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# The Physics PHYA2 thread! 5th June 2013 watch

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1. (Original post by pushkin_)
You are my savior ! Now I'll stop crying because of my chemistry and I'll start doing what I can with physics.
Thaanks again !
so true! life saver!!!
2. (Original post by SAS18)
so true! life saver!!!
your welcome !! just hope its enough to make y'all calm down
3. Does the cladding in an optical fibre have a higher or lower refractive index to the core?

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4. (Original post by masryboy94)
some one posted the experiments we need to know earlier in this thread so credits to them !!! (attachment)

and earlier on in this thread i made some key points (apologies to those who've seen it and think im taking up space).

Here are some key points i made for all of you for PHYA2 to bare in mind:

• Forces acting on an object in equilibrium form a closed loop (Vector triangle)
• Define moment: force x perpendicular distance, and is the turning effect of a force around a turning point.
• Principle of moment: For a body to be in equilibrium, the sum of the clockwise moments about any point equals the sum of the anticlockwise moments about the same point.
• Torque = Force (Turning force) x perpendicular distance between them.
• gives the gradient on a curve to be velocity. and since is acceleration, therefore the stepper the curve the greater the acceleration.
• Newton's First Law: An object will stay at rest or in uniform velocity, unless a resultant foce acts on it.
• Newton's Second Law: Acceleration is proportional to the resultant foce acting on it at a certain mass. F=ma
• Newton's Third Law: When 2 objects interact, they exert an equal and opposite force on eachother.
• Free fall is when the only force acting on an object is gravity.
• For projectile motion: VERTICAL you use SUVAT, for HORIZONTAL you use and in both cases 't' is the same.
• When Friction force = Driving Force object stops accelerating and reaches its terminal velocity.
• Types of friction: Contact (Friction), Fluid (Drag, Air resistance, Fluid Resistance)
• Principle of conservation of energy: Energy cannot be created or destroyed, but can be transferred from one form to another.

• Hooke's Law: Extension is proportional to force given it is within the limit of proportionality.
• Experiment to investigate extension:

1) Object should be supported using a G-clamp
​2) Measure original length using a ruler
3) Masses should then be added in 100g intervals up to atleast
600g.
4) For each mass added, calculate new extension using (new
length - original length).
6) Repeat experiment 3 times taking an average result.
7) Plot a graph of force against extension to show results.

• Elastic and Plastic deformation: If elastic, material will return to original length. Plastic when material will not return to original length
• Young Modulus:
• Young's Experiment:

1) Set up experiment.
2) Add enough mass to straighten wire but not extend it.
3) measure original length using a ruler
4) Add in intervals of 100g of mass up to what it could withstand, and for each mass added read off the new length by using a travelling microscope or vernier calliper.
5)calculate extensions by using (new length - original length)
6) Measure cross sectional area using , to obtain D, you would used a micrometer and measure at different positions on the wire and take an average reading.
7) Repeat experiment 3 times
8) plot a graph of stress against strain and gradient will give young modulus or Young Modulus = .

• Waves are vibrations, they only carry and transfer energy.
• 4 things a wave can do:

1) Reflect - Wave bounces back when hitting a boundary.
2) Refract - wave direction changes as it enters a medium
3) Diffract - wave spread out
4) Interfere - 2 waves co-join

• Phase difference: Amount by which one wave lags behind another wave.
• Transverse waves: Vibrations oscillating at right angles to the direction of travel (Electromagnetic waves, waves on rope, ripple of water ...etc)
• Longitudinal waves: Vibrations oscillating along direction of travel (sound)
• Application of polarisation: Glare reduction (polaroid sunglasses). Improving TV and radio signals by lining up the rods of the receiving aerial to the transmitting aerial.
• Optical fibres: light in optical fibres is used to transmit phone and cable TV signals. Light doesn't heat up fibre therefore little energy loss. No electrical interference. and it is a cheaper alternative.
• Signal loss (reduction in amplitude) in optical fibres are caused by energy lost through absorption and scattering.
• Signal broadening is caused by multi-path dispersion which is when the signal travels straight down the middle and arrives earlier than those undergoing T.I.R.
• Principle of Superposition: When 2 or more waves cross, the resultant displacement equals the vector sum of the individual displacement.
• Constructive interference: when displacement combine to make an even bigger one (e.g. crest plus crest)
• Destructive interference: when negative and positive displacement combine to cancel out (e.g. crest plus trough)
• Stationary or standing wave: is the superposition of two progressive waves with the same frequency and amplitude travelling in opposite directions towards each other. this is when you get fundamental frequency which is . if you double the fundamental frequency you get the second harmonic (first overtone). Triple the fundamental you get third harmonic (second overtone) and so on ...
• Fundamental frequency depends on the length, mass and tension of a spring.

