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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 sarah.102)
Also guys, latest confusion what is potential difference? I understand that it is the measure of energy transfer in between 2 points in an electric circuit or it drives the circuit by forcing the current to flow but still I feel a bit confused. Say if I calculate the potential difference and get 3V what does that actually mean?
3V means that 3J is work done on a coulomb (bunch of electrons) of charge when passing through a component.
2. (Original post by sarah.102)
Also guys, latest confusion what is potential difference? I understand that it is the measure of energy transfer in between 2 points in an electric circuit or it drives the circuit by forcing the current to flow but still I feel a bit confused. Say if I calculate the potential difference and get 3V what does that actually mean?
The 3V is the energy transferred from the source of energy, EMF. 3V from however much the cell provides to the circuit.
3. (Original post by krisshP)
3V means that 3J is work done on a coulomb (bunch of electrons) of charge when passing through a component.
Yaaa^^
4. (Original post by B-Stacks)
The 3V is the energy transferred from the source of energy, EMF. 3V from however much the cell provides to the circuit.
Thats helped my understanding too, thanksss!
5. (Original post by sarah.102)
Also guys, latest confusion what is potential difference? I understand that it is the measure of energy transfer in between 2 points in an electric circuit or it drives the circuit by forcing the current to flow but still I feel a bit confused. Say if I calculate the potential difference and get 3V what does that actually mean?
A potential difference of 3Volts means 3 joules is given to every coloumb of charge, to push it around a circuit. Now if you used 10 volts, you'd give 10 joules to every coloumb, so the current would get a bigger 'push'. But it's important to remember PD is NOT a force, it's just a transfer of energy.
6. I dont understand part (b). On the mark scheme it says that after doing E=hf you have to divide Power(10W) by th energy found. why?
7. (Original post by KBenzema)
I dont understand part (b). On the mark scheme it says that after doing E=hf you have to divide Power(10W) by th energy found. why?
If you don't remember it it's also on the formula sheet, E = hf, the energy of a photon is equal to Plank's constant times its frequency. I've put the rest in spoiler tags, see if you understand it more before looking at the next hint.

Spoiler:
Show
So, by doing 5.5*1014 (the frequency in Hz given in the question) times 6.63*10-34 (Plank's constant) you get the energy of the photon, roughly 3.65*10-19 Joules.

Spoiler:
Show
Now that you know each photon has 3.65*10-19J of energy you can do 10W divided by 3.65*10-19J to tell you how many photons are emitted per second (W/J = J/s / J = *something*/s, in this case photon/s).

Spoiler:
Show
This would tell you that you would have roughly 2.74*1019 photon's being emitted by the screen per second.
8. Can somebody please explain to me polarisation? It baffles me for some reason. Reading more and more into it has confused me.
9. (Original post by sarah.102)
Can somebody please explain to me polarisation? It baffles me for some reason. Reading more and more into it has confused me.
Spoiler:
Show

For polarization what you need to know is that when light passes through a polarizing filter it will become partially polarized meaning the polarizing filter will only let through light which is acting in the same plane, all other planes of light will be blocked up.

However when you pass partially polarized light through a second polarizing filter, the intensity of light will depend on the angle the second filter is rotated. It is denoted by equation I = Io cos^2(pheta)
Where:
- Io is the initial intensity (after passing through the first filter)
- I is the final intensity (after passing through the second filter)
- Pheta is the angle of rotation of the second filter.

Here you would note that if 2nd filter isn't rotated, such that the angle is 0 degrees light would have full intensity. Since cos 0 = 1. 1 squared = 1.

If the 2nd filter is rotated perpendicular to the first filter, such that the angle is 90 degrees no light will pass through. Since cos 90 = 0. 0 square = 0.

Also if the 2nd filter is parallel to the first filter, such that the angle is 180 degrees the light would have full intensity. Since cos 180 = -1. And -1 squared = 1.
10. (Original post by Boy_wonder_95)
x
iirc Malus's law isn't in the Edexcel syllabus, but I could be wrong. May still be useful though
11. (Original post by justinawe)
iirc Malus's law isn't in the Edexcel syllabus, but I could be wrong. May still be useful though
Who's law??
12. (Original post by sarah.102)
Can somebody please explain to me polarisation? It baffles me for some reason. Reading more and more into it has confused me.
http://www.thestudentroom.co.uk/show...352&p=42866490
13. I think I have discovered the angriest Physics lecturer ever.
14. i have a big doubt on part (a)(iii)
why do i get a different answer depending on the formula i use. im baffled.
P=IV : (0.75)*3=2.25w
P=I^2R : (0.75)^2*3.6 = 2.025w
P=V^2/R : (3)^2/3.6 = 2.5w
15. Can somebody help me with section a question 7 on paper January 2012 unit 2 please? I don't get it at all

V=IR
I=V/R

There's constant pd so surely as R increases, I decreases linearly, giving B as the answer?

Thanks
16. In January 2012 paper unit 2 q18a, they said core - cladding. Does this means that light travels from core into cladding, or light travels from cladding? HOW do you know?

Thanks
17. (Original post by krisshP)
3V means that 3J is work done on a coulomb (bunch of electrons) of charge when passing through a component.
Difference between pdf/voltage and emf
18. (Original post by avipsita)
Who's law??
Well, as I said, I'm pretty sure it's not in the Edexcel syllabus.

It's basically the equation the guy I quoted posted.
19. (Original post by Daniel Atieh)
Difference between pdf/voltage and emf
?
20. (Original post by krisshP)
?
lol
better way:
1) define voltage
2) define pdf
3) define emf

haha yea it looks weird ..i just noticed

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