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OCR AS Physics B electricity help please

Hi all,

Please look at the attached screenshot. I do not understand how I would arrive at these values for V1 and V2 (quoting from "Answers" section of the book):

I1 = 0.3
V1 = 3.4
V2 = 2.6

Thank you!
Screen Shot 2016-04-06 at 15.03.52.png33.1KB
So you understand that
Unparseable latex formula:

\sideset{}{_1} I = 0.3

from basic principles.
Now you have 0.3A0.3A going through
Unparseable latex formula:

\sideset{}{_1} V

and 0.4A0.4A going through
Unparseable latex formula:

\sideset{}{_2} V



Conservation proof:
As the loop that contains
Unparseable latex formula:

\sideset{}{_1} V

has to equal
Unparseable latex formula:

\sideset{}{in} V = 3.4

,
Unparseable latex formula:

\sideset{}{_1} V = 3.4


and so
Unparseable latex formula:

\sideset{}{_2} V = 2.6

from the conservation of the entire loops EMF.

Unparseable latex formula:

\sideset{}{_1} I = 0.3


Unparseable latex formula:

\sideset{}{_1} V = 3.4


Unparseable latex formula:

\sideset{}{_2} V = 2.6

(edited 8 years ago)
Reply 2
Avatar for derpz
derpz
5
Original post by londoncricket
Hi all,

Please look at the attached screenshot. I do not understand how I would arrive at these values for V1 and V2 (quoting from "Answers" section of the book):

I1 = 0.3
V1 = 3.4
V2 = 2.6

Thank you!


Voltage across parallel sections is the same so the V1 = 3.4 too, then work out the overall resistance and the resistance of the parallel bit.

Find the difference between them and that is the resistance of V2, then simply use V=IR to get the pd.
Original post by The-Spartan
So you understand that
Unparseable latex formula:

\sideset{}{_1} I = 0.3

from basic principles.
Now you have 0.3A0.3A going through
Unparseable latex formula:

\sideset{}{_1} V

and 0.4A0.4A going through
Unparseable latex formula:

\sideset{}{_2} V



Conservation proof:
As the loop that contains
Unparseable latex formula:

\sideset{}{_1} V

has to equal
Unparseable latex formula:

\sideset{}{in} V = 3.4

,
Unparseable latex formula:

\sideset{}{_1} V = 3.4


and so
Unparseable latex formula:

\sideset{}{_2} V = 2.6

from the conservation of the entire loops EMF.

Unparseable latex formula:

\sideset{}{_1} I = 0.3


Unparseable latex formula:

\sideset{}{_1} V = 3.4


Unparseable latex formula:

\sideset{}{_2} V = 2.6



Original post by derpz
Voltage across parallel sections is the same so the V1 = 3.4 too, then work out the overall resistance and the resistance of the parallel bit.

Find the difference between them and that is the resistance of V2, then simply use V=IR to get the pd.


Thank you both very much for your explanations!

I still do not understand why V1 = 3.4V, though. Could you please explain this a bit further?

Thank you!
Because we are looking at the 'potential difference' between two points when we look at 'voltages' across components.

As the ends of both resistors are connected directly they must be at the same 'potential' and so the potential difference across both resistors must be the same.

I hope that has cleared this up for you, if not I can try to explain further
(edited 8 years ago)

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