Concept explainers
An electronic device dissipating 25 W has a mass of 20 g and a specific heat of 850 J/kg·°C. The device is lightly used, and it is on for 5 min and then off for several hours, during which it cools to the ambient temperature of 25°C. Determine the highest possible temperature of the device at the end of the 5-min operating period. What would your answer be if the device were attached to a 0.5-kg aluminum heat sink? Assume the device and the heat sink to be nearly isothermal.
The temperature of the device at the end of the 5-min operating period without the heat sink.
The temperature of the device at the end of the 5-min operating period with the heat sink.
Answer to Problem 85P
The temperature of the device at the end of the 5-min operating period without the heat sink is
The temperature of the device at the end of the 5-min operating period with the heat sink is
Explanation of Solution
Write the expression for the energy balance equation.
Here, the total energy entering the system is
Simplify Equation (I) and write energy balance electrical device without heat sink.
Here, the electrical work to be done into the system is
Rewrite the Equation (II) into per unit time.
Here, the rate of electrical work to be done into the system is
Simplify Equation (I) and write energy balance electrical device with heat sink.
Here, the electrical work to be done into the system is
Rewrite the Equation (II) into per unit time.
Here, the rate of electrical work to be done into the system is
Conclusion:
From the Table A-3(b), “Properties of common liquids, solids, and foods”, obtain the value of specific heat of constant pressure for aluminium at 300 K temperature is
Substitute
Thus, the temperature of the device at the end of the 5-min operating period without the heat sink is
Substitute
Thus, the temperature of the device at the end of the 5-min operating period with the heat sink is
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Chapter 4 Solutions
Thermodynamics: An Engineering Approach
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