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Finding the parallel impedance in a circuit

  • Engineering
  • Thread starter badaboom
  • Start date
  • #1
23
0

Homework Statement


I need to find the total impedance in a circuit that has:
j60 ohms, 30 ohms, -j60 ohms and 20 ohms in parallel


Homework Equations


Ztotal=1/(Z1+ Z2+ Zn)


The Attempt at a Solution


I've tried doing this, but what I think is the -j60 and the j60 should cancel therefore leaving only the 30 ohms and 20 ohms in parallel, which would equal 1/50, but the answer says it is 12 ohms.

Attached is an image of the circuit and answer.
 

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Answers and Replies

  • #2
gneill
Mentor
20,712
2,714

Homework Statement


I need to find the total impedance in a circuit that has:
j60 ohms, 30 ohms, -j60 ohms and 20 ohms in parallel


Homework Equations


Ztotal=1/(Z1+ Z2+ Zn)


The Attempt at a Solution


I've tried doing this, but what I think is the -j60 and the j60 should cancel therefore leaving only the 30 ohms and 20 ohms in parallel, which would equal 1/50, but the answer says it is 12 ohms.

Attached is an image of the circuit and answer.
So then, 1/30 + 1/20 = 1/50 ? Check your math.
 
  • #3
25
0
Parallel impedance is:

Ztotal=[1/(Z1+ Z2+ Zn)]^-1

Also remember when adding fractions you must have a common denominator.

For example 1/a + 1/b = (a+b)/ab
 
  • #4
23
0
but how does that j5 get up there?
 
  • #5
25
0
To be honest i think that's a mistake. The negative and positive j components cancel out. That should say 5/20 not j5/j20 even though those two ratios are equivalent.

If you're having trouble adding fractions try this page:
http://www.webmath.com/addfract.html
 

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