Energy of a Photon and Planck's Constant?

[Khuzy]

Hello, sorry if this question is out of correct forum or seems silly, I am relatively young (13) and decided to learn Physics but since I am self-teaching I am finding it hard to grasp certain concepts since I am self-teaching myself. When it came to Planck's Constant and Photons I got a bit confused. The book I am reading just put the equation e=hc/wavelength but didn't explain how to do the equation so I went online.

After going on http://www.pveducation.org/pvcdrom/2-properties-sunlight/energy-photon I entered all the numbers into my calculator, after doing 6.626x10^-34 (which is Planck's Constant) x speed of light, I got my answer to be 1.9864x10^-25. Then it said to to convert to eV (electronvolts) so I entered 1.9864x10^-25 x 1.602 x 10^-19 and the website says that the answer should be 1.24 x 10^-6 but my answer was quite different.

So now I am confused, I understand what a photon is (I think, massless and in all light rays?), but I am lost for finding its energy or what Planck's Constant is. Sorry if this question is out of place, I am just confused as I think I am taking the information of the book and that website wrong. It would be helpful if someone explained, thanks!

[Iwikefactz]

I suggest you use

 

(1.99*10^-25)*(6.242*10¹⁸)

 

as 1 joule = 6.242*10¹⁸

[imatfaal]

Why don't you spell out the calculation you are attempting rather than just hint at it - it will be good practice and may well mean you find the error yourself

[Iwikefactz]

Why don't you spell out the calculation you are attempting rather than just hint at it - it will be good practice and may well mean you find the error yourself

 

True. Notice how 1 eV = 1.602 x 10^-19 J, so would you multiply the joules with the joules in an eV or with the eV in a joule?

[swansont]

Hello, sorry if this question is out of correct forum or seems silly, I am relatively young (13) and decided to learn Physics but since I am self-teaching I am finding it hard to grasp certain concepts since I am self-teaching myself. When it came to Planck's Constant and Photons I got a bit confused. The book I am reading just put the equation e=hc/wavelength but didn't explain how to do the equation so I went online.

After going on http://www.pveducation.org/pvcdrom/2-properties-sunlight/energy-photon I entered all the numbers into my calculator, after doing 6.626x10^-34 (which is Planck's Constant) x speed of light, I got my answer to be 1.9864x10^-25. Then it said to to convert to eV (electronvolts) so I entered 1.9864x10^-25 x 1.602 x 10^-19 and the website says that the answer should be 1.24 x 10^-6 but my answer was quite different.

So now I am confused, I understand what a photon is (I think, massless and in all light rays?), but I am lost for finding its energy or what Planck's Constant is. Sorry if this question is out of place, I am just confused as I think I am taking the information of the book and that website wrong. It would be helpful if someone explained, thanks!

 

 

You haven't mentioned what the wavelength of the photon is, or that you included it in the calculation. From the answer, it should have been 1 meter.

 

Your math error is multiplying by 1.602 x 10^-19 when you should have divided by it, as has been mentioned. That's the number of Joules per eV, and you already had something with Joules as a unit.

[Sensei]

Hello, sorry if this question is out of correct forum or seems silly, I am relatively young (13) and decided to learn Physics but since I am self-teaching I am finding it hard to grasp certain concepts since I am self-teaching myself. When it came to Planck's Constant and Photons I got a bit confused. The book I am reading just put the equation e=hc/wavelength but didn't explain how to do the equation so I went online.

After going on http://www.pveducation.org/pvcdrom/2-properties-sunlight/energy-photon I entered all the numbers into my calculator, after doing 6.626x10^-34 (which is Planck's Constant) x speed of light, I got my answer to be 1.9864x10^-25.

It would be right for wavelength = 1m

 

Then it said to to convert to eV (electronvolts) so I entered 1.9864x10^-25 x 1.602 x 10^-19 and the website says that the answer should be 1.24 x 10^-6 but my answer was quite different.

Because you have to divide by e, not multiply..

 

1.9864x10^-25 / 1.602176565*10^-19 = ~1.23984201714482e-006

 

So now I am confused, I understand what a photon is (I think, massless and in all light rays?), but I am lost for finding its energy or what Planck's Constant is. Sorry if this question is out of place, I am just confused as I think I am taking the information of the book and that website wrong. It would be helpful if someone explained, thanks!

Simply, you used multiplication instead of division..

[Khuzy]

It would be right for wavelength = 1m

 

 

Because you have to divide by e, not multiply..

