Elena was a diligent 2º Bachillerato student. She attended all her Physics classes, took meticulous notes, and spent hours in the library. But every time she opened her SM Savia textbook to the section on Campo Magnético (Magnetic Field), her stomach sank.

Her older brother, Carlos, who was in his second year of Engineering, noticed her frustration.

Elena showed him her notebook. The steps were correct, but Carlos spotted the error: she had used the proton's mass in grams (from an old table) instead of kilograms—a factor of ( 10^{-3} ) that ruined everything.

He pulled up a PDF on his tablet. It was the .

"Still stuck?" Elena: "I've done it five times. I don't know where I'm wrong." Carlos: "Show me your process."