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1. The electrical resistance of a wire varies
inversely as the square of its radius.
If the resistance is 0.8 ohm when the
radius is 0.4 cm, find the resistance
when the radius is 0.7 cm.


Sagot :

Answer:

resistance R ≈ 0.26 ohms

Step-by-step explanation:

let the resistance be R and the radius r

given R varies inversely as r² , then the equation relating them is

R = [tex]\frac{k}{r^2}[/tex] ← k is the constant of variation

To find k, substitute R= 0.8 and r = 0.4 into the equation

0.8 = [tex]\frac{k}{0.4^2}[/tex] = [tex]\frac{k}{0.16}[/tex] ( multiply both sides by 0.16 )

0.128 = k

R = [tex]\frac{0.128}{r^2}[/tex] ← equation of variation

When r = 0.7 cm , then

R = [tex]\frac{0.128}{0.7^2}[/tex] = [tex]\frac{0.128}{0.49}[/tex] ≈ 0.26 ohms ( to the nearest hundredth )