 

1.9864x10^-25 / 1.602176565*10^-19 = ~1.23984201714482e-006

 

 

Simply, you used multiplication instead of division..

Oh okay.

So to convert joules to electronvolts you just divide/1.602*10^-19??

Also, so far into the equation the only calculations I have made are Planck's Equation x Speed of Light, and now converted that number into electronvolts. If I am understanding correctly, you are suppose to divide 1.24e-006 by the wavelength to get the answer? After all the equation says e=hc/wavelength and I know that hc=1.24-006..

[Sensei]

Oh okay.

So to convert joules to electronvolts you just divide/1.602*10^-19??

That's correct.

 

m_e= ~ 9.11 * 10 ^{^ -31} kg

 

E=m_ec²

 

You have energy of electron in Joules.

 

Then divide it by e = 1.602176565*10^-19

 

[math]\frac{9.11*10^{-31}kg*299792458^2}{1.602176565*10^{-19}} = 510998.928 \frac{eV}{c^2}[/math]

 

You end up with energy of electron in eV,

and then mass of electron is eV/c².

 

Also, so far into the equation the only calculations I have made are Planck's Equation x Speed of Light,

You need to multiply by speed of light (m/s), then divide by wavelength (m).

 

and now converted that number into electronvolts. If I am understanding correctly, you are suppose to divide 1.24e-006 by the wavelength to get the answer? After all the equation says e=hc/wavelength and I know that hc=1.24-006..

 

electron Volt is energy unit used in quantum physics. It's quantum level equivalent of Joule at macroscopic energy unit.

 

6.62607004*10^-34 J = 4.135667*10^-15 eV

 

It's E=hc/wavelength (not e=hc/wavelength),

as you can easily mix E (energy) with e (elementary electric charge).

Edited October 20, 2016 by Sensei

[Khuzy]

That's correct.

 

m_e= ~ 9.11 * 10 ^{^ -31} kg

 

E=m_ec²

 

You have energy of electron in Joules.

 

Then divide it by e = 1.602176565*10^-19

 

[math]\frac{9.11*10^{-31}kg*299792458^2}{1.602176565*10^{-19}} = 510998.928 \frac{eV}{c^2}[/math]

 

You end up with energy of electron in eV,

and then mass of electron is eV/c².

 

You need to multiply by speed of light (m/s), then divide by wavelength (m).

 

 

electron Volt is energy unit used in quantum physics. It's quantum level equivalent of Joule at macroscopic energy unit.

 

6.62607004*10^-34 J = 4.135667*10^-15 eV

 

It's E=hc/wavelength (not e=hc/wavelength),

as you can easily mix E (energy) with e (elementary electric charge).

Sorry I got a bit confused, where did you get 9.11*10^-31 kg from?

And what does m_{e mean?}

And where do electrons fit into the equation? I thought E=hc/wavelength was to find energy of a photon, so what does that have to do with electrons?

Apologies if all my questions sound childish, I am new to physics and obviously not learning it very well.

[Sensei]

Sorry I got a bit confused, where did you get 9.11*10^-31 kg from?

And what does m_{e mean?}

Mass of electron.

Just example of how to convert mass in kg, to Joules, then to eV energy unit, then to eV/c^2 mass unit.

 

And where do electrons fit into the equation? I thought E=hc/wavelength was to find energy of a photon, so what does that have to do with electrons?

There is pair production.

High energy gamma photon with energy equal or higher than 1.022 MeV after collision with other particle can create pair of electron (matter) and positron (antimatter):

[math]\gamma + 1.022 MeV \rightarrow e^- + e^+[/math]

Mass of electron/positron is ~ 0.511 MeV/c^2.

So you see energy of photon = ( mass of electron + mass of positron ) * c^2.

If incoming photon has higher energy, then newly created electron and positron are accelerated.

 

 

Apologies if all my questions sound childish, I am new to physics and obviously not learning it very well.

No need to apology. Edited October 22, 2016 by Sensei

[Sriman Dutta]

Here's the relationship between joule and electron volt.

 

1 eV = charge of 1 electron X 1 V = 1.6 X 10^-19 C X 1 V = 1.6 X 10^-19 J

[imatfaal]

Here's the relationship between joule and electron volt.

 

1 eV = charge of 1 electron X 1 V = 1.6 X 10^-19 C X 1 V = 1.6 X 10^-19 J

 

To clarify the clarification

 

An electron volt is the energy change when a particle with the charge of an electron moves across a potential difference of one volt