newbie needs help!!!!

Dave

Joined Nov 17, 2003
6,969
I have remerged the threads. Please ensure that replies are constructive to the OP, afterall they are trying to get help with understanding a concept and asked for it in "really, really simple terms".

If we want debate on the deep intricate details of inductance then please feel free to start a new thread.

Dave
 

R!f@@

Joined Apr 2, 2009
9,918
R!f@@,



No, inductance is not a resistance and resistance is not a "force". Inductance and resistance impede (impedance) current by two entirely different mechanisms. A resistor impedes current by dissipating the energy of the moving charges into heat, thereby lowering the voltage (energy density) of the charge. An inductance uses the energy of the moving charges to build a magnetic field where the energy is stored. While the field is increasing, it induces a back voltage that lowers the current through the coil. When the current stops changing, the magnetic field collapses and assists the voltage that was used to first build up the magnetic field. In summary, energy dissipated by a resistor is lost from the circuit as heat, but the energy "borrowed" from the circuit to build the magnetic field is eventually given back for a zero sum gain/loss.



Wrong. Inductance affects (not effects) any circuit when current change is present, whether AC or not.

Ratch

affects: that is my mistake, a simple mistake.

A resistor or inductor heats up, meaning what dude.
Energy lost or transfered.

Beside I do not think the thread starter wanted a huge debate.
If he wanted to go all the way down to voltage transfer, eddy current, collapsing field, magnetic fields and all that he would have not asked for a simple explanation that a beginner would understand.

Don't try to prove me wrong, I know what a beginner needs to understand if he wants to learn the stuff. Once he knew what it is then he can go down the lane to Einstein head or what ever

I rest my case.

Rifaa
 

studiot

Joined Nov 9, 2007
4,998
Unfortunately Riffa, although you have posted many good replies, you slipped up on this one.

As resistance is to DC ( although it is also to AC ), Inductance is mainly a resistive force to AC only
Inductance also comes into play in DC circuits, as in my example with the doorbell.

Further your above statement contains the crux of the paradox I mentioned.

Better not to have introduced AC and DC.
 

R!f@@

Joined Apr 2, 2009
9,918
hmmm!! May be for you guys this is not OK.
When I taught I used this info to give them a basic Idea, once they are comfortable further info is given, advancing as the course progresses.

I don't reply to things that I am not comfortable with.
Still studiot, your way of proving me wrong is better and I welcome your comment.

PS I think you have a typo in my nick

Rifaa



 
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studiot

Joined Nov 9, 2007
4,998
PS I think you have a typo in my nick
Probably, my spulling is known as the worst at AAc. That is why I make so many edits.

The circuit problems posed on many courses are often prefaced

After a long time
or
After the circuit has achieved a steady state

or some such. You may have seen this sort of problem posted here in the homework section.

In fact Transient v Steady State is more fundamental than AC v DC. Elementary treatments tend to avoid transients so students can be lulled into sense of using the simple equations inappropriately.
 

Ratch

Joined Mar 20, 2007
1,070
R!f@@,

A resistor or inductor heats up, meaning what dude.
Energy lost or transfered.
Are those two sentences questions? I am not sure. If they are, post #5 of this thread answers them.

Beside I do not think the thread starter wanted a huge debate.
If he wanted to go all the way down to voltage transfer, eddy current, collapsing field, magnetic fields and all that he would have not asked for a simple explanation that a beginner would understand.
And he received a simple, concise explanation from me in post #6 of this thread.

Don't try to prove me wrong, I know what a beginner needs to understand if he wants to learn the stuff. Once he knew what it is then he can go down the lane to Einstein head or what ever
I believe I do also, that is why I posted #6 of this thread.

I rest my case.
Which is?

Ratch
 

Thread Starter

Ridgy

Joined Oct 14, 2009
3
now I'm really confused, half are saying inductance is like resistance and the other half are saying that's wrong. Isn't reactance like or similar to resistance?
I know that inductance opposes the change in current flow and can store a magnetic charge but I can see the resemblance. Like resistance, I was told if you have a hose with water flowing through it and you squeezed the hose and slowed the water down that is an example of resistance, could someone use an anallergy like that for inductance.( sorry I posted this in the wrong place and thnx again for your help).
 

