# LCR circuits and phase angle

## Homework Statement

A resistor (R), capacitor (C), and inductor (L) are connected in series. What is the complex impedance, Z,
of this LCR series combination?

An AC supply of voltage V(t)=V0eiωt is applied across an LCR series combination.
Derive an expression for the current I(t) in the circuit. From your expression for I(t)
write down expressions for both the magnitude of the current and the tangent of the
phase angle between current and applied voltage.

## Homework Equations

ZL=iωL

ZC=-i/(ωC)

ZR=R

I(t)=V(t)/Z.​

where; ω=Angular frequency of the voltage and i=√(-1).

## The Attempt at a Solution

For the total impedance I got:

Z=R+i(ωL-$\frac{1}{ωC}$),​

and for the current:

I=(V0eiωt)/(R+i(ωL-$\frac{1}{ωC}$)).​

For the tangent of the phase angle it would just be the tangent of the argument of the above equation for the current? If so, I can't get it into a "nice" equation without a trig function of another trig function. Thank you for any help.

Last edited:

## Answers and Replies

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tiny-tim
Science Advisor
Homework Helper
Welcome to PF!

Hi Sleepy_time! Welcome to PF!
For the tangent of the phase angle it would just be the tangent of the argument of the above equation for the current? If so, I can't get it into a "nice" equation without a trig function of another trig function.
tan of the phase angle of a + ib is simply b/a

for (a+ib)/(c+id), use the tan(θ-ψ) formula

Hi tiny-tim, thanks for the help. So what I got is shown in the attachment. Have I got it right?

#### Attachments

• 18.6 KB Views: 313
tiny-tim
Science Advisor
Homework Helper
Hi Sleepy_time!

erm
An AC supply of voltage V(t)=V0eiωt

the tangent of the phase angle between current and applied voltage.​

apart from that, fine!

(remember, impedance, complex current, etc are all constants! )