~~NEEDS HELP on Physics HW!! ~~ Options

[MissHyun90K]

HI everyone! I'm asian too lol, so please don't bash me ^.^
I'm so stuckkkk on my AP PHYSICS problem >< . I've read the chapter but been scabbing my head so much!!
My TEACHER assigned us 2 cases ON THE 1st day of school >_< . The coolest thing is that one of the problems will features on quiz and 1st exam...
I just want to understand the case to better be prepare.
I know this is not appropriate to ask people to help me, but I really need HELP clarifying!! Have to turn in on Monday to get HW credit. Answers are given ~
Anyhow, you don't have to solve everything, just give me hint is enough... Although I'm not a very bright girl, I will try completing the rest..
Much appreciated if you can explain the questions and help me understand. kk !~ >.<

Questions from my teacher's assignment (EXACT WORDS)

Question 1

What uniform magnetic field, applied perpendicular to a beam of electrons moving at (1.1 x 10^6 m/s),
is required to make the electrons travel in a circular arc of radius 0.275 m?

Answer: (2.30 x 10^-5) T

Question 2

An electron is accelerated from rest by a potential diffrence of 350V. It then enters a uniform
magenetic field of magnitude 210MT with its velocity perpendicular to the field.
Calculate a) the speed of the electron and b) the radius of its path in the magnetic field.

Answer: a) (1.11 x 10^7) m/s, b) (3.01 x 10^-4) m

Given constant on Assignment Sheet:
Mass of Electron: 9.11 x 10^-31 kg
Magnitude: 1.6 x 10^-19 c


THANK YOU!! XD ~

[InitialDJay]

wow, you are very honest with yourself when asking for help. lol i'm impress.

since i'm bored today, i help you solving them.. i show you how i do it on paper so you can see it better than typing numerical value can be confusing for you.

you can print it out, study or verify the result or check to see if the answer is right because i'm not sure. don't have scientific calculator atm.

Question 1

What uniform magnetic field, applied perpendicular to a beam of electrons moving at (1.1 x 10^6 m/s),
is required to make the electrons travel in a circular arc of radius 0.275 m?

Answer: (2.30 x 10^-5) T

what it asking you is to find the magnetic field when an electron is travel in a circle.

thanks to Heisenberg or someone i forgot, we're able to see electron behavior without actually directly seeing its particle. when electrons are moving in a champers, we noticed that it emits light when you pour gas into the champers, sort of like an commercial airplane leaving the smoke behind in the sky.

so from this experiment, we see the electron, whatever direction it moving,, it is always perpendicular (ie, 90 degrees angle) to its magnetic field center.

looks at picture...



orange line = electron velocity direction
red line = radius
blue line = magnitude charge

applying the right hand rule, you notice that

magnetic force is just magnetic center multiply magnitude multiply velocity.

in other words,
Fb = B * |q|*V

Fb = magnetic force
B = magnetic field/center
|q| = magnitude ==> it's in absolute bracket because weather it's electron or proton, it has 1:1 same magnitude charge value but it will eliminates negative with absolute bracket (ie, electron is negative but become positive with this | | symbol , proton is already positive anyway)
V = velocity

so you see how all B*|q|*v is perpendicular.

anyhow the real equation for magnetic force is actually

Fb=B*|q|*V*sin90 but since since 90 = 1, so you can eliminate the sin90.

then follow the instruction on paper work i did to see if you understand them.

Question 2

An electron is accelerated from rest by a potential diffrence of 350V. It then enters a uniform
magenetic field of magnitude 210MT with its velocity perpendicular to the field.
Calculate a) the speed of the electron and b) the radius of its path in the magnetic field.

Answer: a) (1.11 x 10^7) m/s, b) (3.01 x 10^-4) m

Given constant on Assignment Sheet:
Mass of Electron: 9.11 x 10^-31 kg
Magnitude: 1.6 x 10^-19 c


THANK YOU!! XD ~

this question is the same as the first one, you just find the Velocity and then Radius.

when it said "electron is accelerating from rest to potential differences"... it means at stationary position or frame of rest, or whatever scientific people used, it means setting equation to zero).

i think you already know that the potential energy is basically U = mgh, and Kinetic energy is K = 1/2mv^2

thus lets U + K = 0 and follow what i did in the paper.

umm one more thing, i dunno if you understand what i just said.. but i advice when solving some physics or mathematical words problem, a good habit is making sure you write out all the constants, then apply some equation that you know and understand.

for both of this cases, what they're asking you is basically an electron is shooting from electromagnetic gun, and the electron is moving in a circular perfectly motion (uniform motion), thus you must apply the right-hand rule to find the magnetic force (Fb = B * |q|*V).





ps: don't tell your teacher i taught you or show you this.

if still confused, maybe check at physicist forum for help.

This post has been edited by InitialDJay: Sep 8 2009, 10:45 PM

[MissHyun90K]

THANK YOU SO MUCH FOR HELPING!!! ^_^ I will print it out and study from your method. XD
Anyway if you don't mind me asking, are you a teacher? >.<
It saves me so much time cuz your method is completely different from mine.
Mine was wrong, I kept getting different answer than the given one. >.<


This post has been edited by MissHyun90K: Sep 9 2009, 08:50 PM

[InitialDJay]

^nope, i'm just a student like you, except i got high easily when doing physics.

anyway...there is only 1 method, i guess you look at a different angle.. maybe confused by words or somethign cuz it quite easy to solve.

