A long straight wire carries 14 A. Find the field at 4 cm and at 7 cm from it.

Step 1. At $4\ \text{cm}$. $B = 2 \times 10^{-7}\ \text{T m/A} \times \dfrac{14\ \text{A}}{0.04\ \text{m}} = 7.0 \times 10^{-5}\ \text{T} = 70\ \mu\text{T}$.

Step 2. At $7\ \text{cm}$. $B = 2 \times 10^{-7}\ \text{T m/A} \times \dfrac{14\ \text{A}}{0.07\ \text{m}} = 4.0 \times 10^{-5}\ \text{T} = 40\ \mu\text{T}$.

Step 3. Compared with the Earth. $\dfrac{70\ \mu\text{T}}{50\ \mu\text{T}} = 1.4$.