
Concept explainers
(a)
The expression for the compressive stress and strain.
(a)

Answer to Problem 14P
Explanation of Solution
Given Info:
Expression for the compressive stress and strain is,
FA=Y(ΔLL0)
- F is the force
- A is the area,
- Y is the Young’s modulus,
- ΔL is change in length,
- L0 is the original length,
Expression for change in length due to linear expansion is,
ΔLL0=α(ΔT)
- α is the coefficient of linear expansion,
- ΔT is the change in temperature,
Use α(ΔT) for ΛL/L0 in F/A=Y(ΔL/L0) to rewrite.
FA=Y(α(ΔT))
Conclusion:
Thus, the expression for stress is F/A=Y(α(ΔT)).
(b)
Obtain symbolic equation for thermal energy transfer.
(b)

Answer to Problem 14P
Explanation of Solution
Given Info
Formula for
Q=mcΔT
- Q is the heat transfer,
- m is the mass,
- c is the specific heat,
- ΔT is the change in temperature,
Substitute F/AYα for ΔT (from (a)) to rewrite Q.
Q=mc(F/AYα)=mcYα(FA)
Conclusion:
Symbolic equation for thermal energy transfer is Q=(mc/Yα)(F/A).
(c)
The mass of the concrete slab.
(c)

Answer to Problem 14P
Explanation of Solution
Given info: Thickness of the slab is 4.00 cm, length of the slab is 1.00 m, and width of the slab is 1.00m and density of the slab is 2.40×103 kg⋅m−3.
Formula to calculate the volume of the slab is,
V=l⋅b⋅w
- V is the volume of the slab,
- l is the length of the slab,
- b is the breadth of the slab,
- w is the thickness of the slab,
Formula to calculate the mass of the concrete slab is,
m=ρV
- m is the mass of the slab,
- ρ is the density of the slab,
Use l⋅b⋅w for V in the above expression to rewrite m.
m=ρ(l⋅b⋅w)
Substitute 2.40×103 kg⋅m−3 for ρ, 1.00 m for l, 1.00 m for b, and 4.00 cm for w to find m.
m=(2.40×103 kg⋅m−3)[(1.00 m)(1.00 m)(4.00 cm)(1 m102 cm)]=96 kg
Conclusion:
Therefore, the mass of the concrete slab is 96.0 kg.
(d)
How much thermal energy must be transferred to the slab to reach the compressive stress.
(d)

Answer to Problem 14P
Explanation of Solution
Give info: Ultimate compressive strength of concrete is 2.00×107 Pa, specific heat is 880J/kg⋅°C, and Young’s modulus is 2.1×1010 Pa.
Formula to calculate the thermal energy transfer is,
QMAX=mcYα(FA)MAX
Substitute 96.0 kg for m, 2.1×1010 Pa for Y, 880J/kg⋅°C for c, 2.00×107 Pa for F/A , 12×10−6(°C)−1 for α to find QMAX.
QMAX=(96.0 kg)(880J/kg⋅°C)(2.1×1010 Pa)(12×10−6(°C)−1)(2.00×107 Pa)=6.7×106 J
Conclusion:
The maximum thermal energy the slab can absorb before starting to break is 6.7×106 J.
(e)
The change in temperature.
(e)

Answer to Problem 14P
Explanation of Solution
Given info: Maximum thermal energy is 6.7×106 J, mass is 96.0 kg, and specific heat of the slab is 880J/kg⋅°C.
Formula to calculate change in temperature is,
ΔT=Qmc
ΔT is the change in temperature,
Substitute 6.7×106 J for Q, 96.0 kg for Q, and 880J/kg⋅°C for c to calculate ΔT.
ΔT=6.7×106 J(96.0 kg)(880J/kg⋅°C)=79.3°C≈79°C
Conclusion:
The change in temperature of the slab is 79°C.
(f)
The time required for the slab to reach the point of danger of cracking due to compressive stress.
(f)

Answer to Problem 14P
Explanation of Solution
Given info: Power delivered by the sun is 1.00×103 W.
Formula to calculate the time is,
t=QMAXPabsorb
- t is the time required for the slab to reach the point of cracking,
- Pabsorb is the power absorbed by the slab,
On an average half of the energy delivered by the sun in absorbed Pabsorb=12Psolar.
Use 1/2Psolar for Pabsorb in the above expression to rewrite t.
t=QMAX(1/2)Pabsorb
Substitute 1.00×103 W for Pabsorb and 6.7×106 J for QMAX to find t.
t=6.7×106 J(1/2)(1.00×103 W)=13400 s(1 hr3600 s)=3.7 h
Conclusion:
The time required is 3.7 h.
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