1 more thing, i guess you're a hs kid (i mean hs girl lol), well be prepared to know most concept in physics you're about to learn in hs is wrong.. *silence*

[MissHyun90K]

^nope, i'm just a student like you, except i got high easily when doing physics.

anyway...there is only 1 method, i guess you look at a different angle.. maybe confused by words or somethign cuz it quite easy to solve.

1 more thing, i guess you're a hs kid (i mean hs girl lol), well be prepared to know most concept in physics you're about to learn in hs is wrong.. *silence*

THANK YOU!!
I received full credit because of your help. XD
Uhm what do you mean concept in HS physics is wrong? >.< Am I wasting time learning them now? lols
Also yes, I'm still in HS. XD
I guess you're a college student. ^_^

This post has been edited by MissHyun90K: Sep 16 2009, 08:40 PM

[InitialDJay]

^nvm, i don't want to ruin your passion for hs physics now.

just know that what you're learn in hs is not always correct.. it misleading in a way.

but the good news is most hs teachers intentionally teaching you this way to prepare for higher physics level later...

for example, like now, i think hs physics are mainly about newtonian classical mechanics and using Neil Bohr's model of atom.

however in upper level of atomic structure, we have to deal with Heisenburg's uncertainty principle, schodinger equation, Einstein-De Haas intepretation, and Richard Feynman's path integral formulation when dealing with subatomic system, such as an electron behavior around the nucleus like in this problem. =p

i know it sounds confusing later and conflicting what you've learned in hs but but don't worry.. everyone does... and it prepares you to understand deeper into the quantum level of physics.

"the more you learn about physics, the less you know about it" --- a great quote by an instructor at caltech.


also, what grade are you in? what major do you plan to go into? if you're not going into physical science, then you don't have to worry about this.

if you have anymore physics hw problem, don't be afraid to ask.. since im about to go back to school.. i need to refresh my brain.

This post has been edited by InitialDJay: Sep 17 2009, 11:50 PM

[MissHyun90K]

^nvm, i don't want to ruin your passion for hs physics now.

just know that what you're learn in hs is not always correct.. it misleading in a way.

but the good news is most hs teachers intentionally teaching you this way to prepare for higher physics level later...

for example, like now, i think hs physics are mainly about newtonian classical mechanics and using Neil Bohr's model of atom.

however in upper level of atomic structure, we have to deal with Heisenburg's uncertainty principle, schodinger equation, Einstein-De Haas intepretation, and Richard Feynman's path integral formulation when dealing with subatomic system, such as an electron behavior around the nucleus like in this problem. =p

i know it sounds confusing later and conflicting what you've learned in hs but but don't worry.. everyone does... and it prepares you to understand deeper into the quantum level of physics.

"the more you learn about physics, the less you know about it" --- a great quote by an instructor at caltech.


also, what grade are you in? what major do you plan to go into? if you're not going into physical science, then you don't have to worry about this.

if you have anymore physics hw problem, don't be afraid to ask.. since im about to go back to school.. i need to refresh my brain.

Great quote!! I also believe in it. Fascinating isn't it?
Uhm it also true that I learn a lot of Newton's law and theory and that you remind me, my teacher does in fact lectures on Borh's model which it gets confusing. >.<
Other concepts you mentioned are new to me, and that I never learned yet.
Can you explain how Borh's model is different in higher physics course? I'm really fascinating with physics even though I'm not really good at it. I admit it. ><
What grade I'M IN? 12th ^_^ AND I do plan on going into biology.
But I need to pass physics first. I think I'm not that good at it, honestly. ><

I LOVE HW HELP! lols
Really appreciate that you took the time to help me.
This week, we are assigned a few more case problems and I do need some help on it. XD
If you don't mind, can you provide hints or procedure to solve it? THANK YOU!!

Question 1:

An astronaut in a small spaceship whose mass is 1.3 x 10^3kg is drifting in outer space with negligible gravitational forces acting on it. If the astronaut turns on a 10kW laser beam, what speed will the ship attain in 3 day because of the momentum carried away by the beam?

I was given hint that the problem related to radiation pressure and total amount of absorption. ><

and total absorption equation is Pr = |I|/c and |I| = P/A with Pr means Radiation Pressure, |I| means Intensity, P means Power, and A means Area.

Okay I know that speed is distance over time, so in my method, I substitute many different variables. Sorrie for confusing you >>

V = A/t - What I did was replacing Area with Intensity * Power.

V = |I|/P divided by t.

V = P/A divided by P divided by t.

V = P^2 divided by A. divided by t.

Hence, V = (10kW)^2 / Area over t.

What I don't get is how should it solves for Area? - In the problem, there was no given amount of Area >.<

Could it be Area = mass x 3 days?

So I could solve with V = (10kW)^2 / 1.3 x 10^3kg * 1)????
What's about time of radiation pressure?


I'm stuck in this part! >.>


Question 2:
Radiation from the Sun reaching Earth, just outside the atmosphere, has an intensity of 1.7 kW/m^2, a) Assuming that Earth and its atmosphere behaves like a flat disk perpendicular to the Sun's rays and that all the incident energy is absorbed, calculate the force on Earth due to radiation pressure, b) For comparison, calculate the force due to the Sun's gravitational attraction.



Extra Credit:

It has been proposed that a spaceship might be propelled in the solar system by radiation pressure, using a large sail made of foil. How large must the surface area of the sail be if the radiation force is to be equal in magnitude in the Sun's gravitational attraction? Assume that the mass of the ship + sail is 2500kg, that the sail is perfectly reflecting, and that the sail is orientated perpendicular to the Sun's rays. See Appendix C of textbook for needed data. (With a larger sail, the ship is continuously driven away from the Sun.)



