Maximum power is transferred when load R = Source R ?

[jaus tail]

Homework Statement

Maximum power is transferred to variable load when:
1) R_L = R_s
2) R_L not equal to 0. and R_s = 0
3) R_L = 0 and R_s not equal to 0
4) R_L = R_s/2

Homework Equations

Maximum power theorem R_L = R_s

The Attempt at a Solution

But if Rs = 0, then current is maximum, so load get maximum power. Shouldn't answer be B. Book answer is A.
I tried with a 4V dc source and 1 ohm source resistance and 1 ohm load resistance.
Power = 2^2 / 1 = 4.
In second case I put source resistance = 0.
Power = 4^4 = 16.

 

[scottdave]

OK, so with R_s equal to zero, what value of R_L gives you maximum power to the load?

 

[jaus tail]

With Rs = 0, R_L can be whatever is needed. That depends on load requirement. Max power = V₂ / R_L. Oh yeah. max power is when R_L = 0, which is R_s

I guess if R_L is not variable when Max power is when Rs = 0.

lol when Rs = 0, max power is infinite and everything goes boom :D

 

[NascentOxygen]

Maximum power is transferred to variable load when:
1) R_L = R_s
2) R_L not equal to 0. and R_s = 0
3) R_L = 0 and R_s not equal to 0
4) R_L = R_s/2

Are you quoting this fully & accurately? If so, then the question gives no indication what they might mean by the terms R_L and R_S?

Making likely assumptions, (2) is undoubtedly correct. Load power is maximised when there are zero source losses.

 

[jaus tail]

Yes the question is same except: Max power is transferred to variable load from source when

But the books have clear indication as to what is R_s and R_L

 

[jaus tail]

 

[NascentOxygen]

It seems like a trick question.

 

[NascentOxygen]

You haven't fully addressed scottdave's question. Suppose R_S is zero ohms, describe the value of load that will make the load as hot as possible.

 

[jaus tail]

You haven't fully addressed scottdave's question. Suppose R_S is zero ohms, describe the value of load that will make the load as hot as possible.

For max power, maximum current which means R_L should be zero. Short circuit.
This value matches of R_s = 0.

 

[NascentOxygen]

Certainly, as resistance approaches zero, power approaches infinity. I'm not sure how to view the exactly zero ohms case.

Do you know which answer your test is marking correct?

There's a big difference between delivering maximum power to the load (as many watts as possible), and delivering maximum proportion of power to the load (i.e., maximum efficiency; lowest loss in source). The former is met by R_L = R_S, the latter by setting R_S=0 (or R_L » R_S).

 

[jaus tail]

Book says right answer is A.
Yeah you're right. For max efficiency, Rs = 0.

 

[anorlunda]

There's a big difference between delivering maximum power to the load (as many watts as possible), and delivering maximum proportion of power to the load (i.e., maximum efficiency; lowest loss in source). The former is met by RL = RS, the latter by setting RS=0 (or RL » RS).

There's a fun anecdote about that difference. When Thomas Edison started the first electric power company, he set RS=0. The electrical engineering societies called him an ignorant fool because they all knew that maximum power is transferred when RS=RL. But those engineers were used to working with a battery where VS and RS are fixed. Edison knew that with his generator, he could adjust VS to be anything he wanted. Edison also knew that the proper question is not maximum power to the load but rather minimum power lost in the source.

 

[donpacino]

There's a big difference between delivering maximum power to the load (as many watts as possible), and delivering maximum proportion of power to the load (i.e., maximum efficiency; lowest loss in source). The former is met by RL = RS, the latter by setting RS=0 (or RL » RS).

As an undergraduate student I had trouble understanding this at first. I remember debating with my professor the exact point jaus tail was trying to make.

 

[anorlunda]

As an undergraduate student I had trouble understanding this at first. I remember debating with my professor the exact point jaus tail was trying to make.

If only our teachers could say it as succinctly and clearly as @NascentOxygen did.