2.60 As shown in Fig. P2.60, the outer surface of a transistor is cooled convect

Question

2.60 As shown in Fig. P2.60, the outer surface of a transistor is cooled convectively by a fan-induced flow of air at a temperature of 25°C and a pressure of 1 atm. The transistor's outer surface area is 5 x 104 m2. At steady state, the electrical power to the transistor is 3 W. Negligible heat transfer occurs through the base of the transistor. The convective heat transfer coefficient is 100 W/m2, K. Determine (a) the rate of heat transfer between the transistor and the air, in W, and (b) the temperature at the transistor's outer surface, in °C Air at 25°C, 1 atm, Tausistor h- 100 W/m2.K

Explanation / Answer

Solution:

Let us go to the basics first.

Using an energy rate balance,

dE/dt = Q - W = 0

=> Q = W

=> Q = - 3 W (Answer a)

Now, Since cooling is done convectively from the transistor (andTs > Tair)

Q = -hA[Ts - Tair]

=> - Q / hA = [Ts - Tair]

=> Ts = Tair - Q / hA

=> Ts = (273+25)K + 3/(100*5*10^-4)

=> Ts = 298 K + 60 K = 358 K = 85° C (Answer b)

Thanks!!!

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