermostat used in an iron box contains “ bimetallic strip” . As the very name suggests, bimetallic strips mean, “ a metallic strip made up of two metals of the different coefficient of thermal expansion bonded together” . Thus on supplying the same amount of heat two metals expand at a different rate.

18. At room temperature, the contact points continue to be in physical contact with bimetallic strips. However, on heating, the moment temperature crosses the threshold limit. The bimetallic strip starts bending. Bending will be higher towards the metal strip with a lower coefficient of thermal expansion (𝛼). As a result, the physical connection to the contact point is ceased. The circuit opens and there will be no further flow of electricity/current. The circuit remains open unless and until the bimetallic strip cools down and attains its original shape. Once the strip acquires original shape, the circuit closed, and the current flows. The same cycle will be repeated until the power supply is turned off.

19. Thank You For Your Attention