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You can calculate the voltages and currents in a simple circuit with Ohm's Law, but not for more complex circuits (such as a decorative net curtain of LEDs). Kirchoff's Laws allow you to write down a series of simultaneous equations which you can solve to find all the voltages and currents. Briefly, (a) the currents flowing into and out of any node in a circuit must be equal, and (b) considering any closed loop in a circuit, the positive and negative voltages around the loop and back to the starting point must sum to zero.