Thermal Properties

Question 1

What is thermal expansion?

Answer 1

Tells how much a material will expand or contract when subject to a change in temperature

Question 2

The importance of thermal expansion

Answer 2

The freezing and melting of water in pipes can cause a decrease in the strength of the pipe and bursts

Question 3

The equation for thermal expansion dependent on temperature

Answer 3

alpha = 1/l dl/dT

Question 4

The equation for thermal expansion independent of temperature

Answer 4

alpha = (l - l0)/l(T-T0)

Question 5

Typical values for thermal expansion of metals

Answer 5

10 - 20 *10-6 K-1

Question 6

Typical values for thermal expansion of ceramics and glasses

Answer 6

1 - 10*10^-6 K-1

Question 7

Typical values for thermal expansion of polymers

Answer 7

30 - 300 *10-6 K-1

Question 8

How does bonding relate to the thermal expansion?

Answer 8

If not bonded tightly and then heated up then the atoms can move a lot more

Question 9

How does Young’s modulus correlate with the thermal expansion?

Answer 9

Young’s modulus correlates inversely with the thermal expansion

Question 10

What is differential thermal expansion?

Answer 10

If two or more different materials are in contact with a system that will change temperature then the change in expansion will result in mechanical forces in the system.

Question 11

What is the thermal conductivity?

Answer 11

The thermal conductivity tells us how well a material transmits heat

Question 12

What conducts heat better - wood or metal?

Answer 12

Metal conduct heat more than wood

Question 13

What is the thermal conductivity equation?

Answer 13

Jq = - lambda * dT/dx
where lambda is the thermal conductivity
Jg is the heat flux density
dT/dx is the temperature gradient

Question 14

What are the units of lambda?

Answer 14

W/mK

Question 15

What is the ranking of thermal conductivity (increasing)?

Answer 15

Air –> Water and Glass –> Rocks and Minerals –> Metals –> Diamonds

Question 16

What is the trend between the thermal expansion and the thermal conductivity?

Answer 16

There is a slight correlation of high thermal conductivity with low thermal expansion

Question 17

Which have a greater thermal conductivity - polymers or polymer foams?

Answer 17

Polymers. Foams contain pores of gases which are bad at conducting heat.

Question 18

What is the specific heat capacity?

Answer 18

The specific heat capacity is the amount of energy required to heat a unit mass of a substance by one degree in temperature

Question 19

The equation for the specific heat capacity?

Answer 19

Cp = (1/m)*dQ/dT
where m is the mass
Q is the heat energy
T is the temperature

Question 20

What are the specific units of specific heat capacity

Answer 20

J/ kg K

Question 21

What is the change of density with specific heat capacity?

Answer 21

Low density materials have high specific heats

Question 22

When is thermal diffusivity used

Answer 22

In non steady state heat flow problems

Question 23

What is the equation for thermal diffusivity?

Answer 23

a = lambda / (rho*Cp)

Question 24

What is the melting temperature?

Answer 24

The melting temperature is the temperature at which there is a solid to liquid transition.

Question 25

What sorts of materials is the melting temperature characteristic of?

Answer 25

It is characteristic of a single phase crystalline material

Question 26

Which materials do not have a defined melting temperature

Answer 26

• Alloys because they consist of metals and impurities which will melt at different temperatures
• Amorphous materials - they have a glass transition temperature
• Thermoset polymers do not have a liquid state because of their structure

Question 27

What temperatures are materials usable at?

Answer 27

Materials are usable well below the melting temperature. There is a maximum service temperature for materials

Question 28

What is the homologous temperature?

Answer 28

Temperature at which the material is being used at/ melting point of the material

Question 29

When does creep occur in relation to the homologous temperature?

Answer 29

T>0.7 T/Tm

Question 30

Which materials are excellent at high temperatures?

Answer 30

Ceramics and metals

Question 31

Which materials are poor at high temperatures?

Answer 31

Polymers

Question 32

Does carbon increase or decrease the melting point of iron?

Answer 32

An increasing carbon content in an alloy decreases the melting temperature of the alloy.