Maximum Power Delivered to Load (P_max = E²/(4r) for variable load R matched to source)

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P_max = E²/(4r) for variable load R matched to source internal r (single source)

What each symbol means

SymbolWhat it stands for
ESource EMF (V)
PPower in external load (W)
RLoad resistance (Ω)
rInternal resistance (Ω)

When to use this

Single EMF with series r; variable R; DC steady.