How do i calculate missing resistance readings in a parallel circuit using R1 and Rt

[MikeGawan]

Hi

I am a current student studying to become an electrician and a question has thrown me. I'm wondering if someone could help, there is a question on one of my home work books. "complete the following table assuming the resistors are connected in parallel?" the example I will give is R1 is 120 ohms R2 ? R3? and Rt is 48 ohms. How would I calculate the missing R2 and R3 readings with no voltage or current to complete the table?

I have spoken to my tutor but I believe there was some miscommunication and got an answer which was relating to another topic. I don't want the answer I would just like to know the formula to help proceed with this if someone could help that would be great.

I understand 1/Rt = (1/R1) + (1/R2) + (1/R3).

 

[SparkyChick]

The problem is that if you have 2 unknowns (R2 and R3), there can be a very large number of possibilities.

What answer did your tutor give you?
[automerge]1585942820[/automerge]
I should add... perhaps message @Dan about getting access to the trainee only area... only trainee's and mentors (and staff) can post in there and there is a lot of useful info

 

[MikeGawan]

I did not receive an answer from my tutor. if I give another example there are 3 resistors R1 R2 and R3 and Rt . R1 40 ohms R2 ? R3 20ohms and Rt is 10 ohms? what formula would I use to calculate the missing resistance reading?

The problem is that if you have 2 unknowns (R2 and R3), there can be a very large number of possibilities.

What answer did your tutor give you?

 

[Andy78]

I'm no maths whizz by any means, but surely the only way that question can be answered accurately is if R2 and R3 are known to be equal, or some other predetermined relationship between them ?

 

[SparkyChick]

@MikeGawan ,

You have to transpose the 1/Rt = 1/R1 + 1/R2.... formula. Which can be done if you know all but one of the values. Unless you made a typo in your original post, if you have more than one unknown value (and assuming there are no assumptions you can make as @Andy78 suggests - which should be stated in the question if there are) you can't work out both missing values because they depend on each other.

 

[MikeGawan]

I have just blown the print screen of the question up to full size on a word document to post in the thread and seen there is a tiny hyphen in R3 which I am guessing means there was no R3 needed to be calculated. I did think 2 unknown values must have some relationship to one another but nothing was indicated. Transposing the equation does make sense! Thank you very much!

@MikeGawan ,

You have to transpose the 1/Rt = 1/R1 + 1/R2.... formula. Which can be done if you know all but one of the values. Unless you made a typo in your original post, if you have more than one unknown value (and assuming there are no assumptions you can make as @Andy78 suggests - which should be stated in the question if there are) you can't work out both missing values because they depend on each other.

 

[Wilko]

Hi - for your first problem with R2 and R3 being unknown - you can calculate what their value in parallel, which is 80 Ohms (from 1/48 - 1/120). Then a reasonable pick might be 160 Ohms each, say.

 

[SparkyChick]

Hi - for your first problem with R2 and R3 being unknown - you can calculate what their value in parallel, which is 80 Ohms (from 1/48 - 1/120). Then a reasonable pick might be 160 Ohms each, say.

I didn't think about it like that. Nice, thanks @Wilko