13. 1 Force on a Wire Carrying a Current in a Mag- netic Field We know the Lorentz force: F=QVX B Now look at a piece dl of wire carrying current 1, a is its cross-section area, n is its density of electron Figure 13-1 Wire of cross-section a carrying a current /. The charges -e move at a velocity v. The magnetic field B is due to currents flowing elsewhere