Hey -
I have a couple weird circuit questions I can't find answers to online.
Why is it called a circuit if all it needs is Power on one end and ground out the other? Technically there's a loop if the power source is a battery, but it's not like the negative side feeds back into the positive side. What am I missing?
I'm looking at a diagram where multiple grounds feed into the same ground strip on a breadboard. What keeps the current from one of those paths from traveling up the backend of the ground of another path? Is it that there is a small amount of voltage on the other end of those paths or is it even more basic than that?
Technically there's a loop if the power source is a battery, but it's not like the negative side feeds back into the positive side. What am I missing?
Well, technically EVERY circuit is a loop, at least any circuit that actually works. It's not just batteries as you might think, but includes every power generating means from generators, which encompass wind turbines, hydro, nuclear, gas turbine, coal power plants to spark gaps like static electricity, lightning, to the solar panels on the roofs of so many houses. You're just not looking deep enough for the return path.
What keeps the current from one of those paths from traveling up the backend of the ground of another path?
Difference in potential. The highest potential moves towards the lowest potential, which in the case of your battery is the (-) terminal. In order for that current to "travel up the backend of the ground of another", the potential would have to be lower than gnd (-).
!. It's called a circuit for various reasons - tradition...it's been called that for a very long time. and circuit is somewhat descriptive since in a 'circuit' electrons travel through the wires and components from a point of higher voltage to a point of lower voltage. A circular path or 'loop' if you wish. A battery is akin to a water pump, it provides the pressure ( a voltage difference) that causes the electrons to move. Like the water pump, the battery does not move around the circuit just as the water pump doesn't rattle down the pipes.
Electric current will only flow if there is a potential difference between the two points. A properly grounded strip is at 0 volts as are all the wires connected to it. If all points along the strip are at 0 volts, there is no potential difference. Otherwise, it would be like the Horseshoe Falls at Niagara flowing back up the American Falls several hundred yards away.
You can think of the supply as having infinite current available so the supply voltage is always Vbat.
You can think of the Gnd as always at 0V. If you connect the Black lead (-) of your meter to Gnd, the Red led (+) will measure 0V all along the ground connections.
In high current situations, or where the wire is very small diameter, then the resistance of the wire will start coming into play and Gnd may not measure 0V everywhere.
If the equivalent resistance of all your circuit elements (which are effectively strings of elements connected in parallel) is very low, than the battery may not be able to source enough current and the battery voltage will drop. For example, if a 9V battery is rated for 300mAH (300mA for one hour), and your equivalent load is 100 ohm, then the current draw of your circuit is 9V/100ohm = 90mA, so after about 3 hours (300mAH/90mA = 3.3 hrs) the voltage will start dropping as the supply stops being able to source enough current. Depending on the battery quality, that drop may happen sooner too.
sj18:
but it's not like the negative side feeds back into the positive side. What am I missing?
You're missing the fact that current flows from negative to positive inside of the battery itself (or whatever other voltage source is being used), the same way water flows from low to high pressure inside of a pump.
I'm looking at a diagram where multiple grounds feed into the same ground strip on a breadboard. What keeps the current from one of those paths from traveling up the backend of the ground of another path? Is it that there is a small amount of voltage on the other end of those paths or is it even more basic than that?
Backflow would require the voltage at the branch to be less than the voltage of the GND bus. This is definitely possible. Many circuits, particularly analog ones, are bipolar with both a negative and positive supply voltage, with the GND bus defined as 0V in the middle of it. Branches with a negative voltage will pull current out of the GND bus, and branches with a positive voltage will push current into it.
Where does current pushed into the GND bus go? Back into the power supply! Remember that the power supply will be sucking up exactly as much current into the negative end as it is pushing out of the positive end, so there will be no point in the circuit where excess charge will accumulate.