document.write( "Question 93127: if z1=2+j, z2=3+j and z3=j3\r
\n" ); document.write( "\n" ); document.write( "what is the equivalent impedance z if \r
\n" ); document.write( "\n" ); document.write( "1/z= 1/z1 + 1/z2 + 1/z3 ?? this was a question given to us for homework and it was not in any text book
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Algebra.Com's Answer #67798 by bucky(2189)\"\" \"About 
You can put this solution on YOUR website!
You are given three impedances:
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\n" ); document.write( "\"z%5B1%5D+=+2+%2B+j\"
\n" ); document.write( "\"z%5B2%5D+=+3+%2B+j\" and
\n" ); document.write( "\"z%5B3%5D+=+0+%2B+j3\" <== notice that this presumes that the impedance is so inductive that the resistance of the device is negligible.
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\n" ); document.write( "The real part of these impedance is resistance and the imaginary part (called the reactance
\n" ); document.write( "and identified as the j terms) are inductances. (You know they are inductances because the
\n" ); document.write( "sign of the j terms is plus. If the sign of a j term is negative, then it is a capacitance.)
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\n" ); document.write( "For the first problem you are asked to find \"z%5Bt%5D+=+z%5B1%5D%2Bz%5B2%5D%2Bz%5B3%5D\". This is a series connection
\n" ); document.write( "of the three impedances and \"z%5Bt%5D\" is the equivalent impedance. You could take out the
\n" ); document.write( "three impedances in a circuit and replace them by the equivalent impedance without effect.
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\n" ); document.write( "In finding \"z%5Bt%5D\" all you need to do is add the three impedances. You do this by
\n" ); document.write( "adding their real parts and then adding their imaginary parts. Where I listed the three
\n" ); document.write( "impedances above, you can just add them in their columns. Adding down the column of real
\n" ); document.write( "parts you get 2 + 3 + 0 = 5. And adding down the column of imaginary parts you get
\n" ); document.write( "j + j + j3 = +5j. (Note j is the same as j1. So the imaginary parts are j1 + j1 + j3 and
\n" ); document.write( "the addition is j(1+1+3) = +j5).
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\n" ); document.write( "So the answer to the first problem is the combination of these two parts, the real part
\n" ); document.write( "which is 5 and the imaginary or reactive part which is +j5. Therefore, we say:
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\n" ); document.write( "\"z%5Bt%5D+=+5+%2B+j5\"
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\n" ); document.write( "You convert this to polar form just as you would in coordinate geometry. You start at
\n" ); document.write( "the origin of an x-y coordinate system and you go +5 units along the x-axis. (That
\n" ); document.write( "represents the real part of your answer.) Then from that point (+5 on the x-axis)
\n" ); document.write( "you go vertically up (the +j direction) +5 units which represents the imaginary or reactive
\n" ); document.write( "part of the answer. [On a graph you are now at the point (5,5).] Now draw a line connecting the
\n" ); document.write( "origin with this point (5,5). What you have now is an Argand diagram of the answer, and the
\n" ); document.write( "Argand diagram consists of the 5 units along the x-axis, the 5 vertical units from that
\n" ); document.write( "point to the point 5,5 and the line connecting the origin to the point (5,5). Just remember
\n" ); document.write( "that the real part of the answer (the resistance) is along the x-axis, and the imaginary part
\n" ); document.write( "(the vertical) represents the j component of the answer.
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\n" ); document.write( "To find the polar form of the answer you need to look at the Argand diagram and find the
\n" ); document.write( "hypotenuse of the right triangle. The hypotenuse will be the magnitude of the answer and
\n" ); document.write( "the angle between the x-axis and the hypotenuse will be the polar angle you need.
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\n" ); document.write( "From geometry you may recall that a right triangle that has two legs that are equal in
\n" ); document.write( "length is a 45-45-90 degree triangle. Therefore, you know that the angle between the
\n" ); document.write( "x-axis and the hypotenuse is 45 degrees. You can use the Pythagorean theorem to find
\n" ); document.write( "the length of the hypotenuse (or you could use trig). The Pythagorean theorem tells you to
\n" ); document.write( "square each of the legs and add these squares to find the square of the hypotenuse.
