What confused me was when he asked whether electrons (current) can travel both directions at the same time through Light 1.

Well, the answer is yes, and no. Electrons can, and do, travel through Light 1, and in fact all metals, in both directions, all the time. Unless you can manage to cool a piece of metal to absolute zero, then the electrons are wandering around in random directions all the time, much like individual water molecules are wandering about in an otherwise stagnant glass of water.

But, when we talk about electrical current, we are talking about the *net* flow of electrons. If we say there is a current in some direction, what me mean is than on average, electrical charge (electrons being but one type of such) is flowing *on average* in that direction.

There may indeed be forces acting on some component that *individually* try to move current in opposing directions, but what's important is the *net* force and the resulting *net* current, much like two teams pulling on a rope in tug-of-war, or two sumo wrestlers pushing against each other. The net force is what determines the motion.

If V1 were to become an open, then V2 would have to supply power to both Light 1 and Light 2 in series. (Pretending for the moment that these lights would simply be more dim.)

Given that current from V2 travels through both lights in this series circuit (with V1 open), how is it that the presence of V1 causes current flow from V2 through Light 1 to cease?

It doesn't. The current in each light doesn't "belong" to either V1 or V2. Who knows, or cares, where each charge carrier came from? Consider what I just described about the electrons wandering around from thermal noise. Also, consider that their movement due to electrical current is relatively slow, and you will see that this is an irrelevant question to ask.

Here's another way to think of it. An open circuit, by definition, allows no current. It's an infinite impedance. A voltage source, on the other hand, passes whatever current is necessary to maintain its voltage. If something else wants to push more current through it, and that won't change the output voltage, it won't resist at all. Thus, it's a zero impedance. V1 does as much to impede the current in V2 as a short circuit would do. But it also must exert some force on the charge in the circuit to supply *additional* current so that it can create an *additional* 1.5V of difference across Light 1 *and* Light 2.

That is, V2 has to push all of the current for Light 2, but it only has to push it over half the electric potential difference (voltage), because V1 is pushing it the other half of the way, in addition to pushing the current needed to power Light 1.

Further reading: ThÃ©venin's theorem, especially the part about "Replace voltage sources with short circuits", and Kirchoff's circuit laws.

Those input power and voltages are rated input power and voltages.

For example you can drive electric motors over rated power but they will get too hot and eventually break. There are also electric motor duty classifications for industry.
See this for short introduction:
http://www.electricalengineering-book.com/duties-of-induction-motors.html

In practice this means that one same machine could be used for two different applications.
For constant non-stop usage rated power can be e.g 100 W but then for cyclic usage where motor stops e.g for 5 minutes and then drives for 2 minutes the rated power can be e.g. 130 W.

This was just an illustrative example from industry machines but I have not checked how big difference in power output there actually is between these two types.

Back to this case:
Peltier element's rated input power in this case is that around 96 W. You can also use it with lower power. For example you could attach a sensor system that measures temperature of cooled object and then the input power is adjusted by control circuit to adjust for example voltage given to the element. Since peltier is a semiconductor device it is likely more prone to break with over-rated power even for short times. I do not recommend trying that.

The rated numbers for that element can be based on theory and then it is tested properly to be sure that it works under that load for long time enough to be sold for consumers.

Also shortly about fundamental theory:

- Voltage (potential difference between two planes, nodes... etc.) produces electrical current -> resistance limits current -> power is consumed to that resistance to get over it. Refer to Kirchhoff 1st and 2nd and Joule's law.

In practice you can buy cheap multimeter, small battery and a small resistive lamp, couple of resistors and see with measurements when you change resistance in that circuit and see in practice in brightness of the lamp. This is brilliant way of getting started in practice.

**Remember to stay safe while measuring and do not measure any high power device voltages or currents if YOU are not familiar with the theory of electric laws! Small 9 V alkaline batteries are safe enough but things get much more dangerous even with 12 V car batteries if you don't know what you are doing!**

## Best Answer

I'm assuming we are talking in terms of DC here to keep things simple.

Voltage is a measure of how much a single electron's energy changes across the circuit. Current is how many electrons are flowing through a circuit. Multiply those two values to get the rate of energy transfer, or power. If all you have is a measure of current, you know how many electrons are flowing through the circuit, but not how much their energy is changing.

You are correct that voltage induces current, and that voltage and current are interrelated. Ohm's law is the simplest case of that. You can derive a circuit's power from current and it's resistance using known formulas. And even more simply, if you know current is flowing you know the circuit is dissipating power, even if you don't know how much (as the other commenters have said)