Cards standing in for solar cells each give 0.5 V and 2 A in bright light. How many cards are needed to build an output of 3 V at 6 A?

1. Cells in series add their voltages, so reaching 3 V needs 3 V / 0.5 V = 6 cards in one string. 2. That string still supplies only 2 A, because a series string carries a single current. 3. Cells in parallel add their currents, so reaching 6 A needs 6 A / 2 A = 3 such strings side by side. 4. The array is therefore 3 strings of 6 cards, which is 18 cards. 5. Its output checks out as (3 V)(6 A) = 18 W. 6. 9 cards can reach either the voltage or the current but never both at once. 7. 12 cards make two strings of six, which give 3 V at only 4 A. 8. 36 cards make six strings of six, which give 3 V at 12 A, twice the current asked for. _Source: OpenStax College Physics (CC BY 4.0), Ch 21 "Circuits and DC Instruments", section 21.2 Electromotive Force: Terminal Voltage_