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# OCR Physics A G482, Electrons, Waves and Photons, 25th May 2012

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1. (Original post by Dale12)
Stationary waves are formed when a progressive wave reflects of a boundary and the two waves travelling in opposite directions superpose and form a standing wave with nodes and anitnodes..

to demonstrate using a microwave transmitter, it reflects of a metal sheet... if u move a probe between the metal and transmitter u get signals of minimum and maxima, these are the nodes and anitnodes...

for air columns the sound wave reflects off the closed end and u can hear a maxima at the tope of the tube, the length between the open end of the tube and closed end is wavelength/4
^This is the experiment for the 2 source interference of microwaves...are they the same?

2. (Original post by sweetascandy)
^This is the experiment for the 2 source interference of microwaves...are they the same?

with 2 source interference, you move the probe along the 'screen' (where the screen would be in youngs double slit) and you'll get the interference pattern...

however to detect a stationary wave you move it between the transmitters and the metal sheet, sort of back and forth and you will be able to detect low signal (node) and high signal (antinode)

also the 2 source interference uses two transmitters connected to one source, for stationary waves only 1 transmitter is used to reflect off a metal sheet... that help?
3. (Original post by Dale12)
with 2 source interference, you move the probe along the 'screen' (where the screen would be in youngs double slit) and you'll get the interference pattern...

however to detect a stationary wave you move it between the transmitters and the metal sheet, sort of back and forth and you will be able to detect low signal (node) and high signal (antinode)

also the 2 source interference uses two transmitters connected to one source, for stationary waves only 1 transmitter is used to reflect off a metal sheet... that help?
Okay, but the progressive waves which form a stationary wave are coherent, right?!
4. (Original post by sweetascandy)
I'm not convinced.
Go on.
5. (Original post by sweetascandy)
Okay, but the progressive waves which form a stationary wave are coherent, right?!
Not really. They are identical, but travel in opposite direcctions. If they were coherent, they would have a constant phase difference, but as they travel in opposite directions, their phase difference is constantly changing. Look at the animation in Stonebridge's sig for more details!
6. can someone plz help me understand this. If sound waves are longitudinal, then how come when you pluck a violin string a stationary transverse wave is produced? So the sound is transverse? i dont understand how sound is longitudinal then? Thanksss
7. (Original post by ebmaj7)
Go on.
Haha.
Firstly, in that equation, if the resistance increases, then as you said, R1 also increases. But doesn't R.total also increase? (I'm assuming R.total=sum of resistances?). Correct me if I'm wrong.
And secondly, only for a metal does resistance increase with a higher temp (which is due to more collisions impeding the electron flow).
Thirdly, how does resistivity come into it? What does resistivity have to do with this?

P.S. Go easy on me please; Physics is my weakest subject, ha.
8. (Original post by Pangol)
Not really. They are identical, but travel in opposite direcctions. If they were coherent, they would have a constant phase difference, but as they travel in opposite directions, their phase difference is constantly changing. Look at the animation in Stonebridge's sig for more details!
Oh okay, cool, that does kinda make sense. Also, who's stonebridge?!
9. (Original post by sweetascandy)
Haha.
Firstly, in that equation, if the resistance increases, then as you said, R1 also increases. But doesn't R.total also increase? (I'm assuming R.total=sum of resistances?). Correct me if I'm wrong.
And secondly, only for a metal does resistance increase with a higher temp (which is due to more collisions impeding the electron flow).
Thirdly, how does resistivity come into it? What does resistivity have to do with this?

P.S. Go easy on me please; Physics is my weakest subject, ha.
You are right, if R1 increases, as does Rtotal.

But say you had 5 out of 10 volts. You'd put 5 / 10 into the equation.

If R1 was then increased by 1 ohm, you'd get 6 / 11.

6 / 11 is bigger than 5 / 10, so you end up with a bigger P.D.

---------------------------------------------------

And yeah, only in a metal, that's a point I forgot, but I mean, most of the time they're talking about metals.

And resistivity comes into it cause OCR have the most random marking points in the mark scheme ever, and they like you to mention resistivity (from what I've seen).

Edit: Also, I'm sorry if I'm appearing abrupt at all. I've spent all day working on this, and spent the last two days going over stuff. The amount of work I've put in is sky high and yet I'm finding this so difficult. Stressed to the max!
10. Is a microwave probe just simply a microwave detector?
And is microwave generator the same thing as a microwave transmitter?
11. (Original post by sweetascandy)
Is a microwave probe just simply a microwave detector?
And is microwave generator the same thing as a microwave transmitter?
I think so Don't think they'll use "probe" in the exam though?

