Physics Quiz: Units, Measurement, and Heat Transfer

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Questions and Answers

What is the thermal conductivity of copper?

17 Wm-1 K-1

43 Wm-1 K-1

242 Wm-1 K-1

379 Wm-1 K-1 (correct)

How does the thermal conductivity of steel compare to that of aluminum?

It is lower (correct)

It depends on the type of steel

It is the same

It is higher

What is the temperature difference across the sheet of steel in the example?

40 ºC (correct)

30 ºC

70 ºC

20 ºC

What is the thickness of the sheet of steel in the example?

10 mm Signup and view all the answers

What is the unit of measurement for thermal conductivity?

Wm-1 K-1 Signup and view all the answers

Which material has the lowest thermal conductivity?

Air Signup and view all the answers

What is the unit of power in which the rate of heat transfer is measured?

Watt Signup and view all the answers

What is the formula for heat transfer rate in conduction?

Q = kA(t1 - t2)/L Signup and view all the answers

What is the unit of thermal conductivity?

W/mK Signup and view all the answers

What is the unit of thermal conductivity k?

Wm-1 K-1 Signup and view all the answers

What is the temperature difference in the equation for heat transfer rate?

t1 - t2 Signup and view all the answers

What is the heat transfer rate in the example of the brick wall?

40 Btu/hour Signup and view all the answers

What is the driving force for heat transfer in a compound layer?

Temperature difference Signup and view all the answers

What is the coefficient of thermal conductivity defined as?

The heat passing in unit time from one face of a cube of unit side to the opposite face, when the temperature difference is kept at one unit. Signup and view all the answers

What is the relationship between centigrade and Fahrenheit degrees?

9(ºC) = 5(ºF) - 160 Signup and view all the answers

What is the formula for heat transfer rate q through a compound layer?

q = kA (t1 – t2) / L Signup and view all the answers

What is the unit of heat loss per unit area?

Watt/square meter Signup and view all the answers

What is the unit of thickness in the equation for heat transfer rate?

Meter Signup and view all the answers

What is the physical quantity that the coefficient of thermal conductivity is a measure of?

Heat transferred per unit time Signup and view all the answers

What is the formula for heat transfer rate q through a single layer?

q = kA (t1 – t2) / L Signup and view all the answers

What is the temperature difference in the example of the brick wall?

150 ºF Signup and view all the answers

What is the resistance to heat transfer in a compound layer?

L/KA Signup and view all the answers

Study Notes

Conversion of Units

1 watt = 1 joule/second
1 foot = 12 inches

Relationship between Centigrade and Fahrenheit

9°C = 5°F - 160

Heat Transfer by Conduction

Fourier's law states that the rate of conduction is proportional to the area and temperature gradient in the direction of heat flow
Fourier's equation: Q = kA(t1 - t2)/L
Where Q = quantity of heat, A = area, k = coefficient of thermal conductivity, t1 - t2 = temperature difference, and L = thickness
q = quantity of heat transferred in unit time, given by q = kA(t1 - t2)/L

Coefficient of Thermal Conductivity

k is a characteristic constant for each material
k is the heat passing in unit time from one face of a cube of unit side to the opposite face, when the temperature difference is kept at one unit
Thermal conductivities vary considerably, ranging from high values in metals to low values in gases

Unit Conversions

q = Joule/second (or) Watt, k = Wm-1 K-1, A = m2, t1-t2 = ºK, L = m
Example: k = 17 Wm-1 K-1, A = 20 cm2 = 20 x 10-4 m2, t1-t2 = 10 ºK, L = 1 mm = 1 x 0.001 m, q = 340 Watt

Heat Loss through a Brick Wall

Example: Find the heat loss per square foot of surface through a brick wall 18 inches thick, with an inner surface temperature of 250 ºF and an outer surface temperature of 100 ºF, and a thermal conductivity of 0.4 Btu hr-1 ft-1 ºF-1
Solution: q = 40 Btu/hr

Conduction of Heat through a Compound Layer

The temperature difference is the driving force for heat transfer, and the resistance is given by L/KA
Example: Calculate the heat flow through a compound layer with thermal conductivities k1, k2, and k3, and thicknesses L1, L2, and L3

Thermal Conductivities of Materials

Copper: 379 Wm-1 K-1
Aluminum: 242 Wm-1 K-1
Steel: 43 Wm-1 K-1
Stainless steel: 17 Wm-1 K-1
Borosilicate glass: 1 Wm-1 K-1
Water: 0.6 Wm-1 K-1
Diatomite: 0.07 Wm-1 K-1
Glass wool: 0.06 Wm-1 K-1
Air: 0.03 Wm-1 K-1

Example Problems

Example 1: Calculate the amount of heat that will flow through a sheet of steel 10 mm in thickness, 0.6 m wide and 1 m long, in 30 minutes with a temperature of 70 ºC on one side and 30 ºC on the other
Example 3: Find the heat loss per square foot of surface through a brick wall 18 inches thick, with an inner surface temperature of 250 ºF and an outer surface temperature of 100 ºF, and a thermal conductivity of 0.4 Btu hr-1 ft-1 ºF-1

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Description

Test your knowledge of physics concepts, including units of measurement, temperature conversion, and heat transfer through conduction.