- #1

bobie

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Energy is expressed in J or eV, but E= hf

As planck constant h is J.s is it possible to express the energy of a photon in h/s? If not, why?

Can we say that the energy of a photon is 2.41 x 10^14 h/s?

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- Thread starter bobie
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- #1

bobie

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Energy is expressed in J or eV, but E= hf

As planck constant h is J.s is it possible to express the energy of a photon in h/s? If not, why?

Can we say that the energy of a photon is 2.41 x 10^14 h/s?

- #2

Drakkith

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- #3

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Your proposal does not.

Your proposal is missing a unit of measure.

For some unit relationships, try

http://en.wikipedia.org/wiki/Planck_constant

- #4

bobie

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Your proposal is missing a unit of measure.

What unit is missing?

A photon with frequency 2.418 x 10^14 Hz has E = 1 eV, is that correct?

h = 4.135 x 10 ^-15 eV.s , h/s = 4,135 x 10^-15 eV

( 2.318x 10^14 x) h/s = (2.428x10^14 x) 4.135 x 10^-15 eV

2.318x 10^14 x h/s = 1

A photon with frequency 2.418 x 10^14 Hz has E = 2.418 x 10^14 h/s

Where do I go wrong?

- #5

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Can we say that the energy of a photon is 2.41 x 10^14 h/s?

'h' is a constant, right?

What does 's' mean in your equation?? Please explain.

- #6

DennisN

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Can we say that the energy of a photon is 2.41 x 10^14 h/s?

Be careful here, you are mixing a

You obviously mean

h: Planck's constant ≈ 6.626×10

s: seconds

but h as a

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- #7

bobie

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If I got it right, h = J.s cannot be modified to h/s =J.

But I read that the Hz is equivalent to 1/s, then in the formula E= h.f => E = h. 1[]/s = aren't we mixing a constant with a unit?

- #8

Bandersnatch

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f=1/T not [1/s]

[1/s] is the unit of f

So, E=hf and the units are h=[J*s] and f=[Hz]=[1/s]

Then E=[J*s*Hz]=[J*s/s]=[J]

- #9

bobie

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- #10

Bandersnatch

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You can substitute 1/T for f if you like, to get E=h/T, where T is the period of the wave.

- #11

bobie

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You can substitute 1/T for f if you like, to get E=h/T, where T is the period of the wave.

So , what is the E of a photon with frequency 2.418x 10^14 wxpressed in h/T?

- #12

Bandersnatch

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f=2.418*10^14 [Hz]

E=fh=2.418*10^14 [Hz]*6.626×10^-34 [J*s]=~16*10^-20 [J]

T=1/f

T=1/(2.418*10^14) [1/Hz]

T=~0.41*10^-14

E=h/T=6.626×10^-34 [J*s]/0.41*10-14

- #13

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bobie,

my post #5 was an attempt to get you to see for yourself the difference between a measure, such as a frequency, f, and the unit utilized to express it, such a cycles per second.

I think that's what the prior posters are explaining,too.

If you write out the UNITS associated with any measure, any formula, you will see whether you have consistency:

for example distance equals velocity times time, right?? :

a distance = ft/sec x seconds = ft and that makes 'sense'.....the seconds cancel.....if you have instead a measure of speed such as [50] miles/hour x [25] seconds your units become mile-second/ hours.....not a standard set of units....so much better to convert the right hand side to either hours, seconds, or whatever....

Note also the unit 'Hertz' [and seconds] has some confusion associated with it:

http://en.wikipedia.org/wiki/Cycle_per_second

[I wasn't even aware of this supposed confusion til I just checked. ]

- #14

jtbell

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how can we express that the scalar of the frequency is always equal to the scalar of the energy of a photon?

We could define a new unit of energy: 1 bobie = 4.135 x 10

Then Planck's constant would be 1 B.s, and a photon with frequency 2.418 x 10

- #15

bobie

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Congrats, jtbell, you hit the nail on the very head!, thanks.We could define a new unit of energy: 1 bobie = 4.135 x 10^{-15}eV = 6.626 x 10^{-34}J. Give it the symbol 'B'. (Or is there another unit with that symbol? I forget...)

Then Planck's constant would be 1 B.s, and a photon with frequency 2.418 x 10^{14}Hz would have an energy of 2.418 x 10^{14}B.

