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Physics help

I’m not sure about this question is it less than 6V because X is 6volts and therefore the total would add up to more
Than 10V?
392FA751-8627-4332-AA71-3F49D2DADDAA.jpg.jpeg
Picture is kind of difficult to read

But the p.d across X is not 6V. The pd across X should be 4V

It is required that the p.d across Y is 6V
so we have Y/(Y+1.2)=6/10
using potential divider
hence solve for Y
Is the resistance of Y 800 ohms?
Original post by Bananasplitxxx
Is the resistance of Y 800 ohms?


No, you should be getting 1800 ohms
It was this part I don’t understand, hopefully it’s a better picture
3623E984-8449-4080-B963-9BB320453877.jpg.jpeg
Hang on how do you get 1800 ohms?
Original post by Bananasplitxxx
Is the resistance of Y 800 ohms?

If you think carefully, you will see that the resistance of Y cannot be less than that of X. The proportion of the p.d taken by Y will be less than that of X. This cannot be true. The p.d across Y is 6V and that across X is 4V. So resistance of Y must be greater than that of X

For the next question, think about what happens the p.d across Y & Z.

Since Y and Z are in parallel, the effective resistance of the parallel combination decreases (and is now less than that of either of the components Y and Z). Hence the p.d across Y and Z must be smaller than 6V.

For the last part, calculate the effective resistance of the parallel combination. Then use potential divider again. [Note: other methods are possible using Ohm's law but taking from the question, it is probably expected to use potential divider]
Original post by Bananasplitxxx
Hang on how do you get 1800 ohms?


It is required that the p.d across Y is 6V
so we have Y/(Y+1.2)=6/10
using potential divider
hence solve for Y

Yes sorry, I didn't realise that you needed help only with the 2nd part. Picture was hard to read haha :smile:
Sorry I’m still confused with part 1, what does the potential divider do?
Can’t i find the current through X using V=IR and then use that current to find the resistance of Y?
Original post by Bananasplitxxx
Sorry I’m still confused with part 1, what does the potential divider do?
Can’t i find the current through X using V=IR and then use that current to find the resistance of Y?


Never mind i must’ve made a mistake I got 1800 this time
Original post by Bananasplitxxx
Sorry I’m still confused with part 1, what does the potential divider do?
Can’t i find the current through X using V=IR and then use that current to find the resistance of Y?


If you haven't learnt about potential divider then you will have to use Ohm's law as you said

Yes you can find current through X using V=IR and then use that current to find the resistance of Y
Can you please explain part 3 again
Original post by Bananasplitxxx
Can you please explain part 3 again


We will use Ohm's law

First find the resistance of the parallel combination using
1/R = 1/Y + 1/Z
where Y and Z are the resistances of Y and Z respectively

Hence from this find the effective resistance in the circuit which is X + the resistance of the parallel combination

Then use V=IR to find the p.d across X. Since the p.d across X + the p.d across parallel combination = 10

you can find the p.d across the parallel combination

the p.d across Y is the same as the p.d across Z since they are in parallel

[You can also use potential divider]
Is the resistance of Y still the same as the last part?
Original post by Bananasplitxxx
Is the resistance of Y still the same as the last part?


Yes
I did this but it can’t be right as it’s more than 6V
C1FCF072-403D-4DC2-8DC3-72C95718E647.jpg.jpeg
Original post by Bananasplitxxx
I did this but it can’t be right as it’s more than 6V
C1FCF072-403D-4DC2-8DC3-72C95718E647.jpg.jpeg


True. The mistake is that the effective resistance of the parallel combination is 1/R=1/4.7+1/1.8
hence R = 1300 ohms

so V = 1300 / (1300+1800) x 10 = 4.2 V

the effective resistance of the parallel combination is 1300 ohms
so you can think of the parallel combination as a single resistor of 1300 ohms. the p.d across this single resistor is then the p.d across the parallel combination. this p.d is equal to the p.d across Y and equal to that across X. hence the p.d is calculated as above
Thanks

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