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### AuthorTopic: math(s) thread  (Read 7231 times)

#### SilentLoner

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•  « Reply #45 on: January 30, 2012, 06:33:13 pm »

That's a horrible idea, and I'm disappointed in you for coming up with such an idea in the first place. Logged

#### Dagnarok

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• • Commander of the USP Talon « Reply #46 on: January 30, 2012, 06:36:48 pm »

It's your funeral.  Logged
Quote from: Tilyami
Shuu is like the monkey's paw if the monkey's paw was a partridge. #### Benxamix2

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• • INACTIVE « Reply #47 on: January 30, 2012, 06:37:42 pm »

* Benxamix2 cries Logged

#### Neko Desu

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•  « Reply #48 on: January 30, 2012, 11:29:15 pm »

By the way, I've been stuck on this physics question for a while now.

When light of wavelength 2.08 * 10^-7 m falls on a photosurface, a voltage of 1.40 V is required to stop the emitted electrons from reaching the anode. What is the largest wavelength of light that will result in the emission of electrons from this photosurface?

I was thinking since V = E/Q then E = 1.40 * 1.6 * 10-19 which gives E = 2.24 * 10^-19 J, then sub in this value for E = hfc and since h is just planck's constant then you just solve for the critical frequency. Then you use λ = c / f where 'c' is the speed of light and 'f' is the critical frequency, resulting in a wavelength of 8.9 * 10^-7 m, but the answer in my textbook says it's 2.7 * 10^-7 m

halp Logged

#### Kwiftee

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• That guy who's always ranting about stuff. « Reply #49 on: January 31, 2012, 01:49:54 am »

By the way, I've been stuck on this physics question for a while now.

When light of wavelength 2.08 * 10^-7 m falls on a photosurface, a voltage of 1.40 V is required to stop the emitted electrons from reaching the anode. What is the largest wavelength of light that will result in the emission of electrons from this photosurface?

I was thinking since V = E/Q then E = 1.40 * 1.6 * 10-19 which gives E = 2.24 * 10^-19 J, then sub in this value for E = hfc and since h is just planck's constant then you just solve for the critical frequency. Then you use λ = c / f where 'c' is the speed of light and 'f' is the critical frequency, resulting in a wavelength of 8.9 * 10^-7 m, but the answer in my textbook says it's 2.7 * 10^-7 m

halp

Well, we know that the stopping voltage is 1.40V. This corresponds to an energy of 2.24*10^-19 J, as you said. That energy represents the maximum possible kinetic energy that an emitted photoelectron can have, when light of the given wavelength is used.

Now, we use EK max = hf - W, in order to get the work function. Or, in this case, we use E = hc/λ -W
2.24*10^-19 = (6.63*10^-34) * (3.0*10^8) / (2.08*10^-7)   -  W.
solving for W gives W = 7.3225*10^-19 J.

Now, we want the cutoff wavelength; the point at which EK max = 0. (ie. electrons aren't really emitted at all.) Hence, hc/λ - W = 0
W = hc/λ
λ = hc/W

so, λ = (6.63*10^-34) * (3.0*10^8) / (7.3225*10^-19)
which is 2.7*10^-7 m.

The mistake you were making is that the energy you had found was NOT the work function. Rather, it was the amount of energy that was left over once you had subtracted the work function from the energy of the given photons.
If that doesn't make sense I'll try and explain it again; these types of problems were always the most difficult we got in our physics exams. You had to make sure you were visualising what type of energy you were referring to correctly. Talking of which; if you know how to use electron volts as your units rather than joules, it makes it a fair bit simpler.
« Last Edit: January 31, 2012, 01:51:28 am by Kwiftee » Logged
zyxfrlpso
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#### Neko Desu

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•  « Reply #50 on: January 31, 2012, 01:54:08 am »

Thanks a lot! Logged

#### Kwiftee

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• That guy who's always ranting about stuff. « Reply #51 on: February 04, 2012, 07:50:42 pm »

for x > 0
x = 1 + 1 + 1 + ... + 1; } x times
x2 = x + x + x + ... + x; } x times

d/dx(LHS) = d/dx(x2) = 2x.

d/dx(RHS) = d/dx(x + x + x + ... + x); } x times
= d/dx(x) + d/dx(x) + d/dx(x) + ... d/dx(x); } x times
= 1 + 1 + 1 + ... + 1; } x times
= x.

but 2x ≠ x.
what Logged
zyxfrlpso
Quote from: Yuletideriz
My god, if I could shit gold and I give it to you, you would still bitch because it was technically crap.

#### Neko Desu

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