\n" ); document.write( "So for this problem we have:
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\n" ); document.write( "\"5%5E2+%2B+5%5E2+=+h%5E2\"
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\n" ); document.write( "and since \"5%5E2+=+25\" the problem becomes:
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\n" ); document.write( "\"25+%2B+25+=+50+=+h%5E2\"
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\n" ); document.write( "To find h just find the square root of 50. You can do it as follows:
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\n" ); document.write( "\"h+=+sqrt%2850%29+=+sqrt%2825%2A2%29+=+sqrt%2825%29%2Asqrt%282%29+=+5sqrt%282%29=7.0711\"
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\n" ); document.write( "Now you know that the magnitude and angle (the polar form) is \"5sqrt%282%29\" or 7.0711
\n" ); document.write( "at an angle of 45 degrees which is often written as \"5sqrt%282%29\"/45 or 7.0711/45 where the
\n" ); document.write( "45 is underlined and is followed by the little \"o\" degrees symbol. (Hard to type the exact
\n" ); document.write( "way it is written.)
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\n" ); document.write( "Now for the second part of the problem. A little tougher because of the math manipulations,
\n" ); document.write( "but pretty straightforward.
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\n" ); document.write( "The given formula is:
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\n" ); document.write( "\"1%2Fz%5Bt%5D+=+1%2Fz%5B1%5D%2B1%2Fz%5B2%5D%2B1%2Fz%5B3%5D\"
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\n" ); document.write( "This is the formula for finding the equivalent impedance of three impedances connected in parallel.\r
\n" ); document.write( "\n" ); document.write( "There are several ways this could be done, but let's crank through a straightforward
\n" ); document.write( "method. Begin by substituting into the formula the values for the three impedances
\n" ); document.write( "to get:
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\n" ); document.write( "\"1%2Fz%5Bt%5D+=+1%2F%282%2Bj%29%2B1%2F%283%2Bj%29%2B1%2F%28j3%29\"
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\n" ); document.write( "Next recognize that the common denominator on the right side of this equation is:
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\n" ); document.write( "\"CD+=+%282%2Bj%29%2A%283%2Bj%29%2A%28j3%29\"
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\n" ); document.write( "Let’s multiply this out combine terms. Remember in multiplying that j times j is j squared
\n" ); document.write( "and by definition j squared is -1.
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\n" ); document.write( "First let’s begin by multiplying \"%282%2Bj%29%2A%283+%2B+j%29+=+6+%2B+j2+%2B+j3+%2Bj%5E2+=+6+%2B+j2+%2B+j3+-1+=+5+%2B+j5\"
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\n" ); document.write( "Next multiply that answer times j3:
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\n" ); document.write( "\"CD+=+%285%2Bj5%29%2A%28j3%29+=+j15+%2B+j%5E2+%2815%29+=+j15+%96+15\" which in more standard form becomes:
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\n" ); document.write( "\"CD+=+-15+%2B+j15\"
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\n" ); document.write( "If we put all three terms over the common denominator, each term's numerator consists of
\n" ); document.write( "the product of the other two terms that were not its denominator. In other words the equation
\n" ); document.write( "becomes:
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\n" ); document.write( "Now let’s multiply out each of the numerators in the three terms.
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\n" ); document.write( "First numerator: \"%283%2Bj%29%28j3%29+=+j9+%2B+j%5E%282%293+=+j9+-3+=+-3%2Bj9\"
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\n" ); document.write( "Second numerator: \"%282%2Bj%29%28j3%29+=+j6%2Bj%5E%282%293+=+j6+-3+=+-3+%2Bj6\"
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\n" ); document.write( "Third numerator: \"%282%2Bj%29%283%2Bj%29+=+6+%2Bj2+%2Bj3+%2B+j%5E2+=+6%2Bj2%2Bj3-1+=+5%2Bj5\"
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\n" ); document.write( "Now add all the numerators over the common denominator:
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\n" ); document.write( "Now we can get the answer for \"z%5Bt%5D\" by just inverting both sides of this equation to get:
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\n" ); document.write( "\"z%5Bt%5D+=+%28-15%2Bj15%29%2F%28-1%2Bj20%29\"
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\n" ); document.write( "Just to make things a little more conventional, let’s factor -1 from both the numerator and
\n" ); document.write( "denominator of this answer so that the real portions of answer are positive. This factoring
\n" ); document.write( "results in:
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\n" ); document.write( "This is not in a useful form yet. We can convert the denominator of this answer by multiplying
\n" ); document.write( "it by its conjugate. The conjugate has the same real part and imaginary part, but the imaginary
\n" ); document.write( "part has the opposite sign. In this case the denominator is \"1-j20\" so its conjugate is
\n" ); document.write( "\"1+%2B+j20\". When we multiply the denominator by \"1%2Bj20\", we must also multiply
