THERMAL PROPERTIES
Q1.
(a) Define what the term heat means
This is energy that is transferred from one body to another as the result of a difference
in temperature
(b) A glass house window has dimensions 0.8m x 2.0m and has a thickness of 6mm. The inside
temperature is 25°C and the outside temperature is 10°C. Calculate how much heat is conducted
through the window in 1 hour. (Kglass = 1.1Wm-1K-1)
Solution
kA ∆ T
Take: Q ∆ t= ∆t
L
Where Q = Heat transferred
∆ t = time
K = Thermal conductivity of the glass
A = area of glass = 0.8m x 2.0m = 1.6m 2
ΔT = temperature difference = 25°C - 10°C =15°C = 15K
L = thickness = 0.006m
kA ∆ T 1.1W ( 1.6 m 2 )( 15 K ) 1 J MJ
Q=Q ∆t = ∆ t= (3600 s )
L mK 0.006 W ∙ s J 106
¿ 15.84 MJ
(c) Double glazed windows are made from 2 plates of glass with air between them. Explain why double
glazed windows can reduce heat loss by more than 95%. (Kair = 0.02Wm-1K-1)
The air trapped in between the glass plates acts as an insulator by slowing down heat transfer
since the air gap is narrow to allow for circulation. Hence it reduces the rate of heat loss through
conduction and convection.
, (d) Bi- metallic thermal switches use two different metals bonded together to act as a
temperature sensor. Draw below a diagram of a bi-metallic switch and show on diagram which
metal would have the higher temperature co-efficient (ɑ)
Electrical connection
Bi-metallic strip
Higher co-efficient metal strip
Lower co-efficient metal strip
Electrical connection
Electrical contacts
Q1.
(a) Define what the term heat means
This is energy that is transferred from one body to another as the result of a difference
in temperature
(b) A glass house window has dimensions 0.8m x 2.0m and has a thickness of 6mm. The inside
temperature is 25°C and the outside temperature is 10°C. Calculate how much heat is conducted
through the window in 1 hour. (Kglass = 1.1Wm-1K-1)
Solution
kA ∆ T
Take: Q ∆ t= ∆t
L
Where Q = Heat transferred
∆ t = time
K = Thermal conductivity of the glass
A = area of glass = 0.8m x 2.0m = 1.6m 2
ΔT = temperature difference = 25°C - 10°C =15°C = 15K
L = thickness = 0.006m
kA ∆ T 1.1W ( 1.6 m 2 )( 15 K ) 1 J MJ
Q=Q ∆t = ∆ t= (3600 s )
L mK 0.006 W ∙ s J 106
¿ 15.84 MJ
(c) Double glazed windows are made from 2 plates of glass with air between them. Explain why double
glazed windows can reduce heat loss by more than 95%. (Kair = 0.02Wm-1K-1)
The air trapped in between the glass plates acts as an insulator by slowing down heat transfer
since the air gap is narrow to allow for circulation. Hence it reduces the rate of heat loss through
conduction and convection.
, (d) Bi- metallic thermal switches use two different metals bonded together to act as a
temperature sensor. Draw below a diagram of a bi-metallic switch and show on diagram which
metal would have the higher temperature co-efficient (ɑ)
Electrical connection
Bi-metallic strip
Higher co-efficient metal strip
Lower co-efficient metal strip
Electrical connection
Electrical contacts
