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Question:
In series LCR circuit resonance occurs at frequency f=fo . If at this moment amplitude of current is I then what is the way less current at that time
Answer:

Voltage and current calculation for this circuit are based on Ohms Law. Our basic expressions are:

«math xmlns=¨http://www.w3.org/1998/Math/MathML¨»«msub»«mi»i«/mi»«mi»L«/mi»«/msub»«mo»=«/mo»«mfrac»«msub»«mi»V«/mi»«mi»L«/mi»«/msub»«msub»«mi»X«/mi»«mi»L«/mi»«/msub»«/mfrac»«/math»     ; «math xmlns=¨http://www.w3.org/1998/Math/MathML¨»«msub»«mi»i«/mi»«mi»C«/mi»«/msub»«mo»=«/mo»«mfrac»«msub»«mi»V«/mi»«mi»C«/mi»«/msub»«msub»«mi»X«/mi»«mi»C«/mi»«/msub»«/mfrac»«/math»     ; «math xmlns=¨http://www.w3.org/1998/Math/MathML¨»«msub»«mi»i«/mi»«mi»R«/mi»«/msub»«mo»=«/mo»«mfrac»«msub»«mi»V«/mi»«mi»R«/mi»«/msub»«mi»R«/mi»«/mfrac»«/math»     ; «math xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mi»i«/mi»«mo»=«/mo»«mfrac»«mi»V«/mi»«mi»Z«/mi»«/mfrac»«/math»

Since this is a series circuit, the value of "i" in each expression is the same. That is

                                                                        «math xmlns=¨http://www.w3.org/1998/Math/MathML¨»«msub»«mi»i«/mi»«mi»L«/mi»«/msub»«mo»=«/mo»«msub»«mi»i«/mi»«mi»C«/mi»«/msub»«mo»=«/mo»«msub»«mi»i«/mi»«mi»R«/mi»«/msub»«mo»=«/mo»«mi»i«/mi»«/math»

and we can use i as our reference value for our calculations. We will also need to know the value of ω (= 2πf) to determine XL and XC.             For f = 10 Hz,

«math xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mi»§#969;«/mi»«mo»=«/mo»«mn»2«/mn»«mi»§#960;«/mi»«mi»f«/mi»«mo»=«/mo»«mn»62«/mn»«mo».«/mo»«mn»831853«/mn»«/math»

Now we can complete our calculations, starting with XL, XC, and Z:

 
 «math xmlns=¨http://www.w3.org/1998/Math/MathML¨»«msub»«mi»§#935;«/mi»«mi»L«/mi»«/msub»«mo»=«/mo»«mi»§#969;«/mi»«mi»L«/mi»«mo»§#8776;«/mo»«mn»125«/mn»«mo».«/mo»«mn»6637«/mn»«mo»§nbsp;«/mo»«mi»§#937;«/mi»«/math»

«math xmlns=¨http://www.w3.org/1998/Math/MathML¨»«msub»«mi»§#935;«/mi»«mi»C«/mi»«/msub»«mo»=«/mo»«mn»1«/mn»«mo»/«/mo»«mi»§#969;«/mi»«mi»c«/mi»«mo»§#8776;«/mo»«mn»53«/mn»«mo».«/mo»«mn»0618«/mn»«mo»§nbsp;«/mo»«mi»§#937;«/mi»«/math»

«math xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mi»Z«/mi»«mo»=«/mo»«mo»(«/mo»«mo»(«/mo»«msub»«mi»X«/mi»«mi»L«/mi»«/msub»«mo»-«/mo»«msub»«mi»X«/mi»«mi»C«/mi»«/msub»«msup»«mo»)«/mo»«mn»2«/mn»«/msup»«mo»+«/mo»«msup»«mi»R«/mi»«mn»2«/mn»«/msup»«msup»«mo»)«/mo»«mrow»«mn»1«/mn»«mo»/«/mo»«mn»2«/mn»«/mrow»«/msup»«mo»§#8776;«/mo»«mn»73«/mn»«mo».«/mo»«mn»2874«/mn»«mo»§nbsp;«/mo»«mi»§#937;«/mi»«/math»

 «math xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mi»i«/mi»«mo»=«/mo»«mfrac»«mi»V«/mi»«mi»Z«/mi»«/mfrac»«mo»§#8776;«/mo»«mn»1«/mn»«mo».«/mo»«mn»37«/mn»«mo»§nbsp;«/mo»«mi»A«/mi»«/math»

 «math xmlns=¨http://www.w3.org/1998/Math/MathML¨»«msub»«mi»V«/mi»«mi»L«/mi»«/msub»«mo»=«/mo»«mi»i«/mi»«mo»§#215;«/mo»«msub»«mi»X«/mi»«mi»L«/mi»«/msub»«mo»§#8776;«/mo»«mn»171«/mn»«mo».«/mo»«mn»4«/mn»«mo»§nbsp;«/mo»«mi»V«/mi»«/math»

 «math xmlns=¨http://www.w3.org/1998/Math/MathML¨»«msub»«mi»V«/mi»«mi»C«/mi»«/msub»«mo»=«/mo»«mi»i«/mi»«mo»§#215;«/mo»«msub»«mi»X«/mi»«mi»C«/mi»«/msub»«mo»§#8776;«/mo»«mn»72«/mn»«mo».«/mo»«mn»4«/mn»«mo»§nbsp;«/mo»«mi»V«/mi»«/math»

«math xmlns=¨http://www.w3.org/1998/Math/MathML¨»«msub»«mi»V«/mi»«mi»R«/mi»«/msub»«mo»=«/mo»«mi»i«/mi»«mo»§#215;«/mo»«mi»R«/mi»«mo»§#8776;«/mo»«mn»13«/mn»«mo».«/mo»«mn»6«/mn»«mo»§nbsp;«/mo»«mi»V«/mi»«/math»

«math xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mi»V«/mi»«mo»=«/mo»«mo»(«/mo»«mo»(«/mo»«msub»«mi»V«/mi»«mi»L«/mi»«/msub»«mo»-«/mo»«msub»«mi»V«/mi»«mi»C«/mi»«/msub»«msup»«mo»)«/mo»«mn»2«/mn»«/msup»«mo»+«/mo»«msubsup»«mi»V«/mi»«mi»R«/mi»«mn»2«/mn»«/msubsup»«msup»«mo»)«/mo»«mrow»«mn»1«/mn»«mo»/«/mo»«mn»2«/mn»«/mrow»«/msup»«mo»§#8776;«/mo»«mn»99«/mn»«mo».«/mo»«mn»9288«/mn»«mo»§nbsp;«/mo»«mi»V«/mi»«mo»§#8776;«/mo»«mn»100«/mn»«mo»§nbsp;«/mo»«mi»V«/mi»«/math»

Allowing for calculator round-off errors through all these calculations, the total voltage V is exactly 100 volts, which is what we initially specified. Therefore our calculations check out and our results are valid.

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