I've tried to solve the first case, but the rest I haven't try yet.! >.<

[InitialDJay]

Great quote!! I also believe in it. Fascinating isn't it?
Uhm it also true that I learn a lot of Newton's law and theory and that you remind me, my teacher does in fact lectures on Borh's model which it gets confusing. >.<
Other concepts you mentioned are new to me, and that I never learned yet.
Can you explain how Borh's model is different in higher physics course? I'm really fascinating with physics even though I'm not really good at it. I admit it. ><
What grade I'M IN? 12th ^_^ AND I do plan on going into biology.
But I need to pass physics first. I think I'm not that good at it, honestly. ><

I LOVE HW HELP! lols
Really appreciate that you took the time to help me.
This week, we are assigned a few more case problems and I do need some help on it. XD
If you don't mind, can you provide hints or procedure to solve it? THANK YOU!!

lol everything is fascinating to me.. even your curiosity is fascinating.

o so you haven't learn those yet... don't worry, you will soon. but since you'll study biology, it might not be a requirement to understand the quantum level.

as for Borh's model...there were a few underlining point of Borh's model which was inconsistent with the newly discover atomic structure.

for example, borh's first point: electron is traveling at certain discrete distance from the nucleus, defining by a circular "fixed orbit".

Borh-Sommerfeld illustration:


well, we know that electron does not moved in Borh's circular nor Sommerfeld's elliptical orbit, but instead at different angle in 3-dimensional structure, like a crystal structure in this illustration...



second point: an electron can only lost and gained energy during the "quantum jump", at illustrated in Borh's atom model above, from n=1, n=2, n=3, etc.. which interpreted when an electron jumps from cell 1 to cell n, it emitted and absorbed electromagnetic radiation (or energy) depending on the cell it was in.

because of the probabilistic nature that Max Born outlining of atomic particle and Weiner Heisenburg's uncertainty principle, we can only "guess" the position and momumtum of electron cell. hence, Borh's interpretation required an electron to be define at a fixed distance and exact cell in order for his total energy absorbing to work (ie E = hv, h is planck constant, v = velocity), and that is inconsistent with the angular momumtum since electron, in fact, absorbed and emitted whenever an external force acts upon it (ie, Einstein proposed a quanta packet called photon). this means only photon can act upon the electron allowed energy consumption to deviate from the total energy absorption.

anyway, there are other points of inconsistency. and we can't really use Borh's model in regard to anything with more than 2 electrons because his model applied only to hydrogen element (which contain only 1 electron, 1 nucleus).

think of Borh's model of atom like the solar system where planet (electron) orbits around the sun (nucleus). for its simplicitic purpose, any elements that have 1 electron (ie hydrogen element = 1 electron, 1 nucleus, prefectly describes electron's "plantary orbital motion").

but as you can see, many other elements in the periodic table have more than 1 electron, thus we required to used some new equation, such as schrodinger equation to calculate energy level cell.

it may sounds confusing to you now, but thats okay. one of the thing that makes quantum physics "cool" is it confused everyone.


----

Okay lets do the HW helps since i promised to help you. lol

Question 1:

An astronaut in a small spaceship whose mass is 1.3 x 10^3kg is drifting in outer space with negligible gravitational forces acting on it. If the astronaut turns on a 10kW laser beam, what speed will the ship attain in 3 day because of the momentum carried away by the beam?

I was given hint that the problem related to radiation pressure and total amount of absorption. ><

and total absorption equation is Pr = |I|/c and |I| = P/A with Pr means Radiation Pressure, |I| means Intensity, P means Power, and A means Area.

Okay I know that speed is distance over time, so in my method, I substitute many different variables. Sorrie for confusing you >>

V = A/t - What I did was replacing Area with Intensity * Power.

V = |I|/P divided by t.

V = P/A divided by P divided by t.

V = P^2 divided by A. divided by t.

Hence, V = (10kW)^2 / Area over t.

What I don't get is how should it solves for Area? - In the problem, there was no given amount of Area >.<

Could it be Area = mass x 3 days?

So I could solve with V = (10kW)^2 / 1.3 x 10^3kg * 1)????
What's about time of radiation pressure?


I'm stuck in this part! >.>

ok i think you did a few thing wrong. this question is not asking for standard velocity (v = d/t), like a car moving with respect to speed and time.

remembered the total absorption of radiation momumtum is total energy / speed of light (pr = U/c). and if radiation (ie light) deflected back out, the radiation momumtum will be twice because it travels twice along the path (back n forth), thus absorption is pr = 2U/c. An easier to remember is if you go from point A to point B, then it's just one forward distance so pr=U/c, and if you go from point A to point B, back to Point A, then it becomes pr = 2U/c.

now, in this question, if the beam of light carried energy U drifting away from the spaceship, then it also has momentum p = U/c.

if P is the power of the laser beam, then the total energy with respect to t (in 3 days, converts to second ok?), is total energy = power * time (ie, U = Pt).

thus the radiation momentum (Pr = U /c) is just Pr = Pt/c.

and with m being the mass of spaceship, and Pr momentum, so calculating the velocity needed to be divided by the mass (ie, V = Pr/m).

okay sum it up, then we know that...

