wbeaty said:
The question is, is IsaacBinary basing his reasoning on the idea that "voltage causes current?"
IssacBinary said:
wbeaty, from all my learning and what I've been taught. I would have to say yes.
Then that's another conceptual sticking point. Yes, in resistors, voltage DOES essentially cause current (meaning that e-fields accelerate the resistor's mobile charges.)
It also works the same way for real-world metal conductors. The current in a wire is always caused by a (tiny) voltage applied lengthwise.
But inductors and capacitors are different. In these, the applied voltage
doesn't directly cause current. L and C devices ...they're very weird.
IssacBinary said:
You can't have a current without a voltage can you?
Sure you can: a bunch of charges flowing in a ring-shaped conductor are analogous to a flywheel.
If you get this 'flywheel' going at high amperes, then disconnect it from everything, the currents persists for a time. If you used ideal zero-ohm wire, the current will continue forever, a flow without a voltage. If you used actual realworld superconductor, the current will keep going, and only decay over billions of years. It's a kind of electromagnetic inertia, fast moving charges, zero drive volts.
IssacBinary said:
So is that understanding wrong?
Yes.
And also no. In capacitors and inductors, the currents and voltages are divorced from each other. It's easy to produce currents without voltage, and voltages without currents.
It's similar to flywheels: they can keep spinning without any drive force. It's similar to balloons, they can remain pressurized without needing any flow. But still you'd need a brief force to initially accelerate your flywheel up to speed. And still you'd need a brief flow of air in order to inflate your balloon initially. The V and the I are separated in time, and don't occur together.
IssacBinary said:
Also wbeaty, I've seems a few places here and there about "current drawing" , "loads" etc but I've never come across any learning material about these effects / properties or anything explained in those ways. So maybe I am missing something?
Bingo, another sticking point. Resistors are associated with two separate concepts:
1. If you have a constant current in an ideal conductor, and then you cut the conductor and insert a resistor, a voltage drop will appear. You started out with a pure current and no voltage. The resistor opposes the flow and causes the voltage-drop to arise. It's like sticking your hand in a rushing creek and experiencing a force.
2. If you have a constant voltage between two conductors, and then connect a resistor across them, the resistor "draws a current." You started out with a pure voltage and zero current. By adding the resistor, you provided a leakage path between the conductors, causing a current to arise. It's like puncturing a balloon and experiencing an air jet.
Resistors can be leakage paths on constant voltage supplies. Or they can be opposers-of-current in a constant-current power supply. (All these concepts are hidden in intro DC engineering texts on "Thevenin equivalent," and CC and CV supplies.)
Mechanical analogies:
1. if a flywheel is spinning, and you let it rub against your finger, this creates a force which makes the flywheel speed start slowing constantly.
2. If you have a pressurized container, and you drill a small hole in it, this creates a flow which makes the pressure start falling constantly.
In education research, the first one is called "current based reasoning," and most of us learn this version in grade school. "Batteries create current electricity." "Light bulbs consume the current." In physics class, the challenge for the teachers then becomes this: get all the students' minds loose from no. 1 above. They have to be freed up so they can learn "voltage reasoning" as in no. 2.
Your question about phase lies within the domain of no. 2, because capacitors have two functions as well.
1. When inserted into a constant current, a capacitor opposes the current and creates a rising voltage-drop across the capacitor.
But here I think is the key you've been missing:
2. when a capacitor is connected across a voltage-based power supply, it "draws a current." Or said more conventionally: the capacitor's current is proportional to the slope of changing supply voltage.
Suppose you connect a capacitor to a triangle wave voltage generator. When the triangle voltage is smoothly rising, the capacitor draws a perfectly constant current. (The current will be larger for a high-value capacitor.) And, when the triangle voltage is dropping, the capacitor draws a negative current. The upshot: apply a triangle-wave voltage across a capacitor, and the capacitor draws a square wave current from the power supply.
Analogy: if you connect a balloon to a regulated supply of air pressure, then slowly adjust the supply pressure upwards, the balloon will draw a constant air flow as it inflates. Next, adjust the regulated pressure downwards, and the balloon will deflate as it pumps a constant unchanging air flow back into the supply.
