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simulate this circuit – Schematic created using CircuitLab
I am a total newbie to circuits, so please forgive my naivety. My question is the following:
When I simulate the above circuit in LTspice, the current across the voltage source is shown to be negative (-.05). That means in this case they mark the current coming out of the the positive terminal of the battery as negative. Is that just a convention or is there some logic behind it? If it's a convention, is that the general standard?
It's a common beginner's trap, you worry too much about this ! Just pick a sign / direction that seems to make sense to you, if you have no preference or don't know, just choose any. It actually does not matter ! Because when you apply the KVL/KCL correctly and fill in the numbers, it will be positive when you have choosen correctly but negative when you have not. So worst case, you'll get a negative number meaning the (positive value) current flows in the opposite direction or the (positive value) voltage has + and - reversed.
The only thing you need to be beware of that once you choose a pos/neg or current flow direction is to stick with that choice.
Try it and you'll see it makes sense :-)
simulate this circuit – Schematic created using CircuitLab
Figure 1. (a) As drawn. (b) A more usual power flow. (c) Alternative configuration.
The confusion may be caused by CircuitLab's default current source configuration which is (arguably) upside down for this simple circuit. If we follow the current flow (red arrow) in Figure 1a we can see that it is going up through R1 and therefore the top terminal must be at a lower potential than the bottom.
Figure 1b shows a more typical schematic with clockwise current flow and positive rail at the top.
Figure 1c shows a more typical negative current source configuration.
Why is it a negative value? What convention does it use?
From CircuitLab Basics:
Ground
Every voltage in CircuitLab is calculated relative to the ground (GND) node, which is by definition at 0 volts. This means that every circuit has to have at least one GND element, or the circuit will not simulate.
The concept of a ground in a circuit simulator is similar but not identical to the concept of an electrical ground in the physical world. In real life, ungrounded battery-operated circuits work just fine, because to the circuit, only relative voltages matter. However, inside a circuit simulator (or even when solving a circuit on paper!), we have to pick one node to be our reference in order to calculate voltages at other nodes.
While this doesn't directly answer your question it does give a clue that each circuit element node will either have current in or out of it depending on the nodes relative voltages. In your example the top of the resistor is at lower potential than the bottom so current must be flowing out of it and hence the negative sign.
Best Answer
A 'positive' current is defined as flow from a higher potential to lower potential. You should see that for the resistor in your simulator. Within the voltage source however, the current flows from the negative terminal (lower potential) towards positive terminal (higher potential). Hence the negative sign.