Pr = U/c, which is just Pr = Pt/c

Pr= radiation momentum
U = total energy
c = speed of light
P = Power
m = mass

then subsititute
V = Pr/m = (P*t)/c over m
V = P*t / m*c

P = 10 kW = 10 x 10^3
t = 3 days = 3 day * 24 hours * 60 mins * 60 second = 259,200 sec
c = 3.0 x 10^8 m/s
m = 1.3 x 10^3 kg

then just enter into calculator and solve it.

Question 2:
Radiation from the Sun reaching Earth, just outside the atmosphere, has an intensity of 1.7 kW/m^2, a) Assuming that Earth and its atmosphere behaves like a flat disk perpendicular to the Sun's rays and that all the incident energy is absorbed, calculate the force on Earth due to radiation pressure, b) For comparison, calculate the force due to the Sun's gravitational attraction.

like i said above, if radiation absorbed, then it just one-forward direction so Pr = U/c. if it absorbs and deflects back out, then it back n forth, so pr = 2U/c.

a) calculate the force of radiation pressure, meaning the intensity of radiation pressure.

so Intensity = energy/time over area = power over area
(ie, I = (U/t) / A) = P/A)

it said it's flat disk, meaning it's a spherical shape object (thats what planet looks like).. therefore the equation for intensity is power/area.

I = P/A, I = P / (4*pie*r^2).

However the cross section area (the diameter, thus we don't need the radius, instead the diameter) of the sun's ray team is (pie*d^2)/4 ...

so I = P / (pie*d^2)/4.

just insert whatever in that value.

P = power
d = diameter which given by cosntant of 2*sigma

b) again, calculate the force of sun's gravitational attraction, which mean calculating the momentum of radiation pressure.

Pr = I/c ==> when you have the answer for part A, replace the I result and divided by speed of light c.

Extra Credit:

It has been proposed that a spaceship might be propelled in the solar system by radiation pressure, using a large sail made of foil. How large must the surface area of the sail be if the radiation force is to be equal in magnitude in the Sun's gravitational attraction? Assume that the mass of the ship + sail is 2500kg, that the sail is perfectly reflecting, and that the sail is orientated perpendicular to the Sun's rays. See Appendix C of textbook for needed data. (With a larger sail, the ship is continuously driven away from the Sun.)


I've tried to solve the first case, but the rest I haven't try yet.! >.<

i'm a little tired today... i guess i help you solve this EC problem tomorrow... when it's due?

anyway, this problem involved the universal gravitational force

rememberd Newton's universal gravitational force? the mass of two objects over the distance between them multiply by universal gravity constant G?

well in this problem it's just that... coool problem btw..

sorry ok. =p

This post has been edited by InitialDJay: Sep 23 2009, 12:37 AM

[MissHyun90K]

lol everything is fascinating to me.. even your curiosity is fascinating.

o so you haven't learn those yet... don't worry, you will soon. but since you'll study biology, it might not be a requirement to understand the quantum level.

as for Borh's model...there were a few underlining point of Borh's model which was inconsistent with the newly discover atomic structure.

for example, borh's first point: electron is traveling at certain discrete distance from the nucleus, defining by a circular "fixed orbit".

Borh-Sommerfeld illustration:


well, we know that electron does not moved in Borh's circular nor Sommerfeld's elliptical orbit, but instead at different angle in 3-dimensional structure, like a crystal structure in this illustration...



second point: an electron can only lost and gained energy during the "quantum jump", at illustrated in Borh's atom model above, from n=1, n=2, n=3, etc.. which interpreted when an electron jumps from cell 1 to cell n, it emitted and absorbed electromagnetic radiation (or energy) depending on the cell it was in.

because of the probabilistic nature that Max Born outlining of atomic particle and Weiner Heisenburg's uncertainty principle, we can only "guess" the position and momumtum of electron cell. hence, Borh's interpretation required an electron to be define at a fixed distance and exact cell in order for his total energy absorbing to work (ie E = hv, h is planck constant, v = velocity), and that is inconsistent with the angular momumtum since electron, in fact, absorbed and emitted whenever an external force acts upon it (ie, Einstein proposed a quanta packet called photon). this means only photon can act upon the electron allowed energy consumption to deviate from the total energy absorption.

anyway, there are other points of inconsistency. and we can't really use Borh's model in regard to anything with more than 2 electrons because his model applied only to hydrogen element (which contain only 1 electron, 1 nucleus).

think of Borh's model of atom like the solar system where planet (electron) orbits around the sun (nucleus). for its simplicitic purpose, any elements that have 1 electron (ie hydrogen element = 1 electron, 1 nucleus, prefectly describes electron's "plantary orbital motion").

but as you can see, many other elements in the periodic table have more than 1 electron, thus we required to used some new equation, such as schrodinger equation to calculate energy level cell.

it may sounds confusing to you now, but thats okay. one of the thing that makes quantum physics "cool" is it confused everyone.


----

Okay lets do the HW helps since i promised to help you. lol


ok i think you did a few thing wrong. this question is not asking for standard velocity (v = d/t), like a car moving with respect to speed and time.

remembered the total absorption of radiation momumtum is total energy / speed of light (pr = U/c). and if radiation (ie light) deflected back out, the radiation momumtum will be twice because it travels twice along the path (back n forth), thus absorption is pr = 2U/c. An easier to remember is if you go from point A to point B, then it's just one forward distance so pr=U/c, and if you go from point A to point B, back to Point A, then it becomes pr = 2U/c.

now, in this question, if the beam of light carried energy U drifting away from the spaceship, then it also has momentum p = U/c.

if P is the power of the laser beam, then the total energy with respect to t (in 3 days, converts to second ok?), is total energy = power * time (ie, U = Pt).

thus the radiation momentum (Pr = U /c) is just Pr = Pt/c.

and with m being the mass of spaceship, and Pr momentum, so calculating the velocity needed to be divided by the mass (ie, V = Pr/m).

okay sum it up, then we know that...

