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# Edexcel Physics Unit 2 "Physics at work" June 2013 Watch

• View Poll Results: The last question - Does resistance increase or decrease?
It increases ( using V=IR or some other method)
70.73%
It decreases using the 'lattice vibrations' theory
29.27%

1. (Original post by alygirl)
Heya, so I agree with you for number 1 as in by agreeing I got there too however 2 and 3 perplex me.

For 2 I thought okay A is closed and so is B meaning some current will flow through to all the resistors right?
I thought the same for 3. However when I'm working it out, I think there's something wrong with my method. I thought the total of the two resistors in series is 2R and then so 1/2R + 1/R = 3/2R? Can you please explain what I'm doing wrong?
For 2, if A & B are closed, no current will flow through the bottom resistor, because current will flow through switch B with zero resistance. ( Current flows through the route with lowest resistance, therefore resistor on the bottom right hand side is not considered ).
2. (Original post by jollygood)
Guys can anyone explain properly Q18 part a(1) and part b. please
What Month/Year?
3. (Original post by CharlieTT)

c) The smallest detail that can be seen on the image is half the length of the ultrasound pulse. The thumbnail on the fetus is 0.50mm thick. The speed of ultrasound in the thumbnail is 2000ms^(-1). Calculate the maximum pulse duration if the thumbnail is to be seen on the image.

v=s/t
t=s/v
=0.0005m/2000ms^(-1)
=2.5x10^(-7)s

Because the smallest detail can be seen at half the length of the ultrasound pulse.

2.5x10^(-7)s x 2= 5x10^(-7)s
i cant get my head around that bit? Can someone please that bit for me a bit better/ dumb it down. I know it says it in the but i cant understand why you have to double the answer, not half?
4. (Original post by ArcRaman)
For 2, if A & B are closed, no current will flow through the bottom resistor, because current will flow through switch B with zero resistance. ( Current flows through the route with lowest resistance, therefore resistor on the bottom right hand side is not considered ).
I think I understand, thanks. What am I doing wrong for 3? My answer is wrong. It's the other way round.
5. (Original post by stealth_writer)
i cant get my head around that bit? Can someone please that bit for me a bit better/ dumb it down. I know it says it in the but i cant understand why you have to double the answer, not half?
If the smallest detail is 0.5 and this is half of the length of the pulse then the length of the pulse must be double.
6. (Original post by CharlieTT)
What Month/Year?
oh sorry forgot to mention. may 2012.
7. (Original post by Tomupcraft)
June 2012?
8. (Original post by jollygood)
oh sorry forgot to mention. may 2012.
For 18)a)i) notice the positive terminals on the two battery's are opposing each other therefore the emfs cancel each other out partly so 15V - 7.6V = 7.4V

For b)i) Because the circuit is in series and it gives you the current in the car batter to be 4.30A therefore the current in the charging batter is 4.30A

V = E -Ir, using the Emf of the car battery which is 15V and the internal resistance which is 0.65 Ohms

V = 15 - 4.30*0.65
V=12.2V
9. Important:

Does anybody have the examiners report for Jan 13 paper?
10. Also, how many sig figs/ dp are we supposed to leave our answers to?
11. What does "initial changing current" mean? Regarding question 18iii June 2012
12. (Original post by alygirl)
What does "initial changing current" mean? Regarding question 18iii June 2012
Total Emf of circuit = 7.4V
Total resistance = 0.65 + 0.005 =0.7Ohms

V=IR
I = V/R
I = 7.4V/0.7Ohms
I = 10.6A

all it is asking for is the initial current in the circuit as it begins to charge.
13. (Original post by jollygood)
oh sorry forgot to mention. may 2012.
18ai) 15V-7.6V=7.4V
aii) 0.65Ohms+0.05Ohms=0.7Ohms
aiii) 7.4V/0.7Ohms=10.6A

b is stumping me, I'll post my answer when I have it.
14. Guys what do we think is going to come up?
15. (Original post by stealth_writer)
Guys what do we think is going to come up?
Physics. Possibly including some stuff on DC Electricity, Waves and Nature of Light.
16. 2013 jan, question 3. can someone tell me how to solve? :/ i dont get it
17. (Original post by arshia)
2013 jan, question 3. can someone tell me how to solve? :/ i dont get it
Efficiency is
useful power/ power input x 100

Pi is power input
Po is power output (useful power)

therefore Po/Pi
18. (Original post by Tomupcraft)
Efficiency is
useful power/ power input x 100

Pi is power input
Po is power output (useful power)

therefore Po/Pi
sorry i meant question 4. sorry lol .. im freaking out :/
19. Could anyone explain question 16 from January 2009 paper please? Both parts if possible and in as much detail as you can!

Here's a link to it: http://www.edexcel.com/migrationdocu...e_20090115.pdf

20. (Original post by arshia)
sorry i meant question 4. sorry lol .. im freaking out :/
mu = refractive index

mu = sin i / sin r

sin i = 35 degrees as it must be angle to normal
sin r = 90 degrees as it is refracted along the boundary

mu = sin35 / 1
mu = 0.57 but this mu is for the glass to the air, it wants it for the air to the glass therefor mu for glass = 1/0.57 = 1.74

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