That's exactly what I meant without aspiring to have a unit named after me., that would make patent the relation of energy to a single oscillation (I posed a similar problem here :https://www.physicsforums.com/showthread.php?t=712514 , it seemed to me that dimensions prevent seeing the forest because of the trees. There I discovered geometrized units and that dimensions are not indispensable).

I have little (or no) experience and I still cannot see the subtle difference between 1B and 1 h/s.

Actually I cannot even grasp what a unit of energy can actually mean when

I understand that power (J/s ) is energy absorbed every second but J.s would correspond to distance ( as compared to velocity, right)?

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- #16

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J.s is the unit for action (and also angular momentum).

- #17

bobie

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Can 2 different entities have the same dimensions?, what has h in common with angular momentum?J.s isthe unit for action(and also angular momentum).

...if h indeed is a unit then it can be mixed with other units and we can use h/s?

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- #18

Bandersnatch

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It is most emphatically not. Nobody has ever said that. It's a constant and it's units are J*s.

h/s is mixing constants with units.

Write J*s/s if you must, although it's obviously just J, if you want to write units. Or write h/T if you want equations. Don't write h/s. It's like writing a=V/s or F=kg*a.

- #19

bobie

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Why do you blame, me, bandersnatch,did you see that I was simply quoting Mr dauto, ?Stop saying that h is a unit, ...Nobody has ever said that

how can I possibly know who is right or wrong?

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- #20

Bandersnatch

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Where, oh, where did he say **h** is a unit? All he said is that **J*s** is a unit.

- #21

bobie

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Sorry, if I misunderstood, but I thought that h=J.s? wiki:Where, oh, where did he sayhis a unit? All he said is thatJ*sis a unit.

h = 1.054571726(47)×10−34

. Can two different units have same dimensions?

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- #22

DennisN

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Why do you blame, me, bandersnatch,did you see that I was simply quoting Mr dauto, ?

how can I possibly know who is right or wrong?

You were not quoting dauto. Dauto said:

J.s is the unit for action (and also angular momentum).

which only was about the unit [Js]. But you said:

...if h indeed is a unit then it can be mixed with other units and we can use h/s?

No, please read my post #6 again. Planck's constant (h) is

It seems to me that you don't understand the difference between a quantity and a unit. I think the posters in this thread have tried to explain this in various ways. But I will try to visualize it in this basic way:

Planck's constant is (full expression)

h ≈ 6.62606957 × 10

h is a

6.62606957 × 10

Js is the

Is this clear? Do you understand that h is

- #23

bobie

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Thanks for your patience, I'll try to make myself clear.h is aquantity(a constant)

6.62606957 × 10^{−34}is thevalue

Js is theunit(joules times seconds)

Is this clear? Do you understand that h isnota unit in itself, and therefore it should not be a part of a unit expression?

T time is not a unit, is a dimension , sec is its unit

Length is not a unit , cm is its unit....

is that right?

if h is only a constant, a quantity , a value, therefore a dimensioneless number, why is it always associated to units such as

α is the fine structure constant and is just a quantity 0.007... but is never associated to units

That is what I do not understand, when I think of h I think of α, where do I go wrong?

- #24

DennisN

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Excellent, no problem! I think we are going somewhere now:Thanks for your patience, I'll try to make myself clear.

Yes.T time is not a unit, is a dimension , sec is its unit

Yes (though normally the unit is m (meter)).Length is not a unit , cm is its unit....

Yes!is that right?

if h is only a constant, a quantity , a value,therefore a dimensioneless number, why is it always associated to units such as J.s, dimensions?

This is where you go wrong (my bolding in your quote). h is

Planck's constant is (full expression) h ≈ 6.62606957 × 10

h is a

6.62606957 × 10

Js is the

A dimensionless quantity is a quantity without a unit. But the

- #25

bobie

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So, if I got it right some constants are dimensioneless and some have units. And we have to create a new unit B to express what I meant by h/s. What would then be the relation of h to B?h ≈ 6.62606957 × 10^{−34}Js

Moreover, you have used a new sign

h

and I am naive enough to think that you can always move around entities from left to right.

But the main problem remains that I have no clue what energy multiplied by time can represent.

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