Pr = U/c, which is just Pr = Pt/c

Pr= radiation momentum
U = total energy
c = speed of light
P = Power
m = mass

then subsititute
V = Pr/m = (P*t)/c over m
V = P*t / m*c

P = 10 kW = 10 x 10^3
t = 3 days = 3 day * 24 hours * 60 mins * 60 second = 259,200 sec
c = 3.0 x 10^8 m/s
m = 1.3 x 10^3 kg

then just enter into calculator and solve it.



like i said above, if radiation absorbed, then it just one-forward direction so Pr = U/c. if it absorbs and deflects back out, then it back n forth, so pr = 2U/c.

a) calculate the force of radiation pressure, meaning the intensity of radiation pressure.

so Intensity = energy/time over area = power over area
(ie, I = (U/t) / A) = P/A)

it said it's flat disk, meaning it's a spherical shape object (thats what planet looks like).. therefore the equation for intensity is power/area.

I = P/A, I = P / (4*pie*r^2).

However the cross section area (the diameter, thus we don't need the radius, instead the diameter) of the sun's ray team is (pie*d^2)/4 ...

so I = P / (pie*d^2)/4.

just insert whatever in that value.

P = power
d = diameter which given by cosntant of 2*sigma

b) again, calculate the force of sun's gravitational attraction, which mean calculating the momentum of radiation pressure.

Pr = I/c ==> when you have the answer for part A, replace the I result and divided by speed of light c.




i'm a little tired today... i guess i help you solve this EC problem tomorrow... when it's due?

anyway, this problem involved the universal gravitational force

rememberd Newton's universal gravitational force? the mass of two objects over the distance between them multiply by universal gravity constant G?

well in this problem it's just that... coool problem btw..

sorry ok. =p

THANKS! XD
I still didn't get it. LOL
It's not your fault, Physics is not my thing at all. ><
Uhmm I'll try to do the HW problem with your calculation.
It's not due until next week. I have plenty of time ^_^
So THANK YOU VERY MUCH again for helping me!!!

[Kaosq]

E=mc²?

[InitialDJay]

THANKS! XD
I still didn't get it. LOL
It's not your fault, Physics is not my thing at all. ><
Uhmm I'll try to do the HW problem with your calculation.
It's not due until next week. I have plenty of time ^_^
So THANK YOU VERY MUCH again for helping me!!!

lol. actually after looking back, i wasn't very clear so it's reasonable that you didn't understand.

i was a little tired so i explained in an unclear way that i don't think any person would understand, including myself. lol

let me try to explain again in an easier, less complicated way of Borh's atomic model inconsistency and inaccuracy compare to the fully developed theory of quantum physics, particularly electron behavior inside the atom.

it was Borh who proposed the theory of atom, and he assumed an electron was roaming around the nucleus at fixed distance. this vision is the same as earth orbiting around the sun.

given that it was an assumption made by Neil Borh, it's nothing more than observational prediction, a scientific hypothesis, and an estimated representation of atomic system.

however as technology improved, experiment verified to be incorrect. though, given the simplicity and less complexity of Borh's model, we are taught at beginning level to visualize electron orbits around the nucleus in closed loop.

the correct model is, of course, much more complicated and requires different intepretation depending on the quantum problem you're working on. for example, heisenburg's matrix, schrodinger equation, and feymann's path integral forumulation are used specifically for calculating quantum amplitude, atomic motion, and molecular diffusion.

perhaps, that's the one main reason why hs students aren't taught the proper method due to the complex mathematical , the degree of randomness, and the quantum uncertainty of the atom.

i was taught the same way you are, except i knew ahead of time that it wasn't right due to reading about it (i had the tendency to read ahead) but back in hs, i didn't want to challenge my hs teacher because 1) i don't understand it yet, 2) he's a teacher, so he knows what he's going.

however due to the success of Borh's model which produced a very similar result to Balmer's formula for visible wavelength (ie, light), it was brilliant, although inaccurate because it's applicable to atom with 1 electron and 1 nucleus such as hydrogen element.

for comparison:

Balmer devised his formula for light emission and absorption with four particular wavelengths that we see often (red, blue, green, purple):
1/lamda = R(1/2^2 - 1/n^2) for n= 3, 4, 5, etc..

Borh derived a similar formula using his atomic model:
1/lambda = R(1/n^2 - 1/n^2) for n = 2, 3, 4, etc..

lambda = 1 oscillating wavelength per frequency.
R = radius constant of atom (assumption)
n = energy cell level

looking at Borh's equation, there's not much different except, we have to know exactly where electron location is (that is in cell n =2), for his equation to work.



an analogy in classical physics...people think of electron like planet earth, nucleus like the sun, the whole solar system like an atom.

using Borh's model, earth moves around the sun in 2-dimensional space, either left/right, up/down, but no height/depth, exactly in the same circular orbit, at precise time.

with these assumptions in mind, if earth orbits in finite path (in reality, it does so don't get confused, macro physics --what we experience directly and micro physics--what we experience indirectly are completely different), you can easily pick out earth from the solar system when you know the position and momentum.

