Potential Divider Problem: Calculating Vout and Thevenin Source Impedance

[boileroo]

potentiometer question

hey there

i hate coming online to ask for help but i am well and truly stuck!

attached is an image containing 2 potential divider setups.
i have a few questions about them, and any help would be appreciated!

a) for the first setup, A, how could the 10k pot be split into 2 resistors to give a Vout of 3v? i am confused by the +-15v source. can i treat this as if the +15v is 30v and the -15 is 0v? making the 2 resistors 9k and 1k? if not, how else is it done; i can find absolutely no examples of potential diviers which aren't grounded?

b) how do i calculate the thevenin source impedance of the potentiometer set to give 3v? I am familliar with thevenin's theorem but don't fully understand the question. as far as i understand, thevenins thereom is used to reduc circuits down to just a voltage source, a resistor and a load; here I'm unsure how this would be done.

c)finally, how do i calculate Vout for the setup B when the effective impedance of the multi meter measuring Vout is reduced to 3.3K? my initial thought was a simple voltage division equation but this came unstuck when i realized that R1 = 0, therefore that doesn't work. I am out of ideas after that!

like i said, i'd really really appreciate some help on this!

many thanks

 

[boileroo]

i've spent all day thinking about the above problems to no avail. can no one help me?

thank you

 

[boileroo]

i think i might be on to something, maybe someone can confirm if this is right, and if it is eplaiin why.

i think it goes +15v into R1 = 5k into r2 = 7.5 k and -15V out. then the output measuered between R1 and R2 = 3v. I've been messing around on a simulator and it works, but i don't know why!

anyone?

 

[boileroo]

i9 realize I've been being a fool.
think the output voltage can vary anywhere between -15V and +15V; a range of 30V.
An output of +3V is therefore 18V above the lower -15 level and 12V below the +15 level. In other words, the potential divider must divide the 30V total in the ratio 12/18. The resistance ratio must be the same; 4 ohm and 6 ohm

part a) done!

some help on b and c would be great still though