Ratch

Joined Mar 20, 2007
1,070
Ridgy,

now I'm really confused, half are saying inductance is like resistance and the other half are saying that's wrong. Isn't reactance like or similar to resistance?
I know that inductance opposes the change in current flow and can store a magnetic charge but I can see the resemblance. Like resistance, I was told if you have a hose with water flowing through it and you squeezed the hose and slowed the water down that is an example of resistance, could someone use an anallergy like that for inductance.( sorry I posted this in the wrong place and thnx again for your help).
Shame on any and all who tell you that inductance is similar or like resistance, or the opposite of capacitance. It's not. Inductance is unique, complementary to capacitance, and supplementary to resistance. There is nothing that can substitute or replace all its properties simultaneously. Much seems to be made of the properties of inductance and resistance causing less current to exist in a circuit, so let me start with that. Resistance restrains the current, while inductance opposes the current. Resistance restrains the current constantly, while inductance only opposes the current for AC, or any other time the current is trying to change. Resistance is constantly draining energy permanently from a circuit. Inductance does not. Inductance opposes current by building up a back voltage. Resistance restrains the current by dissipating the circuit energy as heat, thereby lowering the voltage across the resistor. Inductance causes a phase change between current and voltage. Resistance does not. Inductance causes a frequency dependent reactance. Resistance is theoretically independent of frequency. Inductance is a energy storage element. Resistance is not.

So do you still think inductance is "like" resistance? If so, think; no, not, or negative. If you still don't know exactly what inductance is, perhaps you now know what it is not.

Ratch
 

Wendy

Joined Mar 24, 2008
23,415
Actually capacitors and inductors both have reactance, in opposite ways. A coil resists change in current, a capacitor resists change in voltage. Both can resist AC or DC, but they do it different ways, the math is different for each. They have one thing in common about their reactive resistance though, it generates no heat, unlike a resistor. For this reason both can be used to limit current in an AC circuit, such as a flourescent lamp (generally a coil is used for this application). As you may have noticed, reactance resembles resistance, it can be used instead of resistance in many cases, but it isn't true resistance. Many cases this reactance is expressed in ohms, I'm not sure how you could do this differently, but it is the convention.

A capacitor can pass AC and block DC, a coil can pass DC and block AC. When a circuit is made using both the equations can be quite complex, often involking the square root of a -1. Both store energy, the coil does it with a magnetic field, the capacitor does it with an electric field.

This is one of those areas you start simple, with simple concepts, and build on knowledge as you advance. To start simple, look how they are used. As a suggestion, pick an application you are interested in, and ask how they might improve what you are trying to do. Break it down into small pieces, digest the info before getting more. As you might have guessed by now, it is a large part of electronics.

Start with these concepts.

XL (Reactance in ohms) = 2 * PI * Frequency * L (in Henry's, the unit of inductance)
XC (Reactance in ohms) = 1 / (2 * PI * Frequency * C (Farads, the unit of capacitance))

The part we're not talking about is phase shift, which is where the hairy math comes in.
 
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bundick

Joined Dec 19, 2007
97
I had to laugh at Bill's reply. I think we spent a week on that in class.
I remember seeing a Resister laid out in all of its individual componants and formulas.

I made a not to myself about stopping at the Tech level. That diagram was a mind muddler for me.
"Resistance" is as far as I wanted to go. :)
 

Wendy

Joined Mar 24, 2008
23,415
It is a lot simplier. I've been wondering if we wouldn't have been better just going over the part, and leaving the comparisons aside. The comparisons would come later, after a deeper understanding of the part by itself.
 

Thread Starter

Ridgy

Joined Oct 14, 2009
3
so is inductance the opposition to current change or is it the induced voltage it puts back into the circuit? sorry for the delay as in time difference as I am in western australia. thnx again
 

Wendy

Joined Mar 24, 2008
23,415
Actually aspects of both. A coil is an electromagnet (and visa versa), but so small we don't really notice the magnetism (usually). As you have current go through the coil a magnetic field develops. This is stored energy. If you change the current the field has to change, so you're going to add or subtract more energy, and there is a bit of something that resembles inertia doing this. The magnetic field actually grows or shrinks with the change in current.

If you break the current suddenly, as with a switch, the magnetic field collapses though the coils windings, and all that energy shows up as a high voltage spike. A flyback transformer cultivates this effect, the voltages it reaches are quite impressive. I used to take a simple relay and make a buzzer out of it, the coil of the relay will definitely tingle from the high voltage spikes.

A coil resists current change. If you force it to change it generates voltage. This voltage can go both ways, adding or subtracting to the power source voltage feeding the coil. This process is very well understood, and can be expressed with the right math.

Coils and caps are easy to make, every kid makes an electromagnet using a nail and wire at some time. This is an inductor, pure and simple. One of the several ways it gets complex is different materials (such as the nail) act differently for different frequencies, but that is something that comes later.

You want to see how something like this can be used, check out CMOS 555 Long Duration LED Flyback Flasher.

Remember that relay I was talking about? If you don't design for it, that high voltage spike can actually damage electronics inside a circuit. The thing to remember is it was the current that set this up, the voltage is a side effect.

When you have a coil in series with a resistor there is a charge curve, showing how the current stabilizes in the coil. It doesn't happen immediately, but it is extremely predictable.

Part of what got us in trouble is caps are almost a mirror image of coils mathematically speaking. I could rewrite the paragraph above with capacitors, and mostly the verbiage would be extremely similar, except I would exchange magnetic field for electric field, current for voltage, and voltage for current.
 
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