"Power drain"?

[Hemorrage]

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Can someone explain the term power drain from TBR physics electric circuits? I initially thought this meant higher Resistivity leading to more joules being disappeared per unit time but apparently not.

 

[NuttyEngDude]

Not sure which you're referring to exactly (page #'s?) but in general, power drain is the same power that the p=i^2r refer to.

 

[Hemorrage]

Not sure which you're referring to exactly (page #'s?) but in general, power drain is the same power that the p=i^2r refer to.

Can you take a look at the example on page 175 of physics 2? I don't understand their explanation

 

[NuttyEngDude]

Ahh ok. P = IV ( and V=IR). so they are saying that the voltage is fixed so you can only worry about the I in the equation (current). It's the same. How much power does the resistor use? P = IV How to increase I? increase the conductivity

 

[milski]

They mean the power dissipated in the resistor, same as R.I^2., or what is more practical to use in this case, VI. V is give as a constant, so you want to maximize I. The only way to do that is to decrease resistance and that's the same as increasing conductivity. The other three options are more or less obvious way to not change it or keep it the same.

 

[NuttyEngDude]

Yeah, perhaps your confusion could be if you are trying to increase the resistance in the wire but that would cause less voltage to be dissipated from the resistor itself.

 

[dudewheresmymd]

Can you take a look at the example on page 175 of physics 2? I don't understand their explanation

Holy crap I just saw this thread and I just read that page while I was studying on my own haha. Small world. As previous post mentioned, P=IV gives the right intuition, if V is held constant, I must increase (conductivity must increase or resistivity must decrease since they're reciprocals) and therefore I increases, increasing Power "drain" through the resistor. 🙂

 

[kmp0410]

So the relationship is decrease resistance and increase power drain?

 

[milski]

Yeah, perhaps your confusion could be if you are trying to increase the resistance in the wire but that would cause less voltage to be dissipated from the resistor itself.

That's a good point. Since P=RI^2=VI=V^2/R, people ask which one of the formulas to use. If you want to understand if power increases or decreases, you want to use the one which has one of the variables constant, otherwise you have to think what happens when one increases and the other increases.

In this example, VI is what works - V is constant, so P changes the same way as I. Trying to use RI^2 is not so helpful - R goes down when I goes up, so the effect on power is not so obvious.

 

[dudewheresmymd]

+1

 

[dudewheresmymd]

that's a good point. Since p=ri^2=vi=v^2/r, people ask which one of the formulas to use. If you want to understand if power increases or decreases, you want to use the one which has one of the variables constant, otherwise you have to think what happens when one increases and the other increases.

In this example, vi is what works - v is constant, so p changes the same way as i. Trying to use ri^2 is not so helpful - r goes down when i goes up, so the effect on power is not so obvious.

+1

 

[milski]

So the relationship is decrease resistance and increase power drain?

Yes, if the voltage is being held constant. (which is the case in this problem)

If in some way the current was held as a constant, you'll have to increase the resistance to increase the power.

 

[milski]

+1

2?

 

[dudewheresmymd]

Haha accidental post. But yes you deserve +2 for clarity 🙂

 

[milski]

Haha accidental post. But yes you deserve +2 for clarity 🙂

Hehe, I just could not resist showing off my math skillz. 😉

 

[Hemorrage]

Ahh ok. P = IV ( and V=IR). so they are saying that the voltage is fixed so you can only worry about the I in the equation (current). It's the same. How much power does the resistor use? P = IV How to increase I? increase the conductivity

Oh I see. Ok so if P = IV, holding V constant P must go up so I has to go up. Got it. I forgot about these equations. I was simply trying to deduce it using ohms law by thinking increased resistance = higher dissipation of energy into thermal energy but I guess that's not the case?

What exactly is the point of a resistor then? I thought the point was to decrease current flow?

 

[dudewheresmymd]

Oh I see. Ok so if P = IV, holding V constant P must go up so I has to go up. Got it. I forgot about these equations. I was simply trying to deduce it using ohms law by thinking increased resistance = higher dissipation of energy into thermal energy but I guess that's not the case?

What exactly is the point of a resistor then? I thought the point was to decrease current flow?

Yeah resistors oppose the flow of current and drain power from circuits. The purpose of resistors is to create voltage drops in circuits. Think of it as a kinked garden hose, you may not want all of the water to flow onto your precious hibiscus plants so you'd create "kinks" or resistors in the hose's pathway to lessen the flow of "water" current so that the hibiscus plants aren't damaged by the entire flow of water from the pipe. Similarly, circuits need resistors in certain areas of the circuit to make the circuit work for its intended purpose.