# Homework Help: Confusion RLC circuit

1. May 21, 2013

### k31453

1. The problem statement, all variables and given/known data

Hi i got this question ?

[Broken]

have to find Vr and VL

so can i use this formula :

VR = IR
VL = I * XL???

confuse ?? need help ??

am i on right track?

Last edited by a moderator: May 6, 2017
2. May 21, 2013

### ehild

Impedance is used for AC circuits, when the current and voltage have sinusoidal time dependence. Here the time dependence of the current is different from a sine function. Go back to the definition of the voltage across a capacitor (in terms of charge) and inductor (in terms of derivative of current).

ehild

3. May 22, 2013

### k31453

so answer is 0.002 * (20/1) = 0.04 for Vl

right?

4. May 22, 2013

### ehild

??? You have to show the time dependence of voltages. The answer is not a number.

ehild

5. May 22, 2013

### k31453

whaaattt???

are you kidding mee !!

6. May 22, 2013

### ehild

I try to help and I am not kidding. Read the problem text, please.

ehild

7. May 22, 2013

### k31453

yeah i know its time dependency .. this is the graph for voltage in inductor vs time in ms

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Last edited by a moderator: May 6, 2017
8. May 22, 2013

### ehild

You sent the plot of voltage across the resistor. The shape is correct, but watch out the magnitude. The current is given in A (ampers) The resistance is 2Ω. What is the maximum voltage across the resistor?

ehild

9. May 22, 2013

### k31453

[Broken]
Got it right?

Last edited by a moderator: May 6, 2017
10. May 22, 2013

### Staff: Mentor

No, your diagrams are not correct.

Can you write the general expressions (definitions) relating voltage and current for a resistor and an inductor?

11. May 22, 2013

### ehild

No. The maximum voltage for the resistor is about right now, but the shape is not. You remember Ohm's Law: The voltage across the resistor is U=IR, proportional to the current. The shape of the U(t) function follows the shape of I(t).

As for the inductor, remember Faraday's law about induced emf in a coil and inductance. How was it defined?

ehild

Last edited by a moderator: May 6, 2017
12. May 22, 2013

### k31453

I know deal with resistor but i have no clue about inductor

13. May 22, 2013

### ehild

The voltage across the inductor is U=L(dI/dt) (it is proportional to the time derivative of the current). Determine the derivatives at the different sections and multiply by L.

ehild

14. May 22, 2013

### barryj

Along with U=L(dI/dt) , We should point out that the voltage across the capacitor v = (1/C) integral (i(t)) and of course voltage across the resistor is v = IR. The plot of current is simple enough that taking the integral or the derivative is simple.

15. May 22, 2013

### technician

I agree with you. If the graph is current against time then this is the voltage across the inductor for the first part of the graph AND the last part of the graph. This voltage will be constant over these time intervals.
For the middle part it will be -0.04V.
For the flat bits of the graph there is no change of current with time....what will the voltage across the inductor be ??

Last edited: May 22, 2013
16. May 22, 2013

### ehild

The current changes 2 A in 1 ms,( Edit:20 A in 1 ms) so the voltage on the inductor is not 0.04 V.

ehild

Last edited: May 22, 2013
17. May 22, 2013

### technician

It looks to me like 20A in 1ms .....

18. May 22, 2013

### ehild

I lost a zero, thanks. I edit the previous post. But still, UL is not right.

ehild

19. May 23, 2013

### k31453

so it will be this graph right because di/dt is derivitve !!!

[Broken]

Last edited by a moderator: May 6, 2017
20. May 23, 2013

### ehild

Excellent! Good solution, nice picture. (Only the unit V is missing from the vertical axis.)

ehild

Last edited by a moderator: May 6, 2017
21. May 23, 2013

### k31453

Yeah i solve the question but i forget how i did it and why is like that !!???

:(

22. May 23, 2013

### ehild

Go to post #13

ehild

23. May 23, 2013

### k31453

gotchya for 1 ms liner becomes flat and than flat becomes no gradient follow on right?

24. May 23, 2013

### ehild

The flat part has zero gradient so zero induced voltages.

ehild

25. May 23, 2013

### k31453

Nice !!! thanks for all the help !!