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# As physics help please watch

1. What is the relevance of this derivation I don't understand it and also kirchhoffs laws second one
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2. (Original post by be patient)
What is the relevance of this derivation I don't understand it and also kirchhoffs laws second one
It's better to post physics questions in the physics section.
I've moved this one for you.
3. Not entirely sure what your asking in the first question. The wave equation is a really important part of AS physics, but rather than just tell you that this equation is true, the book is explaining where it comes from and why it is true. It simply starts off with the idea that speed = distance / time, and then says "What if distance = one wavelength? This makes the time = the period of the wave." So, in this case, speed = wavelength / period, and as the period of any wave is 1 / frequency, this gives the wave equation; speed = frequency x wavelength.

The important thing to remember for A Level is that, unlike most equations, you are expected to be able to derive the wave equation in the above manner in an exam.

If you're actually asking something else, do say so, and I'll have another go!
4. For the first one, The equation for speed is distance divided by time. For a wave think of the wave speed (in metres per second) as V, Distance as lambda (the flipped y symbol represents wavelength. ie the distance if you want to think of it as that) and time as the period(the time taken for one oscillation). From that you get V= Lambda/Period. Lambda divided by period can be written as 1/period X lambda (This 1/period is the same as frequency, frequency is the number of oscillations per unit time). There you sub in F (frequency) for 1/period and you get V(wavespeed)= Lambda(wavelength) multiplied by F (frequency).

For the second one, Kirchhoff's first law states that the sum of the currents entering a point is equal to the sum of the currents leaving that same point. So in your diagram i1 and i2 are entering P and i3 is leaving P, so i1 + i2 = i3. And Kirchhoff's second law sates that the sum of the emf's in a closed loop (ie the sum of the voltages of the batteries in a loop) is equal to the sum of the potential differences in the same loop. So find the 2 of the currents using this and you can use them to work out the 3rd current.

Hope this helps
5. As for the second question... the last time I looked at a thread like this, Stonebridge showed me a really quick and elegant way to solve these problems, which I have now forgotten! So this might not be the best method, but here goes...

First - use Kirchhoff's Second Law to work out the potential difference across the 30 ohm resistor. You can then work out I3.

Next - use Kirchhoff's Second Law again to work out the potential difference across the 10 ohm resistor. You can then work out I1.

Can you finish off from here? And don't forget to think carefully about the directions of the currents and the orientation of the cells.
6. I can do it partially but then I get stuck should I show you?
7. Yes, But if your really struggling watch this video on youtube because its hard to explain with text: http://www.youtube.com/watch?v=zUEvc...29A5B7&index=9
8. (Original post by Pangol)
As for the second question... the last time I looked at a thread like this, Stonebridge showed me a really quick and elegant way to solve these problems, which I have now forgotten! So this might not be the best method, but here goes...

First - use Kirchhoff's Second Law to work out the potential difference across the 30 ohm resistor. You can then work out I3.

Next - use Kirchhoff's Second Law again to work out the potential difference across the 10 ohm resistor. You can then work out I1.

Can you finish off from here? And don't forget to think carefully about the directions of the currents and the orientation of the cells.

http://www.thestudentroom.co.uk/show...&page=2&page=2
9. Thanks
10. (Original post by be patient)
Thanks
think im a bit late...but here http://www.thestudentroom.co.uk/show...1#post42204505

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