well, not really. here's why... in quantum interpretation of reality, when you know the position, then momentum changes, and vise versa. if you try to find both at the same time, wave-function collasped. this experiment often known as double-slit experiment. in addition, electron appears randomly inside the atom, instead of going in circle with precision.

generally speaking, Borh's model is not applicable to higher physics. it is too simple when newly discovered showed atomic structure is much more complex, not in circular orbit nor calculating the momumtum and position make sense.. there is a course in Quantum Electrodynamics which will teach you a completely different physics if you're interesting.

anyway do you get it now? and do you know why we need to know this?

there are people who can be very good in physics without being good at mathematics, but not the other way around.

oftentimes, people who do physics, they know the mathematical framework to solve problems but doesn't know why they're doing it.

asians, for instance, are notable example. we're good at memorizing the mathematical framework in solving a quantum problem but doesn't know why we do it because of our lack in conceptual talent to visualize the big picture.

to me, appreciative of the concept is more important than finding the result. i recommend you do the same, whether your major is biology or physics.

however in reality whenever you do something, you have to ask, what's the point of doing this or that? it's the one fundamental question that concerns society the most.

any scientists who got fund into a research should explained why or how some real-world phenomenon happens AND what's the usefulness of knowing it.

to give you an idea the importance of knowing what i just said (sorry for lecturing you ).

with these information of Borh's atomic model combined with Einstein's equation E=mc^2, human can produced the first nuclear bomb, and numerous discoveries in chemistry and biology that can be studied for medical treatment (for example, Quantum Electrodynamics raised the idea of anti-matter which then used in medical science to scan a copy of human-biological system because we're made of matter; it then allowed doctors to see the atomic structure in human body, and treat biological virus effectively.

furthermore, observing and learning quantum physics, such as photonic light and electron behavior, is a huge technological leap in quantum computing, like nanotechnology whose Richard Feynman (my 2nd favorite physicists of all-time) was a pioneer.

the reason i'm telling this is to encourage and motivate you to continue studying biology, one of classic field of science, as you can make a difference in the world. =p

a) calculate the force of radiation pressure, meaning the intensity of radiation pressure.

so Intensity = energy/time over area = power over area
(ie, I = (U/t) / A) = P/A)

it said it's flat disk, meaning it's a spherical shape object (thats what planet looks like).. therefore the equation for intensity is power/area.

I = P/A, I = P / (4*pie*r^2).

However the cross section area (the diameter, thus we don't need the radius, instead the diameter) of the sun's ray team is (pie*d^2)/4 ...

so I = P / (pie*d^2)/4.

just insert whatever in that value.

P = power
d = diameter which given by cosntant of 2*sigma

b) again, calculate the force of sun's gravitational attraction, which mean calculating the momentum of radiation pressure.

Pr = I/c ==> when you have the answer for part A, replace the I result and divided by speed of light c.

lol also, sorry, i told you i was tired so i try to solve a little too quick.

THIS IS WRONG. so don't turn it in....yet

----
Question 2:
Radiation from the Sun reaching Earth, just outside the atmosphere, has an intensity of 1.7 kW/m^2, a) Assuming that Earth and its atmosphere behaves like a flat disk perpendicular to the Sun's rays and that all the incident energy is absorbed, calculate the force on Earth due to radiation pressure, b) For comparison, calculate the force due to the Sun's gravitational attraction.
----
a)
okay, the radiation momemtum we know is
Pr = Intensity over speed of light (Pr = I/c)

now, to calculate the force of radiation pressure...

this equation is given by F = Radiation Pressure * Area. (ie, F = Pr & A)
because it said flat-disk (implies circle), so the area is = pie*R^2

so,
F = I/c * A
F = I/c * pie*R^2
F = (I * pie * R^2) / c

I = intensity of radiation = 1.7 x 10^3 W/m^2 ==> i used this because 1 K = 10^3 power.
R = the radius of earth = it was something in 10^6 but i don't remember exactly ==> check the web for earth's radius
pie = 3.14
c = speed of light = 3.0 x 10^8 m/s

for forgot for part B.. lol

b) earth gravitation attraction meaning comparing the force of Sun's gravity that is pulling on earth.

this is the univseral gravity that newton demonstrated

F = G (M * m) / d^2

G = gravitational constant = 6.67 x 10^-11 Newton/meter per second
M = mass of sun
m = mass of earth
d = distance between them

you can find all these constant on the web...

for the EC problem..
-----
Extra Credit:

It has been proposed that a spaceship might be propelled in the solar system by radiation pressure, using a large sail made of foil. How large must the surface area of the sail be if the radiation force is to be equal in magnitude in the Sun's gravitational attraction? Assume that the mass of the ship + sail is 2500kg, that the sail is perfectly reflecting, and that the sail is orientated perpendicular to the Sun's rays. See Appendix C of textbook for needed data. (With a larger sail, the ship is continuously driven away from the Sun.)
-----

what he's asking you is to find the surface A with respect to the force of radiation pressure equals the sun's gravitational attraction, in other words, the pull of sun's gravity on the spaceship.

sun's gravitational attraction => F1 = G (M * m) / d^2
force of radiation pressure = F2 = 2IA/c ==> the reason for 2IA/c, instead of IA/c is becaused light hits the spaceship and deflects back.. go twice as much along the path.

so set them equal to each other, and use algebra to find Area...

