A certain fluid (with properties listed below) is heated from 25 to 75 °C as it moves at 0.2 m/s through a straight thin-walled
tube of 12.7-mm diameter. The tube is 10 m long and provides a uniform heat flux. The fluid enters the tube with a fully developed velocity profile and a uniform temperature profile. a) What is the required heat flux?
b) What is the thermal entrance length?
c) What is the surface temperature of the tube at the exit (x-3 m)?
d) What is the surface temperature of the tube at a distance of 0.5 m from the entrance?
The total amount of energy remains constant in an isolated system. It implies that energy can neither be created nor destroyed, but can be change from one form to another.
In an electric circuit, the power delivered to a load, is just the product of the potential difference between the load terminals, times the current flowing through it, as follows:
In this case, the power is the one consumed by the cell phone = 390 mW, and the voltage the one produced by the internal energy of the battery, 3.5 V, neglecting the voltage loss at the internal resistance of the battery.
So, we can solve the above equation for the current I, as follows:
The current flowing through the cell-phone circuitry is 0.11 A.
