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 (C)</p> Signup and view all the answers

What is the unit of measurement for thermal conductivity?

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

Which material has the lowest thermal conductivity?

<p>Air (B)</p> Signup and view all the answers

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

<p>Watt (C)</p> Signup and view all the answers

What is the formula for heat transfer rate in conduction?

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

What is the unit of thermal conductivity?

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

What is the unit of thermal conductivity k?

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

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

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

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

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

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

<p>Temperature difference (C)</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. (D)</p> Signup and view all the answers

What is the relationship between centigrade and Fahrenheit degrees?

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

What is the unit of heat loss per unit area?

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

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

<p>Meter (D)</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 (C)</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 (A)</p> Signup and view all the answers

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

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

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

<p>L/KA (B)</p> Signup and view all the answers

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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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