F1 = F2

G (M * m) / d^2 = 2IA/c

M = sun's mass
m = spaceship mass
I = Sun intensity
d = distance of sun to earth



This post has been edited by InitialDJay: Sep 26 2009, 12:04 AM

[MissHyun90K]

Thanks for everything. ^_^
I didn't completely understand because particle concept is very confusing to me. >.<
In class, my teacher had never taught us anything about the uncertanty principle or that you mentioned.
However, I have learned Newton's force, dipole, magnetism, acceleration force, so on.
What I don't get is if we can't tell what atom position and all that, how does all those people can find the result?? Do you get what I mean?
Sorrie >.< my physics knowledge is not very good.
Currently in class lab, we did this experiment where we find the magnetic field in bar wire and direction of M-field.
Have you do that when you're in HS?
But thank you for your help, I was able to learn a lot. XD
Next time if I need help, can I bug you? Do you have msn? XD lols

[InitialDJay]

Thanks for everything. ^_^
I didn't completely understand because particle concept is very confusing to me. >.<
In class, my teacher had never taught us anything about the uncertanty principle or that you mentioned.
However, I have learned Newton's force, dipole, magnetism, acceleration force, so on.
What I don't get is if we can't tell what atom position and all that, how does all those people can find the result?? Do you get what I mean?
Sorrie >.< my physics knowledge is not very good.
Currently in class lab, we did this experiment where we find the magnetic field in bar wire and direction of M-field.
Have you do that when you're in HS?
But thank you for your help, I was able to learn a lot. XD
Next time if I need help, can I bug you? Do you have msn? XD lols

lol you are a slow sluggish snail!!! =p jk perhaps i should have explained more clearly by giving you a history lesson on quantum physics. =p

a particle concept is similar to a pixel you see on the computer screen; only difference is particle has variaty of property that distinguished from one another by its charge, spin, mass, and lifespan.

similar wise, an image compressed into thousand, if not millions or even billions of individual pixels, to form a digital image.

maybe that will help you visualize what's particle better?

anyway, it's a difficult concept to explain because not many people can comprehend the eccentricity of expediency since quantum phenomenon that light quanta experience defied common-sense in everyday reality. we see things as our eyes see.

it is one of the most heated debate in the 20th century, the nature of light, a primary characteristics of atom.

though, no one ever saw an atom directly before. it's only an imaginary diagram theoretical scientists used to develop their atomic theory.

anyhow to introduce you to an abstract concept of quantum physics, the study of infinitesimal molecule, atom, or subatomic particles, we must rewind back to the historic debate between Issac Newton (whose favored light property as particle) and Christian Huygens (whose favored light property as wave).

this kick started the scientific literacy about light property and splitted the scientific community back then into two groups: particle vs wave. most people favored Newton because he's Newton, therefore, people assumed light property was particle.

then in mid 1800s, when James Clerk Maxwell discovered light was propagating electromagnetic wave in his theory of electrodynamics that encompassed the unifying of electricity and magnetism, it contradicted classical theory of light as particles. so is light really wave or particle?

evidently, this revived an intense debate in the early 1900s when Max Planck, the first founder of quantum field theory, re-defined classical theory of light in his proposition that light-wave consisted of minuscule single packet of energy called quanta, in which he concluded the size of his proposed quanta constant denoted as "h". (ie, "h" = Planck constant, size of quantum dot).

this replicated the theory Issac Newton previously proposed that light was particle.

later, Albert Einstein derived similar conclusion of particle-like nature in the discovery of photoelectric effect, with Neil Borh and Somerfield further investigated into this quanta matter and revealed that it made of atom with neutron, proton, and electron.

soon later, Somerfield and Borh developed their model of hydrogen atom illustrating electron in circular/elliptical orbit to signify discrete atomic mechanism, although it produced inaccurate result for multiple-electrons-protons model.

thus we say the initial development of quantum physics is a byproduct or based on Newtonian classical mechanics, so the classical premise doesn't stand the test of time when Weiner Heisenberg formulating his uncertainty principle.

and along with other 20th century newcomers of quantum mechanical Nobel laureates, Edwin Schrodinger, Max Born, Paul Dirac, and Louis de Broglie have worked on and describe light had both wave and particle property alike.

this particle-wave interpretation is a result of the double-slit experiment, which demonstrated individual subatomic particle can distributed simultaneously at different position known as superposition, that led to producing interference pattern despite single photonic light was conducting in the experiment, and that can only be occurred if light behaved as wave.

the tricky part is whenever an observer tried to test if light really behaves as wave, then the observer effect totally disturbed the wave interference pattern, and light pattern resumed back as particle.

this led to a few mind-blogging debate about the philosophical implication about reality we lived in, however most agreed that the experiment confirmed the fundamental of particle-wave duality, that governing how molecular and microscopic stuffs behaved at quantum scale.

the realistic application, that affects how technology improved by the dual nature of particle-wave, led to brilliant invention for electrical circuit, nano chip, and optical fiber we used in modern electronic device and transmittal communication.

anyhow, i'm more inclined to believe in particle-wave complementary as opposed to duality but that's another discussion. =p

anyway continue... this uncertainty interpretation implies random result, which doesn't flow well with Einstein, Borh, and all those old school of thought from classical physics. nonetheless, the Heisenberg uncertainty principle is flawless in nature.

noticed, at this point, we're talking about wave even though we know light also has particle-like nature. light, in small packet is quanta, travels in space as wave.

and wave has infinite wavelength, meaning light goes on forever. though, wavelength can sketched out, but forever moving forward in space.

so if wavelength moves infinitely, therefore finding the position, or tracking certain point/location of light movement is impossible. thus if no initial states of position can be traced, it implies uncertainty.

