## Sapling Week 2,3,4 Q 9

Kayla Booker 1C
Posts: 40
Joined: Wed Sep 30, 2020 9:43 pm

### Sapling Week 2,3,4 Q 9

As you may well know, placing metal objects inside a microwave oven can generate sparks. Two of your friends are arguing over the cause of the sparking, with one stating that the microwaves "herd" electrons into "pointy" areas of the metal object, from which the electrons jump from one part of the object to another. The other friend says that the sparks are caused by the photoelectric effect. Prove or disprove the latter idea using basic physics.

Suppose the typical work function of the metal is roughly 4.810×10−19 J.
Calculate the maximum wavelength in angstroms of the radiation that will eject electrons from the metal.

I know that I need to find Energy of Photons in order to find the wavelength, but I'm not sure how to do that with just the work function. Can someone please walk me through this?

Posts: 49
Joined: Wed Sep 30, 2020 9:40 pm

### Re: Sapling Week 2,3,4 Q 9

Since the problem asks for the maximum wavelength that will reject electrons, KE=0, so the work function is equal to the energy of the photon. Therefore, you can set 4.810 x 10^-19 J equal to hv. Since we want to find wavelength, you can make v=c/lambda, making the equation 4.810 x 10^-19 J=hc/lambda. From there, you can find the wavelength by solving for lambda, and then convert to angstroms.

Rob Tsai 2F
Posts: 41
Joined: Wed Sep 30, 2020 10:09 pm

### Re: Sapling Week 2,3,4 Q 9

Akshata Kapadne 1C wrote:Since the problem asks for the maximum wavelength that will reject electrons, KE=0, so the work function is equal to the energy of the photon. Therefore, you can set 4.810 x 10^-19 J equal to hv. Since we want to find wavelength, you can make v=c/lambda, making the equation 4.810 x 10^-19 J=hc/lambda. From there, you can find the wavelength by solving for lambda, and then convert to angstroms.

I am currently having the same problem solving this particular question. I've solved the equation in multiple ways (1. Plugging into p=E/c, then plugging p into lambda=h/p. 2. Plugging my E(photon) value into v=E/h and then plugging into lambda=c/v), and I get the same answer no matter how I do it. I think I'm making some kind of conversion error when I'm converting to angstroms, but I can't figure out exactly what I'm doing wrong. I get a value x 10^-7, so I'm just moving my decimal place three to the left. Can someone help me figure this out? My work function value is 3.250x10^-19 J. Thanks!

Brianne Conway 1H
Posts: 41
Joined: Wed Sep 30, 2020 9:36 pm

### Re: Sapling Week 2,3,4 Q 9

Rob Tsai 2F wrote:
Akshata Kapadne 1C wrote:Since the problem asks for the maximum wavelength that will reject electrons, KE=0, so the work function is equal to the energy of the photon. Therefore, you can set 4.810 x 10^-19 J equal to hv. Since we want to find wavelength, you can make v=c/lambda, making the equation 4.810 x 10^-19 J=hc/lambda. From there, you can find the wavelength by solving for lambda, and then convert to angstroms.

I am currently having the same problem solving this particular question. I've solved the equation in multiple ways (1. Plugging into p=E/c, then plugging p into lambda=h/p. 2. Plugging my E(photon) value into v=E/h and then plugging into lambda=c/v), and I get the same answer no matter how I do it. I think I'm making some kind of conversion error when I'm converting to angstroms, but I can't figure out exactly what I'm doing wrong. I get a value x 10^-7, so I'm just moving my decimal place three to the left. Can someone help me figure this out? My work function value is 3.250x10^-19 J. Thanks!

Your method seems to be correct, but if you're getting a value x 10^-7, you should be moving the decimal place three to the right to convert to angstroms.

Rob Tsai 2F
Posts: 41
Joined: Wed Sep 30, 2020 10:09 pm

### Re: Sapling Week 2,3,4 Q 9

Brianne Conway 1H wrote:
Rob Tsai 2F wrote:
Akshata Kapadne 1C wrote:Since the problem asks for the maximum wavelength that will reject electrons, KE=0, so the work function is equal to the energy of the photon. Therefore, you can set 4.810 x 10^-19 J equal to hv. Since we want to find wavelength, you can make v=c/lambda, making the equation 4.810 x 10^-19 J=hc/lambda. From there, you can find the wavelength by solving for lambda, and then convert to angstroms.

I am currently having the same problem solving this particular question. I've solved the equation in multiple ways (1. Plugging into p=E/c, then plugging p into lambda=h/p. 2. Plugging my E(photon) value into v=E/h and then plugging into lambda=c/v), and I get the same answer no matter how I do it. I think I'm making some kind of conversion error when I'm converting to angstroms, but I can't figure out exactly what I'm doing wrong. I get a value x 10^-7, so I'm just moving my decimal place three to the left. Can someone help me figure this out? My work function value is 3.250x10^-19 J. Thanks!

Your method seems to be correct, but if you're getting a value x 10^-7, you should be moving the decimal place three to the right to convert to angstroms.

It's that late night brain freeze. You're totally right, because I'm making my answer into a smaller value. Thank you!