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# Problems with some areas of AS Physics

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1. HI, I'm posting because I have some problems with certain topics, hopefully you guys can help.·

General:
1. Significant figures.Do I always presume it’s the same as the SF of the values in the question? What about calculations do I use the rounded figure for these always? When there is no SF mentioned do I just take 2SF to be the standard?
Optics:
1. For a double slit experiment, why is it necessary that waves be coherent, Surely you can find certain non-coherent waves that can form an IP if they’re given the right WL. Secondly white light is composed of light of differing wavelengths, yet it would still show an IP, so coherence isn’t necessary here? There are differing definitions for coherence, some suggest it's CPD + frequency others CPD + wavelength, which is it?
2. Exam question: Calculate FS by using a figure with a ruler below (with 25 maxima), I got 5.24mm which was correct, it then asks to determine diffraction spacing (Given WL as 628nm and DtS as 2.5m), using dsin(theta) = n x WL. Maybe I'm missing some understanding here but I get that sin(theta) = tan(theta) which = FS/DtS However in the answer they use FS as 0.13/2.5, where is this value from, I get it's 5.24mm x 25? But I feel like I'm missing something, why does n = 25? I get that's the number of maxima, but surely n would be 12 or 13 because the fringes are repeated on either side, and on the diagram there are 25 maxima but if I set n = 25 aren't I calculating 25 maxima on one side of the IP?
3. Exam question: when a spectrometer is rotated from an initial angle of zero degrees a spectrum is first seen when the spectrometer is turned to 50 degrees. it asks you to determine the number of orders of the spectrum, how?! The next question states what is the difference in light emerging from the 50 degree point to the light emerging from the zero degree point? Then using dsin(theta) = n x Wlstyle question they set theta as 51 degrees, what is going on here?
4. How does intensity varies as you increase the number of slits, Why for 1 slit is the central fringe 2x the size of the outer fringes? Why does i ntensity blend in 1 slit? why is WL/2 dark fringes for multiple slits and bright fringes for 1 slit
5. What are orders of diffraction gratings? what is a zero-order maximum? Why set n as the number of fringes and not no of fringes/2 (as they are repeated)?
Particles:
1. I'm confused on Interactions and Decay differentiation. Exam question: a pion + proton = kaon +X (another strange particles), which interaction is involved here. I get strangeness separates WNF from SNF . The answer isSNF but what if the X was +1 strangeness. How can I just assume it’s -1?
2. Why is a Neutron decay called a B- Decay, yet a Proton is termed stable when a B+ decay exists?Why do questions asking for Neutron decay only require B- Decay, aren’t there many ways it could decay? What separates decay and interaction in an equation?
3. In Photoelectricity it is stated that the no of electrons emitted is proportional to the intensity of the light, but light intensity = no of photons/per sec and =energy of photons/per sec. Wouldn’t no of photons/per sec not change the rate of electron emission?
4. For excitation the specification states it is an electron moving from a lower energy state to a higher energy state Isn’t this just one form of excitation? This is probably AS Scope but surely then it would be best to state: describe excitation in terms of electrons. Not just describe excitation?
Electricity:
1. How does adding multiple resistors slow down the current at all points in the circuit.
2. How and why is potential difference shared for components in parallel.
3. Why is total emf the sum of potential drops around the circuit why does a charge flow, do some work one one comp and the rest of it on the other comp? why not vice versa? Why not do only the work required?
Energy:
1. Efficiency is written as useful energy/energy supplied. One question it states that an exercise bike pedals against a brake force of 30N at speed 15ms, I know P = Fv, but why isn't useful energy transferred the kinetic energy?
2. How do I determine which is the input power and what is the output power, easy for say a bulb or a machine much harder for something organic.

2. (Original post by WanderSotC)
HI, I'm posting because I have some problems with some select areas of certain topics........
Hello.

TSR is full of people willing to help but I doubt anyone will answer you with the length of this post.

I politely suggest you split this up into separate (posted individually) questions and précis each to make it more approachable.

Thanks.
3. Sorry I'm not very good at brevity I will try to cut it down. Do you mind unquoting the post so there aren't two walls of text
4. (Original post by WanderSotC)
Sorry I'm not very good at brevity I will try to cut it down. Do you mind unquoting the post so there aren't two walls of text
Done.

Also, always quote your replies (see my post #2 to you) as the person/s replying will then automatically get a flag that you replied otherwise they won't know unless they click on the thread again.
5. I hope that's better.
6. (Original post by WanderSotC)
Electricity:
1. How does adding multiple resistors slow down the current at all points in the circuit.
2. How and why is potential difference shared for components in parallel.
3. Why is total emf the sum of potential drops around the circuit why does a charge flow, do some work one one comp and the rest of it on the other comp? why not vice versa? Why not do only the work required?
1) You are thinking about current in the wrong way. Do not think in terms of kinetic energy. It's not the speed at which electrons move through the circuit which provides the energy.

We know that like charges attract and unlike charges repel. Electrons will therefore all repel each other. The power supply provides a surplus of electrons (net negative charge) at the cathode and a deficit of electrons at the other (net positive charge). Thus an energy potential exists between the nodes, which is realised if a conduction path (circuit) is created between the two.

Now think about electrons in a conductor. An analogy to use is billiard balls crammed into a long tube. Push on one ball at one end of the tube (supply) and all of the balls in the tube move. Any resistance at any point in the tube will propagate that resistive force in both directions throughout the tube back to the person pushing (supply) and also to the end where the balls pop out (return on supply). But the work is done at the point of resistance which heats up.

Another way is to think about it is with a chain on a bicycle: push on the pedal (supply) and the chain tension increases throughout the chain. The road wheels provide resistance and its the friction with the road is where the work is done.

And so it is with electrons in a conductor. Create a circuit and the electrons pushing at the cathode (surplus of electrons) will create a pressure (voltage potential) which ripples all the way through the conductor such that the electrons at the anode end will now recombine with atoms of net positive charge (space for electrons in their outer valence shell).

Any resistance will cause energy to be expended at that point - no matter where in the circuit the resistance is placed.

2) Refer to the explanation above and also that Voltage-potential is defined as Joules per Coulomb of charge potential (analogous to water pressure). Think of components in parallel as if they were the T junction of water filled pipes. The water pressure at the junction is the same and so it is with voltage pressure. The Voltage pressure is the same for all components connected to the same junction when referenced back to the supply common return.

3) Charge flow: see previous explanation.
Voltage drops: potential difference is a measure of the energy expended between two points in a circuit. If it's a resistor then the available Joules per Coulomb of charge at the entry to the resistor will be less than the Joules per Coulomb of charge available to all resistances down-stream of that point. i.e. work has been done, energy was expended.
7. (Original post by uberteknik)
1) You are thinking about current in the wrong way. Do not think in terms of kinetic energy. It's not the speed at which electrons move through the circuit which provides the energy.
Thank you very much that cleared up a lot, it was very articulate and well worded, seriously this is what I needed, I really need help with the optics if anyone knows that specifically those 2 exam problems.

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