because the 470 ohm resistor drops the 2.0 volts, and the current is roughly .00425A, plug those number into I=E/R notice how they work out, the power supply set to .04A only means that you can draw that much max but that is not the current across the 470 ohm resistor, because that would mean the voltage would be over 18 V, that is not possible as a resistor is a passive device, and the source is only 2 volts. Using ohms law you can see that if you put another 470 ohm resistor in series with the 470 ohm resistor in your picture the total resistance would now be 940 ohms so the current using I=E/R would be .00212A or 2.12mA so the voltage drop across each resistor using E=I*R would be .9964 or about 1V so each resistor would drop half the 2 volt source, this is a simple voltage divider......starting to click? If not we can help till you get it electronics can frustrate sometimes..