A closed box contains no electric charge anywhere inside it. If 11 electric field lines enter through one side of the box, how many electric field lines must exit through the rest of the box?

1. Electric field lines can only begin or end on a charge. 2. Because the box has no charge inside it, no field lines can start or stop anywhere within the box. 3. Every field line that enters the box must therefore continue on and exit the box somewhere else. 4. So the number of lines exiting the box must equal the number entering it, which is 11. _Source: OpenStax Physics (CC BY 4.0), Ch 18 "Static Electricity", section 18.3 Electric Field_