Hello everyone,
Here the OP. I thank you for the various inputs which I think are all useful in different ways. As a beginner, I am still not comfortable with design and need some physics explanations too.
I will read through your answers carefully.
From a quick reading, it seems that voltage is what causes current, in general. A current operated device seems to be one where the flow of current in a specific part of the device enables the device to function the way it is supposed to function, fulfilling the purpose it was designed for. For instance, if a transistor is current operated, that current will make the transistor acts as a switch and block or pass current in some other area...Same reasoning goes for voltage controlled devices.
A LED is defined as a current operated device in the sense that the light brightness is dependent on the current in the LED, in a more correlated way than the voltage. That is the functional definition of why the LED is called current operated. But that current is surely generated by tweaking a voltage, even if the relation current-voltage is not linear...
Thanks,
Kavan
Hello again,
I dont want to confuse you, but the current and voltage relationship come sort of at the same time. So it is arguable whether or not the voltage came first or the current came first. But to simplify, it easier to think of EITHER the current or the voltage is what comes first depending on the application. Let me give a couple examples...
The inductor. When we apply a voltage, at the instant of application there is zero current. But an infinitesimally short time later current appears.
The capacitor. When we apply a current, at the instant of application there is zero voltage. But an infinitesimally short time later voltage appears.
But there is no such thing as an infinitesimally short time period, except in theory. A human being can never experience an infinitesimally short time period. So the question comes up as to whether or not nature actually allows infinitesimally short time periods or time is quantized like everything else. And if you try to get an infinitesimally short time period, you end up with zero time, which is no time at all.
So if we apply a signal at t=0 we havent actually done anything yet. This is evident also in noting that at t=0 there can be no power because it's impossible to start a current flowing at t=0 because again no time has yet passed, and every distance in nature inherently contains inductance and so no matter what the device is there is at least some small inductance ever present, and some capacitance as well, and some resistance.
So lets look at an application...
We have a switch and battery to power a resistor wired up. When we turn on the switch, the resistor gets current and voltage from the battery. That's the simplified view.
But what really happens is that when we press the switch, the contacts come closer and closer together, which of course means for a tiny amount of time we have a capacitor connected at one end of the switch, and it is getting larger and larger in value as the contacts come closer and closer to each other. Since the cap has some voltage across it (from the battery through the resistor and wires) that means the pseudo cap will start to conduct. So before the switch actually closes we have a tiny conduction current. It's not zero, but some small value.
So it appears that the voltage was there first, then the current started to flow. But that's not really true either is it? Because the switch contacts were always there, they didnt just suddenly burst into existence. So there must have been capacitance there too (again distance comes into the picture).
So before we even start this experiment, we had voltage AND current. But it's safe to say that the current in the switch had gone to zero some time after the battery was first connected into the circuit, and that had happened some time in the past. So the actual start of the experiment then is when we first touch the switch, and at least one of the switch contacts begins to come closer to the other. That increases the capacitance and so current starts to flow again.
So we started with a set of charges that were arranged in a certain distribution in space, which must have had an electric field associated with it. Then we applied a force to the switch, and that changed the capacitance of the switch. The changing capacitance of the switch caused a current to flow again. So it was our finger that caused the imbalanced charge distribution to change.
But we're not done breaking this down yet. When we push the button, we dont immediately get a change in the distribution of charge. We first have to overcome the parallel capacitance of the wires and the series inductance of the wires. That means initially we're back to square one where the change we cause doesnt have any effect until after some inductance was finished impeding the current flow and the capacitance has stopped preventing the voltage from rising.
So we find we're back to the same situation, where the current getting to the device is impeded by the inductance and the voltage cant appear immediately because of the capacitance (of the device or just the wires). Analyzing this electrically, we see that as the voltage application appears the current begins to rise as well as the voltage to the device, but it doesnt seem possible to determine which came first (as seen by the device, which is what this thread is all about).
To see this in light of past work in physics, there is nothing in Maxwells equations to suggest that one comes before the other. In spite of this there are other views that say that there might be a delay from one to the other.
I hope i've explained this well enough and not confused you at all. This is a subject that requires a very detailed look at nature, where we might be concerned with behavior down in the attoseconds of time to fully (hopefully) understand this.