If R is the total resistance of three resistors, connected in parallel, with resistances R1, R2, R3, then 1 R = 1 R1 + 1 R2 + 1 R3 . If the resistances are measured in ohms as R1 = 100 Ω, R2 = 25 Ω, and R3 = 10 Ω, with a possible error of 0.5% in each case, estimate the maximum error in the calculated value of R. (Round your answer to three decimal places.) Ω

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Answer:

maximum error is 0.03333

Explanation:

given data

R1 = 100 Ω,

R2 = 25 Ω,

R3 = 10 Ω

1/ R = 1/ R1 + 1/ R2 + 1 /R3

possible error = 0.5%

to find out

maximum error

solution

we know

1/ R = 1/ R1 + 1/ R2 + 1 /R3

put all value R1, R2 and R3

1/ R = 1/ 100 + 1/ 25 + 1 /10

R = 20/3

now take derivative  

dR/dR(i) = R²/R(i)² for i = 1, 2, 3

we have given error 0.005

so dR(i) = 0.005×R(i)  for the i = 1,2,3

so the equation will be

dR = dR/dR(1) ×dR(1) +dR/dR(2) ×dR(2) + dR/dR(3) ×dR(3)

dR = R²/R²(1) ×dR(1) + R²/R²(2) ×dR(2) +  R²/R²(3) ×dR(3)

put the value dR(1) and dR(2) and dR(3) and R

dR =  (20/3)²/R²(1) ×0.005×R(1) +  (20/3)²/R²(2) ×0.005×R(2) +   (20/3)²/R²(3) ×0.005×R(3)

dR =  (20/3)²/R(1) ×0.005 +  (20/3)²/R(2) ×0.005 +   (20/3)²/R(3) ×0.005

dR =  (20/3)²/100 ×0.005 +  (20/3)²/20 ×0.005 +   (20/3)²/10 ×0.005

dR =  (20/3)² ( 0.005/100 + 0.005/25 + 0.005/10)

dR = 0.033333

maximum error is 0.03333

The maximum error in calculating the value of equivalent resistance, R is 6.667 ohms.

The given parameters;

  • first resistor, R₁ = 100 Ω
  • second resistor, R₂ = 25 Ω
  • third resistor, R₃ = 10 Ω
  • error in each measurement; = 0.5% = 0.005

The equivalent resistance of the three parallel resistors is calculated as;

[tex]\frac{1}{R} = \frac{1}{R_1} + \frac{1}{R_2} + \frac{1}{R_3} \\\\\frac{1}{R} = \frac{R_2R_3\ + \ R_1R_3 \ + \ R_1R_2}{R_1R_2R_3} \\\\R = \frac{R_1R_2R_3}{(R_2R_3) + (R_1R_3)+(R_1R_2)} \\\\[/tex]

The maximum error in calculating the value of R is estimated as follows;

[tex]R = \frac{0.005(R_1R_2R_3)}{0.005(R_2R_3) + 0.005(R_1R_3)+0.005(R_1R_2)}[/tex]

[tex]R = \frac{0.005(100\times 25 \times 10)}{0.005(250) \ + \ 0.005(1000)\ +\ 0.005(2500)}\\\\R = \frac{125}{18.75} \\\\R = 6.667 \ ohms[/tex]

Thus, the maximum error in calculating the value of equivalent resistance, R is 6.667 ohms.

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