Worked example: An electron (q = −e) moves at v = 2 × 10⁷ m/s along +x. Find B at a point 1 nm away on the +y axis. (a) magnitude, (b) direction. Step 1: B = (μ0/4π)·|q|v/r² = (10⁻⁷)(1.6 × 10⁻¹⁹)(2 × 10⁷)/(10⁻⁹)² = (3.2 × 10⁻¹⁹)/(10⁻¹⁸) = 0.32 T. Step 2: v × r̂ = x̂ × ŷ = +ẑ for positive charge; electron is negative, so B points −z (into the page if x is right and y is up). Step 3: Perspective — 0.32 T from a single electron at atomic distance is enormous (a fridge magnet is ~0.01 T), but it falls as 1/r², so at 1 mm it is 10⁻¹² times smaller — negligible. This is why atomic fields matter only at atomic scales.