My Teacher Couldn't Answer This...

[misssweden]

Got a hand-out today in and got stuck on a question. Asked my teacher how to work it out and he couldn't give me an answer. (Well, he could, but it was: "it's Friday, I can't think. I'll get back to you on Tuesday.").

Don't need the answer, just want a nudge in the right direction as to how I work it out without knowing either current or voltage please.

Two resistors are connected in parallel to produce a total wattage of 4000W. One resistor has a resistance of 45 ohm. Calculate:

a) the total resistance of the circuit

b) the current flow

c) the supply voltage

d) the value of the second resistor

If someone could please just give me an idea of how to work out current or voltage that would be much apprechiated thank you.

 

[Andy™]

dont think you can answer that without more info, i.e one of your answers... or its friday and i cant think either

 

[ProDave]

W = v2/R (Voltage squared divided by resistance)

W= I2 * R (Current squared times resistance)

So transposing those will let you work out the voltage and current IF you know the total resistance and the total power.

The bit that makes this sound like a trick question to me, is the fact it's two resistors in parallel. one of which is 45 ohms.

But what value is the other? It could be anything, say 1 ohms, or say 1000 ohms

So with that being unknown, there is no one answer.

I would make the assumption that both resistors are 45 ohms so you calculate the parallel resistance of two 45 ohm resistors in parallel, and use that to work out your voltage and current.

Could your tutor be expecting you to work out what value second resistor would make it 4000W at a voltage of 230V? if so he should say so. Taking thw wording of the question literally, there is no one answer.

 

[misssweden]

Thank you very much Dave

I wouldn't presume that the second resistor is 45 ohm as d) is asking for the value of the second resistor. So really there isn't an answer?

My teacher better have a bloody good explanation on tuesday! I'll let you know what he says.

 

[ProDave]

My guess is he is expecting you to work it out for a voltage of 230V, since that's the mains voltage we work with. But as I say, he really needs to specify that.

Let us know what he says.

 

[Andy™]

you could always go with 400v, so you get different answers to what he probably expects. at least if your math is correct then he cant complain

 

[steptoe]

you could always go with 400v, so you get different answers to what he probably expects. at least if your math is correct then he cant complain

yer, I like that,

if he questions you using 400v tell him you are going to be a proper spark and not a domestic installer.

 

[misssweden]

Hahaha! Ok, I'm definately gonna do it on 400v then! If it's not what he intended then I'll just tell him I sought wisdom elsewhere

 

[Andy™]

go for it. ive already worked out the answers for 400v, so post your's and ill say if they are right

 

[steptoe]

you go for it, show him that you can think a bit more than the usuall follow the herd lot that he normally gets.

 

[Andy™]

wonder if he will figure it out when he reads answer C ?

 

[misssweden]

go for it. ive already worked out the answers for 400v, so post your's and ill say if they are right

Ok I'm just off for a night behind the bar so I'll post them on here tomorrow

 

[misssweden]

Ok, just had a go at this based on 400v and I got stuck.

Current:

I=P/V

I=4000/400

I=10A

Total resistance:

R=V/I

R=400/10

R=40ohm

If one resistor is 45 then the total resistance can't be 40! What have I done wrong?

If I knew the resistance of the other resistor I would use 1/R=1/R1+1/R2 but I don't see how I can apply this with the information that I've got. Where have I gone wrong?

 

[ProDave]

What's the problem?

One resistor is 45 ohms.

the total most certainly CAN (and will) be less than 45 ohms.

You have worked out the total combination of the two parallel resistors needs to be 40 ohms, so you just have to work out with your formula, what value resistor, when connected in parallel with 45 ohms, gives a total of 40 ohms.

The 1/R = 1/R1 +1/R2 +1/R3.... is the generic formula for any number of resistors in parallel.

But you may find it easier to use the simpler formula that only applies to two resistors in parallel:

R = (R1 * R2) / (R1+R2)

 

[Sharpend]

Better still work it out on both 230 & 400v. Covers all standard voltage bases.

Before you give him your answer or workings insist that he demonstrates how to Answer the question with the info provided.

 

[Andy™]

Ok, just had a go at this based on 400v and I got stuck.

Current:

I=P/V

I=4000/400

I=10A

Total resistance:

R=V/I

R=400/10

R=40ohm

If one resistor is 45 then the total resistance can't be 40! What have I done wrong?

If I knew the resistance of the other resistor I would use 1/R=1/R1+1/R2 but I don't see how I can apply this with the information that I've got. Where have I gone wrong?

Total current is correct, as is total resistance

the resistance of the other resistor, together in parallel, will reduce the total resistance

re-wokring Dave's formula above, you get Rt = (R2 x Rt) / (R2-Rt). which should get your answer

and as Sharpend says, you need to get your lecturer to answer the question first. if he says voltage is 230, 400 or whatever, tell him thats an answer, and how did he get that from the info given

 

[misssweden]

What's the problem?

One resistor is 45 ohms.

the total most certainly CAN (and will) be less than 45 ohms.

You have worked out the total combination of the two parallel resistors needs to be 40 ohms, so you just have to work out with your formula, what value resistor, when connected in parallel with 45 ohms, gives a total of 40 ohms.

The 1/R = 1/R1 +1/R2 +1/R3.... is the generic formula for any number of resistors in parallel.

But you may find it easier to use the simpler formula that only applies to two resistors in parallel:

R = (R1 * R2) / (R1+R2)

Ah, had a Swedish moment! Will try again, not giving up until I get it right Give me a few minutes. Thank you so much for your help and patience, I really apprechiate it.

 

[ProDave]

re-wokring Dave's formula above, you get Rt = (R2 x Rt) / (R2-Rt). which should get your answer

Almost. R1 = (R2 x Rt) / (R2-Rt) :coat

 

[misssweden]

Current: 10A

Rt: 40ohm

R1: 45ohm

R2: 360ohm

Am I right? (This is gonna get embarrassing if I'm wrong again ).

 

[ProDave]

That's the answers I get.