# Physics electronincs

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#1
Hi all

Can you look at the attachet picture and tell me how the unequal resistances of R1 and R2 will affect the current - will it be inversely I assume, that is if 2R2 = R1 then the current through R2 will be twice that through R1 and the voltage will be half?

Is this correct?
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#2
Ok if the cell voltage is 9.0V, the current at the series resistor is 0.75A, and that resistor's resistance is 4 Ohm, and the far right hand side's resistancer is 24 Ohm, (and the left one is R) then I work out that R = 12. This seems alright to me... Now I just need to work out the current, which I'd assume is 0.5A through R.
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16 years ago
#3
(Original post by mik1a)
Hi all

Can you look at the attachet picture and tell me how the unequal resistances of R1 and R2 will affect the current - will it be inversely I assume, that is if 2R2 = R1 then the current through R2 will be twice that through R1 and the voltage will be half?

Is this correct?
the voltage accross them will be the same. the current through R1 will be R1/(R1+R2) of the total current, the current through R2 will be R2/(R1+R2) of the total current.
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#4
Are you sure that's right? That seems to imply that the current favours the path of higher resistance.
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16 years ago
#5
(Original post by mik1a)
Are you sure that's right? That seems to imply that the current favours the path of higher resistance.
ah, sorry, thats the potential divider. for parallel, they are just inverses of the potential divider, so the current through R1 is (R1+R2)/R1, the current through R2 is (R1+R2)/R2
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#6
...which holds which Ohm's law. Thanks, that what I was hoping to hear. I had a think about it while waiting for a response and half figures it out for myself, just what you said cleared it up completely.

Cheers,

Mike
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16 years ago
#7
(Original post by mik1a)
...which holds which Ohm's law. Thanks, that what I was hoping to hear. I had a think about it while waiting for a response and half figures it out for myself, just what you said cleared it up completely.

Cheers,

Mike
hold up, i think i might have got that wrong too, (got a bit confused with the duality principle, sorry)

the current through R1 will be R2/(R1+R2) of the total current, the current through R2 will be R1/(R1+R2) of the total current.

that should be right now (hopefully - but dont take my word. ive probably made another horrible ghastly mistake.)
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#8
No that makes sense - as the resistance increases in a thermistor in parallel with a fixed resistor, the voltage gobbled up by it increases but the current through it decreases because it's taking increasingly more voltage to pass through. R = V/I, so as R increases, you'd naturally expect a proportional drop in I to the rise in V.
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