Noob Questions about Inductors

[Sillyboy]

So, I am trying to read about transformers, which led me to reading about Inductors, which lead to ... well, complete confusion.

Lemme see if I understand this. If I was to take a length of wire, and plunge both ends into random socket in my house... well, the lights would go off. Because I would draw so much current that I would trip the breaker.

However, if instead I first wrap the wire a few dozen times around a nail... and THEN stick it into the outlet... all would be okay?

Assuming that is really all that is required to make an inductor... that is some serious black magic.

I would have created a magnetic field that pushes against the flow of electricity? I assume the strength of this affect changes with the number of wraps (and I read it changes depending on what is in the center of my coil).

Am I understanding this madness correctly? Could it really draw almost no current (assuming I don't present another inductor to it w/ a load... making a transformer...)

[youyoung21147]

Theorically, you are right, but not to allow the 50Hz of the mains to pass, you'll need a really huge inductor with a ferrite core to increase magnetic flux. I'd suggest you not to try

Seriously, an inductor creates a magnetic field in the coil. This magnetic field stabilizes the current flowing in the wire forming the coil, i.e, when you start to send current, it will take time to establish because the inductor creates the magnetic field. When the current flowing decreases, the magnetic field will oppose and act as a generator keeping the current flowing.

Consider a sine wave : tension varies, and so does intensity flowing through the "resistor" formed by the wire. The wire is shaped as an inductor : assume it has a high enough value not to let the current vary as fast as the sine wave makes the intensity vary : almost no current can flow ! this happens in transformers.

It is well-simplified though but I hope it is clear enough

[AndrewT]

Hi,
an inductor is a winding that has both resistance and impedance.

Your straight length of wire is almost all resistance and tiny inductance.

Your nail and turns is now resistance and a little more reactive inductance (at 50/60Hz).

However I think you are misunderstanding two different concepts.
The inductor's two impedances do indeed reduce the total current through the wire, but not to zero. If you were to reduce the resistance to zero and have pure inductance (impossible) then the current would appear to be zero at the maximum voltage and a power meter would indeed measure no power consumption. But there is still current flowing, but lagging 90degrees out of phase. This is because the current and voltage happen at different times.

[lndm]

An inductor is essentially a storage device. It's main behaviour is to resist changes in current.

The way it does this is: when current through it is increased, it puts it aside (so to speak) in the form of a magnetic field around the inductor. When current is reduced, the inductor attempts to keep it flowing by using its reserves in the magnetic field.

In practice, the inductor is the converse of the capacitor. Its bevaviour is theoretically the same but also opposite. ie current and voltage relationships are swapped. High and low frequency behaviour are swapped.

An ideal inductor (with zero DC resistance) does not get hot as it does not dissipate power, it just shuffles it around. If the mains was DC, it would not be possible to do this shuffling and the inductor would act like a short circuit.

[Sillyboy]

Do you have any idea how much this all sounds like black magic?

I am embarrased to admit that before this thread, I had no idea resistance and impedance were different.

So, a wall wart, that is plugged into mains, but not the device it normally drives. So, in theory it would cost me no money? And this is because it would have ... what... infinite impedance (and no resistance) ?

It is the fact that it has some resistance that ends up costing me money (for which i get some heat, I assume).

I am realizing that i have no idea what power/current is. Is that bad? ;-)

maybe a question that is stupid in the most epic of scales, but... how does ohm's law apply to inductors? (or maybe ohm's law isn't somethig used for AC?)

i = v/r. so, the current of a perfect inductor... should in theory be infinite (110v/ 0 ohms) !?

[AndrewT]

Hi,
Ohm's law works for AC, but you have to be very careful applying that law to caps and inductors. They draw current that varies with time and Ohm's law cannot be used for that type of variation (maybe?).
I was never too good at this real and imaginary stuff.

Recently another thread had a discussion on the inductance of a transformer and what kind of load it presented to the mains. I think the other posters went silent when I suggested that a transformer is a substantially resistive load on the mains, otherwise the power companies would have had it outlawed for overheating their distribution systems with no revenue from us consumers.

[lndm]

Ohms law does always work.

The thing is, as AndrewT was saying, there is a complex relationship between reactive components (inductors/capacitors) and resistors.

To illustrate, FWIW. If you have an inductor, and you choose a frequency where that inductor is expressing 1 ohm of impedance, and you put it in series with a 1 ohm resistor, the total impedance will be 1.4 ohms.

I have found that inexpensive wall warts will buzz and hum and get hot. Apart from what's already been said, their energy can be wasted through the movement of the core laminations producing sound and heat.

I think that a wall wart's ability to be quiet and remain cool shows it's suitability to remain plugged in for extended periods.

[damo21]

To add a little more to these explanations:

Magnetic field and electric field are orthogonal to each other so ohms law applied to these two things simultaneously needs to account for this fact.
In a more mathematical framework, this means you can't just add inductance and resistance in a linear fashion, ie R+L=Z doesnt work.
What happens in reality is that energy is stored in components such as inductors and capacitors, to be released at a later time, but this exact amount of energy is not something which you can necessarily measure or observe within the electrical circuit as a dissipation of energy (like for example in a resistor). It can be associated to the circuit by means of a second component j such as a vector in two dimensions would have 2 components.
Then the "complex" j component refers to the "storage" of energy at a particular time and the "real" component refers to the actual component which dissipates energy in the circuit.
Try a textbook on electrical circuits it's really interesting stuff!

[Sillyboy]

Ya, I am slowly working through a few electronics books now, like a good boy

Thanks to everyone for their replies. Got your pm, Cal, and no -- I didn't realize the wire in an inductor was "special" (coated and magnetic)