A motorcyclist is making an electric vest that, when connected to the motorcycle's 12 V battery, will warm her on cold rides. She is using 0.25-mm-diameter copper wire, and she wants a current of 4.2 A in the wire. Part A What length wire must she use

L = 8.35 m

Explanation:

The lenght of a wire L can be calculated using the following expression:

L = R A/ρ  (1)

Where:

R: resistance of the wire

A: Cross section area of the wire

ρ: resistivity of the copper wire.

With this expression we realize that we do not have the area of the cross section, and the resistance of the wire either.

To calculate the area we can use the following expression:

A = πr²    (2)

If the diameter is 0.25 mm, then the radius is half, 0.125 mm. Converting this in meter it will have to be:

0.125 /1000 = 0.000125 m

Replacing we have:

A = π(0.000125)²

A = 4.91x10⁻⁸ m²

The reported resistivity of a copper wire is 1.68x10⁻⁸ Ω.m, so we just need to determine the resistance, which can be found using Ohm's law:

R = V/I  (3)

Replacing (3) into (1) we have:

L = (V * A) / (I * ρ) (4)

So finally, the length of the copper wire will be:

L = (12 * 4.91x10⁻⁸) / (4.2 * 1.68x10⁻⁸)

Hope this helps