Why Does Adding a Coil Decrease Current in a Circuit?

[GoodEngineering]

TL;DR

Why does the current decrease if added a coil?

Hello, so today I in my electrical machines class the teacher said that if we add a coil on a cicuit the total current will be less compared with a circuit with no coil included. Also if we replace a coil with (for example) 100 spirals with another with 300 spirals, the total current going through the circuit will be less. Example:

The teacher hasnt explained to us the math behind this, so right now I'm unable to solve this, Nevertherless, that's not what I'm looking for. He told us while we were in the class that this occurs because the magnetic field slows the current down, later on when the class was over I asked him for more information about this phenomenon, & told me that the current actually converts into a magnetic field.

Of course this second declaration left me confused & this is why I post this.

So can anyone tell me clearly why & how the current decrease when we add a coil in the circuit?

 

[hutchphd]

Nowhere do I see the magic acronym "A. C." or mention of frequency. You do understand that these effects all require dynamics, yes?
Also a "magnetic circuit" describes a pathway for magnetism, so the nomenclature is usually a circuit with inductors.

 

[anorlunda]

Those examples are terrible and wrong because the don't mention time.

The voltage across an inductor (coil) is proportional to the time rate of change of current. So it slows down the rate of increase of current. If the current is steady, the coil does nothing.

 

[berkeman]

Hello, so today I in my electrical machines class the teacher said that if we add a coil on a cicuit the total current will be less compared with a circuit with no coil included. Also if we replace a coil with (for example) 100 spirals with another with 300 spirals, the total current going through the circuit will be less.

Have you had differential equations in your studies so far? The relationship between the voltage across an inductor and the current through it are related by the well-known differential equation:

$$v(t) = L [\frac{di(t)}{dt}]$$

That explains why more turns (higher inductance) lowers the AC current through a coil...

 

[GoodEngineering]

Well I think I forgot to say that it is DC, not AC.
This is why I was curious. Of course I know that coils in AC have a resistance, rather inductive reactance.

 

[Tom.G]

Oh my!

An instructor that needs instructing?

Who would have thought!

 

[DaveE]

I think you may have misunderstood something, or perhaps your instructor didn't explain what he really meant. As others have said, this doesn't really make sense for a DC circuit.
It's hard for us to explain what an inductor is and how it works if you aren't at that point in your studies. Unfortunately, there are a lot of "practical" or "vocational" electronics classes that do a really poor job of teaching what inductors and capacitors really are. These components respond to the rate of change of the voltage and/or current in the circuit. This means that the best understanding of them comes after you have studied a little bit of calculus.
This link may be helpful if you want to know more:
https://www.khanacademy.org/science...-and-forced-response/a/wmc-inductor-in-action

 

[davenn]

Well I think I forgot to say that it is DC, not AC.

OK

Of course I know that coils in AC have a resistance, rather inductive reactance.

no, not correct, the other way around

Coils in DC have resistance rather than inductive reactance that they do with AC

As others have said, this doesn't really make sense for a DC circuit.

of course it makes sense with DC
Adding a coil, is just adding extra resistance, and therefore the current through the circuit drops
as stated in post #1. There will be other effects as the magnetic field is generated

It seems everyone that responded has wrongly assumed AC