# Confusion RLC circuit

## Homework Statement

Hi i got this question ?

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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?

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ehild
Homework Helper
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

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

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

right?

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

right?

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

ehild

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

ehild

whaaattt???

are you kidding mee !!

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

ehild

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

ehild

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

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ehild
Homework Helper
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

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

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Got it right?

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gneill
Mentor
No, your diagrams are not correct.

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

ehild
Homework Helper
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Got it right?

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

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

ehild
Homework Helper
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

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.

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

right?

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 ??

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ehild
Homework Helper
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 ??

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

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

ehild
Homework Helper
It looks to me like 20A in 1ms .....

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

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

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

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

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

ehild

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

ehild

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

:( ehild
Homework Helper
Go to post #13 ehild

Go to post #13 ehild

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

ehild
Homework Helper
The flat part has zero gradient so zero induced voltages.

ehild

The flat part has zero gradient so zero induced voltages.

ehild

Nice !!! thanks for all the help !!