Question

Experiments carried out on the television show Mythbusters determined that a magnetic field of 1000 gauss is needed to corrupt the information on a credit card's magnetic strip. (They also busted the myth that a credit card can be demagnetized by an electric eel or an eelskin wallet.) Suppose a long, straight wire carries a current of 5.0 A .
How close can a credit card be held to this wire without damaging its magnetic strip?

Answer 1

Answer:

his distance is too small (r = 0.01 mm), therefore the cut can be at any distance

Explanation:

For this exercise let's use the ampere law.

Let's use a cylinder as the circulating surface

          ∫ B. ds = μ₀ I

in this case the field is circular and ds is circular therefore the angle between them is zero and cos 0 = 1

          B 2π r =  μ₀ I

          r =  $\frac{\mu_o I}{2\pi B}$

The field needed to demagnetize the card is B = 1000 gauss = 0.1 T

          r = $\frac{4\pi 10^{-7} 5.0 }{2\pi \ 0.1}$

           r = 2 10⁻⁷  5.0/0.1

          r = 1  10⁻⁵ m

this distance is too small (r = 0.01 mm), therefore the cut can be at any distance