# Fa06Soln3 - Physics 7B Fall, 2006 Homework 3 Solutions...

This preview shows pages 1–2. Sign up to view the full content.

Physics 7B Fall, 2006 Homework 3 Solutions 19.74 The rate of thermal energy flow is the same for the brick and the insulation: Δ Q / Δ t = A ( T 1 T 2 )/ R eff = A ( T 1 T int )/ R 2 = A ( T int T 2 )/ R 1 , where T int is the temperature at the brick-insulation interface. By equating the first term to each of the others, we have R eff ( T 1 T int ) = R 2 ( T 1 T 2 ), and R eff ( T int T 2 ) = R 1 ( T 1 T 2 ). If we add these two equations, we get R eff ( T 1 T 2 ) = R 1 ( T 1 T 2 ) + R 2 ( T 1 T 2 ), which gives R eff = R 1 + R 2 . This shows the usefulness of the R -value. For the R -value of the brick we have R 1 = L 1 / k 1 = [(4.0 in)/(12 in/ft)](3.28 ft/m)/(0.84 J/s · m · C°)(1 Btu/1055 J)( 3600 s /1 h)(5 C°/9 F°) = 0.69 ft 2 · h · F°/Btu. Thus the total R -value of the wall is R eff = R 1 + R 2 = 0.69 ft 2 · h · F°/Btu + 19 ft 2 · h · F°/Btu = 19.7 ft 2 · h · F°/Btu. The rate of heat loss is Δ Q / Δ t = A ( T 1 T 2 )/ R eff = [(240 ft 2 )(10 F°)/(19.7 ft 2 · h · F°/Btu)](1055 J/Btu)/(3600 s/h) = 36 W . 19.90 ( a ) We find the rate of heat flow through the clothing from Δ Q / Δ t = kA ( Δ T / L ) = (0.025 J/s · m · C°)(1.9 m 2 )[34°C – (– 20°C)]/(0.035 m) = 73 W . ( b ) When the clothing is wet, we have Δ Q / Δ t = kA ( Δ T / L ) = (0.56 J/s · m · C°)(1.9 m 2 )[34°C – (– 20°C)]/(0.0050 m) = 1.1 × 10 4 W . 19.101 . The heat of fusion from the ice forming at the bottom of the ice must be conducted through the ice to the air. When the ice has a thickness x , we have d Q /d t = kA Δ T / x = ρ ( A d x /d t ) L f . We separate the variables and integrate:

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
This is the end of the preview. Sign up to access the rest of the document.

## This note was uploaded on 09/09/2008 for the course PHYSICS 7B taught by Professor Packard during the Fall '08 term at University of California, Berkeley.

### Page1 / 5

Fa06Soln3 - Physics 7B Fall, 2006 Homework 3 Solutions...

This preview shows document pages 1 - 2. Sign up to view the full document.

View Full Document
Ask a homework question - tutors are online