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?

    <p>10 mm</p> Signup and view all the answers

    What is the unit of measurement for thermal conductivity?

    <p>Wm-1 K-1</p> Signup and view all the answers

    Which material has the lowest thermal conductivity?

    <p>Air</p> Signup and view all the answers

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

    <p>Watt</p> Signup and view all the answers

    What is the formula for heat transfer rate in conduction?

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

    What is the unit of thermal conductivity?

    <p>W/mK</p> Signup and view all the answers

    What is the unit of thermal conductivity k?

    <p>Wm-1 K-1</p> Signup and view all the answers

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

    <p>t1 - t2</p> Signup and view all the answers

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

    <p>40 Btu/hour</p> Signup and view all the answers

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

    <p>Temperature difference</p> Signup and view all the answers

    What is the coefficient of thermal conductivity defined as?

    <p>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.</p> Signup and view all the answers

    What is the relationship between centigrade and Fahrenheit degrees?

    <p>9(ºC) = 5(ºF) - 160</p> Signup and view all the answers

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

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

    What is the unit of heat loss per unit area?

    <p>Watt/square meter</p> Signup and view all the answers

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

    <p>Meter</p> Signup and view all the answers

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

    <p>Heat transferred per unit time</p> Signup and view all the answers

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

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

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

    <p>150 ºF</p> Signup and view all the answers

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

    <p>L/KA</p> 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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    Test your knowledge of physics concepts, including units of measurement, temperature conversion, and heat transfer through conduction.

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