The resistance of a wire varies directly as its length and inversely as the square of its diameter. A 32-ft wire with a 0.8-in diameter has a resistance of 0.0125 .

Find the resistance of a 62-ft wire with a diameter of 0.4 in.

1 answer

Let the resistance \( R \) be modeled by the equation

\[ R = k \frac{L}{d^2} \]

where \( L \) is the length of the wire, \( d \) is the diameter of the wire, and \( k \) is a constant of proportionality.

Given:

  • For the first wire, \( L_1 = 32 \) ft, \( d_1 = 0.8 \) in, and \( R_1 = 0.0125 \) ohms.

We can solve for \( k \) using this information:

\[ 0.0125 = k \frac{32}{(0.8)^2} \]

Calculating \( (0.8)^2 \):

\[ (0.8)^2 = 0.64 \]

Substituting that into the equation gives:

\[ 0.0125 = k \frac{32}{0.64} \]

Calculating \( \frac{32}{0.64} \):

\[ \frac{32}{0.64} = 50 \]

Now we can rewrite the equation:

\[ 0.0125 = k \cdot 50 \]

Solving for \( k \):

\[ k = \frac{0.0125}{50} = 0.00025 \]

Now, we want to find the resistance \( R_2 \) of a second wire with \( L_2 = 62 \) ft and \( d_2 = 0.4 \) in:

\[ R_2 = k \frac{L_2}{d_2^2} \]

Calculating \( d_2^2 \):

\[ (0.4)^2 = 0.16 \]

Now substituting \( L_2 \), \( d_2 \), and \( k \):

\[ R_2 = 0.00025 \frac{62}{0.16} \]

Calculating \( \frac{62}{0.16} \):

\[ \frac{62}{0.16} = 387.5 \]

Thus,

\[ R_2 = 0.00025 \cdot 387.5 = 0.096875 \]

Therefore, the resistance of the 62-ft wire with a diameter of 0.4 in is:

\[ \boxed{0.096875} \text{ ohms} \]