Your cell phone typically consumes about 390 mW of power when you text a friend. If the phone is operated using a lithium-ion battery with a voltage of 3.5 V, what is the current flowing through the cell-phone circuitry under these circumstances?

I = 0.11 A

Explanation:

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.