Our college gave us loads of practice papers with the old syllabus stuff on! I was wondering why I wasn't understanding half of it Do we need to know how to work out conductivity? (I know resistivity)
12. (Original post by Joseppea)
I think so Don't think they'll use "probe" in the exam though?

Our college gave us loads of practice papers with the old syllabus stuff on! I was wondering why I wasn't understanding half of it Do we need to know how to work out conductivity? (I know resistivity)
conductivity is just 1/resistivity
13. (Original post by SamXi)
conductivity is just 1/resistivity
Thank you!!! Now just gotta hope it comes up tomorrow...

Also, question for whoever knows it, is an absorption spectrum mostly black with a few coloured lines or mostly coloured with a few black lines? o.O
14. (Original post by jake9382)
can someone plz help me understand this. If sound waves are longitudinal, then how come when you pluck a violin string a stationary transverse wave is produced? So the sound is transverse? i dont understand how sound is longitudinal then? Thanksss
Sound is created by the medium therefore the air vibrating not the string. The string vibrates the air which produces a longitudinal Sound Wave. Hope this helps your question
15. (Original post by Joseppea)
Thank you!!! Now just gotta hope it comes up tomorrow...

Also, question for whoever knows it, is an absorption spectrum mostly black with a few coloured lines or mostly coloured with a few black lines? o.O
Coloured with a few black lines

Think of it like this ... all ranges of Em radiation are been shone through but the electrons only absorb certain wavelengths/frequency which is why you get the black bands
16. (Original post by SamXi)
conductivity is just 1/resistivity
Srsly? Are we meant to know this though? Well, even if we don't, I've learnt it now anyway.
17. (Original post by karanrucks)
Sound is created by the medium therefore the air vibrating not the string. The string vibrates the air which produces a longitudinal Sound Wave. Hope this helps your question
thankss soo if vibrating air produces longitudinal wave then how does the whole transverse wave come into the situation? Is there a longitudinal and tranverse wave?
18. (Original post by davie18)
Hmm seems fine to me. What happens when you try to open it?
Massive implosion... followed by an explosion.

......it actually works fine for me
19. oh dreading this exam tomorrow, anyone know anything that will almost definetly come up?
20. Gonna dump all the experiments i know ... someone correct me if i miss something out pls !

Photoelectric effect
-one photon acts with one electron
-if the photon is below the threshold frequency no electrons are displaced even if the intensity is altered
-if the photon is above the threshold frequency a surface electron is freed
-when the frequency is above the threshold the amount of electrons displaced is effected by intensity
-if the kinetic energy transfered from the photon to the electron is great enough it will travel through the vacuum to complete the circuit

Stopping potential
- The stopping potential can be found by passing a current the opposite direction to the freed electron
- To find the max KE of an electron we need to know find voltage is needed to cancel out the photoelectron
- We then multiply the voltage by e (electron charge / 1.6x10^-19) to find energy (Max KE)

Two slit - light
- Coherent Light shone at a screen through two slits
-where the light meets at the screen an odd number of half wavelengths apart (or out of phase) it creates min intensity which is destructive interference
-where the light meets and even number of half wavelengths apart (or in phase) it creates max intensity which is constructive interference

Two slit - microwave
Same as light but microwaves shone through 2 slits in a metal screen at a microwave detector

Find the speed of sound
- Vibrate a tuning fork of known frequency above a measuring cylinder
- Add water to the cylinder until resonance occurs (sound becomes loudest)
- Measure the coluomb of air above the water this is the fundemental frequency (1/4 a wavelength)
- times the length of air by 4 to find the wavelength then use V=FxLambda to find V

Finding planks constant, threshold frequency and the work function
-Alter the P.D. across LED's of different frequencies until they become visible
-Multiply the P.D. by e (Electron charge / 1.6x10^-19) to change it to energy
-Plot a graph of frequency against energy
-x intercept = threshold frequency
-y intercept = work function

De Broglie diffraction
-An electron beam is shone through a sheet of material
-Where the electrons diffract the most is the point where the wavelength of the electron is aprox equal to the atomic spacing in the material
-by changing the amount of energy in the electron we can change the wavelength
-We can use this experiment to determine the atomic structure of materials

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Updated: December 11, 2012
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