Applied Electricity and Circuit Theory
Explainer
When Does a Load Get the Most Power?
A load gets maximum power only when it matches the source resistance.
Quick check · tap an answer to continue
Lesson notes
Key points
- Model any source by its Thevenin equivalent, Vth in series with Rth, feeding a load resistance R_L.
- Differentiating this power with respect to R_L and setting it to zero gives the peak at R_L equal to Rth.
- At this matched point only half the power reaches the load, so efficiency is just fifty percent.
- So signal systems like amplifiers and antennas use matching, while power grids avoid it to minimise losses.
Takeaway: Max power when R_L = Rth, but efficiency drops to 50%.
Quick check: Rth = 8 Ω. Which load receives maximum power?
- 8 Ω
- 4 Ω
- 16 Ω
Source modelR_L = Rth50% efficiencyMatching vs efficiency
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