In the same X-ray tube, an electron of charge -1.602 x 10^-19 C starts at rest at the source and crosses the 0.10 m gap in a field of E = -100 x 10^3 N/C. Using K_f = qE(x_f - x_i), what kinetic energy does the electron gain?

1. Conservation of energy gives K_f = qE(x_f - x_i) for an electron starting at rest. 2. Substitute the charge and field, both negative: K_f = (-1.602 x 10^-19 C)(-100 x 10^3 N/C)(0.10 m - 0). 3. The two negative signs multiply to give a positive result: K_f = 1.602 x 10^-15 J. 4. Kinetic energy must come out positive for a real accelerating charge, so option B, which keeps a negative sign, must be wrong; forgetting that both q and E are negative here is the trap. _Source: OpenStax Physics (CC BY 4.0), Ch 18 "Static Electricity", section 18.4 Electric Potential_