1) if length increases, frequency decreases
2) if mass increases, frequency decreases
3) if tension increases, frequency increases

• Application of stationary waves: Microwaves, sound waves.
• you get greatest diffraction if the slit size is equal to
• Laser is monochromatic (has a single wavelength) and coherent (same frequency and constant phase difference).
• laser beams are powerful and can cause damage to eyesight, this can be prevented by wearing safety laser goggles or removing any reflective surfaces.
• Path Difference: how much further a wave has travelled than the other wave.
• When you get constructive interference, at your first order(s) [bright fringes] your path difference is where is an integer this also means that the phase difference is a multiple of .

• where you get [dark fringes] between say zero and the first order or first and second order, the path difference is where the phase difference is a multiple of (for it to be perfectly out of phase).
• non - coherent light such as white light will have wider maxima containing different colours with central white fringe. light is continuous range of frequencies
• Young's double slit experiment: . w is fringe spacing. is wavelength, s is spacing between slits, and D is distance from slits to screen.
• Diffraction grating: They have more slits causing bright bands to be brighter and narrower and dark fringes to be even darker. monochromatic light is used causing interference patterns to be sharper and more accurate of a measurement.

Hope it helps !!!!!

Thanks, this is a big help. Question, what's the difference between diffraction grating and fringe spacing?
5. Higher. Otherwise TIR won't occur.
6. (Original post by Felix Ivers)
Does the cladding in an optical fibre have a higher or lower refractive index to the core?

Posted from TSR Mobile
Lower, always Lower
7. (Original post by masryboy94)
can i just stress that for SINGLE SLIT DIFFRACTION the intensity on an intensity graph will reduce. whilst for a DOUBLE SLIT DIFFRACTION the intensity on an intensity graph will stay the same or similar to the maximum central. not totally sure why, but theres a good chance it could come up and you need to realise the difference between the two !!!!
Why does it always reduce in single slit diffraction when we narrow the slit?
8. June 2011 5d please?
9. (Original post by OliverG)
Higher. Otherwise TIR won't occur.
See this other dude's saying lower so I don't know what to think.

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10. (Original post by Qari)
Thanks, this is a big help. Question, what's the difference between diffraction grating and fringe spacing?
im guessing you mean difference for diffraction grating is when you have thousands of slits per mm whereas fringe spacing is for 2 slits. big difference is diffraction gratings give a much sharper maxima than
11. (Original post by Claree)
But the mark scheme says that if you were to alter the velocity or acceleration the horizontal force would be affected. What I don't understand is how the ship's speed changing could affect the horizontal force.
you mention the change is velocity will affect the horizontal motion. That is correct. But, if you say velocity will change the horizontal motion, that is incorrect. Ship changing speed would mean that the drag force from the water is increased and so the air resistance, so it is a factor that causes change in the horizontal force/motion.
12. (Original post by Pirateprincess)

That depends on what the question is exactly. IF they tell you the frequency then rearrange v=fλ to get λ= v/f, where v is the speed of light (3.00x 10^8 ms^-1)
If you're using the laser for a double slit experiment then rearrange spacing of maxima= wavelength x distance from slits to screen
slit spacing

You could use a number of other methods but it depends on what info they give you. Did you have a specific question in mind?

I was talking about the 6 marker!
13. What happens to the intensity graph/pattern for Young's double slit experiment when the width of the two slits is reduced? NOT the slit spacing, but the slit width.
14. (Original post by Felix Ivers)
See this other dude's saying lower so I don't know what to think.

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I'm really sorry. I quoted it the wrong way around. It must be lower, otherwise there will be no TIR. Sorry!
15. question: what is the difference from saying you get a maxima\minima or bright\dark fringes or zero\first order?
16. (Original post by Felix Ivers)
Does the cladding in an optical fibre have a higher or lower refractive index to the core?

Posted from TSR Mobile

(Original post by masryboy94)
im guessing you mean difference for diffraction grating is when you have thousands of slits per mm whereas fringe spacing is for 2 slits. big difference is diffraction gratings give a much sharper maxima than
Thanks and is the answer to the quoted question above lower or higher?
17. (Original post by NedStark)
What happens to the intensity graph/pattern for Young's double slit experiment when the width of the two slits is reduced? NOT the slit spacing, but the slit width.
It's taken as a given that slit width is negligable, otherwise you would not get bright and dark minima and maxima. You would get a white middle maxima, of double intensity and double width, then normal dark maxima, then of half intensity blue fringes on the inside, then red fringes on the outside, then repeat ad finitum, with intensity halving each time.
18. Only thing I'm not 100% on is the 6 marker. Anyone have any brief ideas on what it could be?
19. (Original post by NedStark)
What happens to the intensity graph/pattern for Young's double slit experiment when the width of the two slits is reduced? NOT the slit spacing, but the slit width.
Why? Is there a question on the slit width? I have never heard of that thing.

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