# Why If P=Pmax Ri (source) should be eqaul to Ra (load) in the following circuit?

#### Papabravo

Joined Feb 24, 2006
12,704
In the following circuit:
View attachment 156501

Why do we get the most energy, if we make Ra=Ri? Can you explain it to me?

Thanks

https://en.wikipedia.org/wiki/Maximum_power_transfer_theorem
One way to prove it is to use the calculus and write an equation for the power delivered to the load as a function of a fixed Ri and a variable Ra. Now take the first derivative and set it equal to zero. This will give you the maximum of the original function.

https://en.wikipedia.org/wiki/Maximum_power_transfer_theorem