I think I lost the fundamental knowledge on this kind of stuff, so hope you guys
ID: 2019077 • Letter: I
Question
I think I lost the fundamental knowledge on this kind of stuff, so hope you guys explain to me in a very detailed way. Thank you very much.
Given the circuit as above. The switch has been closed for a long time before being opened at t = 0.
This is the explaination in the book
Since the switch has been closed for a long time prior to t so the voltage across the inductor must be zero at t=0-. Therefore the initial current in the inductor is 20 A at t=0-
This is what I'm still confused about
1. I think that the resistor will absorb some energy form the current source, so the current in the inductor will never reach 20A. So why the book said it is 20A.
2. I thought that if an electrical charge flows, it must have a difference in voltage between two points. So if the voltage across the inductor is zero, why the current is still 20A at the same time?
3. This is my knowledge about the inductor. Please check it if it's right or not. When a current go into an inductor, the inductor induces a voltage which produces another current in opposite direction to the initial current. That's because the inductor does not want change in current passing through it.
4. Following (3), this is what I'm very confused. How many currents in and voltages across the inductor? The current going into the inductor (i1), the voltage making i1(v1) , the i2in opposite direction and v2 making i2. So there is 2 currents and 2 voltages right? So which one is the v = L(di/dt)?
I think I must be crazy because of this matter. Any help would be appreciated. My exam is comming soon :(. Thank you in advance.
Explanation / Answer
I think part of your confusion stems from that item on the left: the 20 amp current source. That is an imaginary component that, no matter what else happens in the circuit, equals whatever voltage+load it takes to make 20 amps flow. Doesn't really exist in the real world. The inductor is just a piece of wire after the current has been on a long time. That's why there is no potential difference across it -- no more than there is a potential difference between two points on a wire. The current flows because there is potential difference and a load elsewhere, and the wire (or inductor) is just a conduit. Since it is just a piece of wire, its theoretical resistance is zero -- so all the current will flow through it and none through the 0.1 ohm resistor. Yes, in real life the inductor's resistance would be 0.001ohm or something -- but not in this example.