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A-level Physics (Advancing Physics)/Current/sigma
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That's the Greek letter Sigma. It means 'sum of', so Kirchoff's First Law is stating that the sum of all the currents going into a point equals the sum of all the currents leaving the point. So:
Iin1 + Iin2 + Iin3 + ... + Iinn-2 + Iinn-1 + Iinn = Iout1 + Iout2 + Iout3 + ... + Ioutn-2 + Ioutn-1 + Ioutn