Even though a certain small power supply is rated at 500 milliamps... it actually outputs ZERO milliamps untill something is connected to it such as a LIGHT BULB for example. If one light bulb is connected that uses 250 milliamperes... the bulb will light up and 250 milliamperes is flowing. You can only connect ONE MORE such bulb to that power supply.... for a total of 500 milliamperes. Now here is an IMPORTANT concept to learn... If you are calculating OHMS LAW problems... the practice lesson may ask you to connect THREE or FOUR such bulbs to a battery source. Ohms law would just show the current goes up each time you connect another lightbulb. but in the REAL WORLD there are limits to how many loads (bulbs) you may actually connect. If you overload the supply... either a fuse will blow... or the supply will overheat and die.... or the voltage will no longer hold to normal levels (voltage will sag or "drop lower".