# Homework Help: Electronics: Finding current etc. in an RCL parallel circuit with complex numbers? Ma

1. Apr 24, 2012

### EVriderDK

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

50 volts, 60 Hz

I have to find the current through R, C, and L.
Total current.
Totalt impedance.

2. Relevant equations

I=(U/Z)

3. The attempt at a solution

For I_R it must be (50/150)=0.333. And as it is a resistor, the angle must be 0, so 0.333∠0 ?

On my TI89 i can sum all the currents on complex form, but how to do it in mathcad? On the TI89 it is just (3∠90)+(4∠90) etc.

2. Apr 24, 2012

### Staff: Mentor

Re: Electronics: Finding current etc. in an RCL parallel circuit with complex numbers

In MathCad you can work directly with complex numbers. Just assign R, ZL, ZC variables the appropriate complex values, then write whatever expressions you want with them. Use the |x| operation to find the magnitude of the complex value x. Similarly use the arg(x) function to find the argument, that is angle, of the complex value.

Examples:

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3. Apr 25, 2012

### EVriderDK

4. Apr 25, 2012

### Staff: Mentor

Re: Electronics: Finding current etc. in an RCL parallel circuit with complex numbers

The red highlight that you're seeing on the "j" variable indicates that you're referencing an undefined variable.

When entering i or j to represent $\sqrt{-1}$ then enter them as 1i or 1j, that is, a numeric 1 followed immediately by i or j. The '1' will disappear when the equation is not selected. This serves to distinguish these special variables from mundane variables i and j that you may define and use.

5. Apr 26, 2012

### EVriderDK

Re: Electronics: Finding current etc. in an RCL parallel circuit with complex numbers

Thank you! Do you know how only to calculate on polar form?

And is there away to reset all defined variables? Like r:=r with r and all:=all with all? :)

Last edited: Apr 26, 2012
6. Apr 26, 2012

### Staff: Mentor

Re: Electronics: Finding current etc. in an RCL parallel circuit with complex numbers

I'm not aware of any built-in way to force a result to be displayed in polar form, at least not with the older version of Mathcad that I'm using.

You can "undefine" individual variable numerical values by using them in unfinished assignment statements:

r :=
b :=

etc.

They can be turned into unassigned variables for the symbolic processor by equating them to themselves as you've done. I'm not aware of any way to de-assign all variables in one step.

7. Apr 26, 2012

### EVriderDK

Re: Electronics: Finding current etc. in an RCL parallel circuit with complex numbers

THank you for all your help!

8. May 28, 2012

### NemoReally

Re: Electronics: Finding current etc. in an RCL parallel circuit with complex numbers

You can define functions to do the conversions. In Mathcad 15, you can define them in such a way as to allow you to put them in the unit placeholder to display results in the way you want.

NR

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• ###### phys - 12 05 28 complex number polar notation.zip
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9. May 28, 2012

### EVriderDK

Re: Electronics: Finding current etc. in an RCL parallel circuit with complex numbers

Thank you very much again Nemo. That was helpful. Now i can save a couple of steps henceforth ;)