E = hf to calculate the highest frequency level of radiation ...

This isThis is in the edexcel spec: Recognise and use the expression E = hf to calculate the highest frequency of radiation that could be emitted in a transition across a known energy band gap or between known energy levels .... no idea where to begin, help?

Reply 1

HighlandDancer

What are the values for both sides of the energy band gap?

Reply 2

Munrot07

18

Original post by LadyFireRose

This isThis is in the edexcel spec: Recognise and use the expression E = hf to calculate the highest frequency of radiation that could be emitted in a transition across a known energy band gap or between known energy levels .... no idea where to begin, help?

If you have, lets say, the energy levels of -1.62 x 10^-19J and -4.24 x 10^-19J you can work out the energy difference, 4.24 - 1.62 = 2.62 x 10^-19J

then use E = hf

f = E/h

f = (2.62 x 10^-19)/(6.63 x 10^-34)

and then you have the frequency :smile:

I hope that makes sense

Reply 3

krisshP

14

Original post by Munrot07

If you have, lets say, the energy levels of -1.62 x 10^-19J and -4.24 x 10^-19J you can work out the energy difference, 4.24 - 1.62 = 2.62 x 10^-19J

then use E = hf

f = E/h

f = (2.62 x 10^-19)/(6.63 x 10^-34)

and then you have the frequency :smile:

I hope that makes sense

Why are these numbers always given as negative numbers?

Reply 4

Munrot07

18

Original post by krisshP

Why are these numbers always given as negative numbers?

The electrons are bound in the atom, you need to give them energy for them to escape so -4.24, this means an electron needs to gain 4.24 :smile:

Reply 5

krisshP

14

Original post by Munrot07

The electrons are bound in the atom, you need to give them energy for them to escape so -4.24, this means an electron needs to gain 4.24 :smile:

No.

I am asking for like why the numbers are always negative? Why not positive?

Reply 6

Munrot07

18

Original post by krisshP

No.

I am asking for like why the numbers are always negative? Why not positive?

The concept here is repeated in the sections on gravitational and electric fields.
Consider an electron and the nucleus of an atom. They attract each other due to their opposite charge. As a result of this attraction the electron has potential energy, E_p.
As the electron moves away from the nucleus, this potential energy increases (in the same way as the potential energy of a stone increases as it moves away from the surface of the Earth). However, as the electron moves away from the nucleus, the increased distance makes the force due to the electrical attraction smaller. At infinity, the attraction between the two is zero. No attraction - no potential energy.
So at the biggest distance from the nucleus, where you would logically expect to have the biggest value of potential energy, you find that the E_p value is in fact zero. Doh!! That means that the biggest value possible is zero. Therefore, as you move back towards the nucleus and lose potential energy, you must be going below zero potential energy, i.e. you must have negative potential energy.
For this reason, all the electron energy levels in diagrams are given a negative value, showing that the potential energy at that point is less than it is at infinity.

(sorry it's hard to explain so copied for S-cool)

E = hf to calculate the highest frequency level of radiation ...

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