The RL circuit:

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rattus

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Crossman has given us an example of the step function response of an RC circuit. Well how about an example of an RL circuit?

Consider a battery with voltage, V, connected through a switch to a resistor, R, in series with an inductor, L.

Who cares to explain with words and/or mathematics what transpires after the switch is closed?

We are especially interested in the voltages and currents at times 0+ and infinity.
 
Re: The RL circuit:

Bob,

I already know. I want the members to take a stab at it. These links are very good though, usually.

Rattus
 
Re: The RL circuit:

I'll take a stab at it before I look at the link.

Should be the opposite of the capacitor. When the switch is closed, the inductor acts like a "nearly open" circuit. This is because the quickly rising voltage... errr...

Fact = the current will start at a low level and will build up and hold steady at the current level that flows thrugh the "resistance" of the wire in the coil.

Now for the "why"... has something to do with the magnetic field, as it first starts to build, it opposes the source voltage a bunch, but then the cemf goes down as the change in current reduces... heck, somebody explain it to me.

Basically, the cemf produced by the inductor is proportional to the rate of change of the current flow. If the current is steady, then there is no CEMF and only the resistance of the wire opposes the current. But if the current is rising, CEMF is produced which opposes the change in current. The faster the rise, the greater the CEMF.
 
Re: The RL circuit:

There was an overly-simplified memory aid that was presented early in my EE classes. It was a way of remembering what happens to capacitors and inductors in a DC circuit, after enough time had passed to create a steady state condition. It was also a way to remember which was which.

The answer to the ?which is which? is that a capacitor becomes an open circuit, and that an inductor becomes a short circuit. The memory aid starts with the circuit symbol used for each. A capacitor is shown as two parallel lines. An inductor is shown as a coiled line. Imagine grabbing the symbol, with one hand on each end of the symbol, and pulling your hands apart. As you pull apart the capacitor symbol, the two plates get farther and farther apart, making it look very much like an open circuit. As you pull apart the inductor symbol, you stretch the coiled line into a straight line, making it look very much like a simple wire (i.e., short circuit).

It is not a precise model. It does not account for any resistance or the influence of electrical and magnetic fields. But as a memory aid, it has worked well enough for me.
 
Re: The RL circuit:

In a coil the wires lay right next to each other. As the current changes the flux produced in the wire changes, thus causing said lines of flux to cut through the wire next to it, giving you generator action. Without a picture you'll just have to believe me, but if you used all the different little hand rules you can show that the emf generated due to the flux is counter to the applied voltage, hence counter emf. As the rate of change in current changes so does the rate that the lines of flux are generated. A large change in current gives a large flux which gives a large cemf. When the current is steady there is no change so there is no cemf.
 
Re: The RL circuit:

I am so old that I don't remember ELI the ICE man. I have to remember -L(di/dt) and that integral thing!
 
Re: The RL circuit:

This sketch might help illustrate what those guys just said above.

Ed

Phase8.gif
 
Re: The RL circuit:

That's OK Crossman,

Ed is demonstrating the formula,

cemf = -L(di/dt)

and the sine wave makes a good example.

Nice diagram Ed. Do you have one for a triangular wave?
 
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