\n" ); document.write( "the numerator by \"1%2Bj20\". So we get:
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\n" ); document.write( "\"z%5Bt%5D+=+%2815-j15%29%2F%281-j20%29+%2A+%281%2Bj20%29%2F%281%2Bj20%29\"
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\n" ); document.write( "Multiplying the denominator by its conjugate results in:
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\n" ); document.write( "Then multiplying the numerator by the conjugate of the denominator:
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\n" ); document.write( "This further multiplies out to:
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\n" ); document.write( "\"15+%2B+j300+%96+j15+-+%28-1%29%28300%29\"
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\n" ); document.write( "Subtracting the two j terms shortens this to:
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\n" ); document.write( "\"15+%2Bj285+-+%28-1%29%28300%29\"
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\n" ); document.write( "The last term involves minus a minus term which is positive:
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\n" ); document.write( "\"15+%2B+j285+%2B+300\"
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\n" ); document.write( "The two real parts add and the numerator becomes:
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\n" ); document.write( "\"315+%2B+j285\"
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\n" ); document.write( "This gets put over the real denominator which we found to be 401:
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\n" ); document.write( "\"z%5Bt%5D+=+%28315+%2B+j285%29%2F401+=+315%2F401+%2B+j%28285%2F401%29\"
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\n" ); document.write( "Dividing the two terms by 401 results in:
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\n" ); document.write( "\"z%5Bt%5D+=+0.7855+%2B+j%280.7107%29\"
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\n" ); document.write( "The Argand diagram will start at the origin and go to the right on the x-axis 0.7855 units.
\n" ); document.write( "From that point it will go vertically upward 0.7107 units. You are now at the point
\n" ); document.write( "(0.7855, 0.7107). Draw a line from the origin to that point. Again, as in the first
\n" ); document.write( "problem, this is the Argand diagram for the equivalent impedance.
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\n" ); document.write( "Next we will use the Argand diagram to find the polar form. The polar form requires that
\n" ); document.write( "you have a magnitude and an angle. The magnitude is the length of the hypotenuse in the
\n" ); document.write( "Argand diagram, and you can use either the Pythagorean theorem or trig functions to get
\n" ); document.write( "the magnitude. This time let’s use trig. We know that the tangent is defined by the side
\n" ); document.write( "opposite divided by the adjacent side of the angle. In this problem the side opposite the
\n" ); document.write( "angle we are looking for is the vertical side and its length is 0.7107. The adjacent side
\n" ); document.write( "is the horizontal side of the triangle and its length is 0.7855. So the tangent of the angle
\n" ); document.write( "we are looking for is:
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\n" ); document.write( "\"tanA+=+0.7107%2F0.7855+=+0.9048\"
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\n" ); document.write( "That means that the angle has a tangent of 0.9048. Using the the tan^(-1) function key
\n" ); document.write( "on a calculator we find that the angle with a tangent of 0.9048 is 42.1388 degrees. So
\n" ); document.write( "we have half of what we need for the polar form. We still need the hypotenuse and
\n" ); document.write( "we can get that using either the sine or cosine function. Let’s use the sine. The
\n" ); document.write( "definition of the sine is:
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\n" ); document.write( "\"sinA+=+%28side+opposite%29%2F%28hypotenuse%29++\"
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\n" ); document.write( "If we multiply both sides of this equation by the hypotenuse we get:
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\n" ); document.write( "\"%28hypotenuse%29%2AsinA+=+side+opposite\"
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\n" ); document.write( "Then divide both sides by \"sinA+\" to get:
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\n" ); document.write( "\"hypotenuse+=+%28side+opposite%29%2FsinA\"
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\n" ); document.write( "Now we can plug in values. We know the side opposite is 0.7107 and \"sintA\"
\n" ); document.write( "is the sine of 42.1388 degrees. From a calculator this is 0.6709. So our equation
\n" ); document.write( "with these values is:
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\n" ); document.write( "\"hypotenuse+=+0.7107%2F0.6709+=+1.0593\"
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\n" ); document.write( "So the polar form of this impedance is 1.0593/42.1388 which is read as 1.0593 ohms at 42.1388
\n" ); document.write( "degrees.
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\n" ); document.write( "Lots of work. Hope this all makes sense to you and you can figure out from this the basics of
\n" ); document.write( "finding equivalent impedances for series and parallel circuits.
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