consequently Heisenberg's mathematical formula for uncertainty (X*P > h/2), position and momentum always greater than Planck constant divided by whatever real value. also experimental analysis verified repeatedly that he's correct, quantum mechanics is all about random chance, and end the longtime debate about behavior and property of light.

here is a simple way to look at what the uncertainty principle actually means...

assuming that you can pinpoint an electron at some position... in order to know where the position is, wouldn't you agree that you have to shine light into the spot? without light, how can we see?

however, anytime an experimenter or observer shoots a proton (ie, light) to interact with an electron at that position, the momentum of electron changes.

in other words, whenever you take a beam of light and shine onto the spot at that particular electron location, it interferes and kicks an electron to some random position, and thus its speed changes.

as a result, the relationship with respect to position (X) and momentum (P), or i'll use speed for better explanation, of particles, depend on one another, so the transitional relationship devised as X * P.

a significant key to this standard deviation relies on that X * P MUST BE greater than the sum of "h" (Planck constant), otherwise the wave-function breaks down. that's why you cannot find X & P at the same time because they are dependable on each other, and trying to measure both will caused the result to be less than Planck constant as it takes longer to measure both position and momentum due to indeterminacy; hence, measurement collapsed producing garbage answer.

the mathematical statement of indeterminacy results under the rule of quantum mechanics, and abide within principle of uncertainty.

due to the quantum indeterminacy, position and momentum of particles depend on the probabilistic states and quantum symmetry.

asking the question of "how can we measure the location and speed of particle are meaningless because indeterminacy of the quantum states; instead the question should be how do we approximate them?"

approximation involved guessing, and measure implies precise calculating. there is a major difference in the thought principle here and make sure you know the distinction.

in this case, Schrodinger equation is used to solve statistical probability of quantum symmetrical problem by approximating the highest probability of one particle location, speed, and amplitude by taking random guess where one is positioned, and you can do that by squaring the wave-function.

what do i mean by wave-function? it is a complex mathematical conjugation of vector space V = (x1, x2, x3) that corresponding to space, position, and time. (ie, V^2 = highest probability, basis of Schrodinger equation)

in simple mathematical demonstration, one represents the function f(x) = x and let x be any arbitrary number of certain symmetrical position.

for instance, in a thought experiment, a ball represents any subatomic particles (electron, proton, positron, photon). if one place a ball at position A and another at Point B, and by symmetrical scale, calculating the distance of two different positions in quantum states, you will need to square up the function f(x)^2 = x = squareroot(x), instead of adding and subtracting the function f(x) = x -+ x.

likewise, the same can be performed on momentum P.

in addition, a foremost subfield of quantum mechanics is quantum electrodynamics (QED), and this particular subject introduces a whole kind of bizarre physics and has not integrated into QM.

under QED, we approach an multifaceted mathematical glimpse into quantum quantification and require a guy named Richard Feynman to interpret random weirdness of subatomic matter in a way that even today nobody fully understand and are still confused and baffled.

Feynman once said something like "if you don't like how nature works here, you can go somewhere else (in other universe) when nature is less complicated, and nature works the way you want it". --this quote reflects unpredictability about nature in general.

actually now i felt i confused you even more, so it is okay that you don't get anything. I hope that i make some sense to you. lol

but if you do, you already learn over 90% of quantum physics just by knowing what uncertainty is.

the motive and logic behind unlocking the mystery in nature at the smallest scale is light because light is energy, substance to our survival perhaps? and also because information/property of light never decomposed.

we are like an amateur kid getting trap inside the jungle and trying to figure out the symmetry/pattern in order to escape.

and, how long has the scientific revolution begin? maybe only 400 years when human civilization had been around for at least 5,000.

that's a short timeline, but i have great hope for science when the LHC operating again and we can test some modern theories (most notably particle physics of standard model, as well as a few minor insinuations in string theory).

one of the hypotheses i anticipated in LHC experiment is the finding of "extra dimension".

theoretically speaking, string theory is the leading candidate for theory of quantum gravity, and quantum gravity is an esoteric force and holds the key to unify all forces under one called the "superforce".

however, for string theory to get scientific credibility, extra dimension needed to proven with falsifiability, both experimentally and observationally. it'll be interesting to find out, but i doubt we find extra dimension because it sounds like coming out of science fiction. lol

if you need physics help, i can help you, just post question here or pm me. i check once in a while since i'm more busy now. i don't use msn much, sorry, you won't find me there.

i'll help you seeing that i think it is essential to exchange scientific information to fellow member, that is if you care and read what i wrote. =p but umm it's fun that you enjoy this.

also currently, i'm researching on the problem of universal symmetry such as galactic formation and the balance of electromagnetic strength.

what i mean is how some galaxies are in confinement shape like spiral, elliptic, and sphere, and if it has anything to do with symmetry rather than gravitational effect, which appears like an elusive force.

and how electricity and magnetism have two balancing negative and positive charge, north & south pole, strong & weak spin, and what invisible force applying that makes them counter-interact each other.

i think i'm going crazy if i continue to talk about this to you. lol

have a good day and good luck with class.

This post has been edited by InitialDJay: Oct 10 2009, 01:03 AM

[Littlegirl]

THANK YOU SO MUCH FOR HELPING!!! ^_^ I will print it out and study from your method. XD
Anyway if you don't mind me asking, are you a teacher? >.<
It saves me so much time cuz your method is completely different from mine.
Mine was wrong, I kept getting different answer than the given one. >.<

This post has been edited by Littlegirl: Jul 1 2010, 01:05 AM