## When to Use De Broglie

$\lambda=\frac{h}{p}$

David Chibukhchian 2G
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Joined: Wed Sep 30, 2020 9:50 pm

### When to Use De Broglie

Hey guys, I had a question about when to use the De Broglie equation. When you're solving a photoelectric effect problem and the wavelength of an ejected electron is given, do you still have to use the De Broglie equation rather than the E=ch/(wavelength) equation? I think the latter is only for photons and nothing else. So does that mean that even when it's a photoelectric effect problem, the ejected electron's wavelength is found with De Broglie? I would just get confused because I'd always associate this type of problem with the E=ch/(wavelength) equation. Any info would be really helpful!

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### Re: When to Use De Broglie

If given the wavelength of an ejected electron, you would use the DeBroglie equation because the electron has mass. You could figure out the velocity and then from there figure out the kinetic energy and relate that back to the incident light, depending on what the question asks for. You are correct that E=ch/(wavelength) is only used for photons.

Hope this helps!

JonathanSung_2G
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### Re: When to Use De Broglie

Yup, just always associate DeBroglie's with electrons since they have mass and light does not. Typically when you also see an electron's velocity in the problem you will most likely have to use DeBroglie's wave equation at some point as well.

Julianna Shimabukuro 1D
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### Re: When to Use De Broglie

As previously stated by the other commenters, you use de Broglie with electrons since electrons have a mass while photons do not use de Broglie as they have no mass. Instead, you would use e = hc/lambda for photons, which is E = hv and c = lambda(v) combined. In c = lambda(v), v is isolated to give v = c/lambda, and this is plugged in for v in E = hv. Hope this helps!

Xavier Herrera 3H
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### Re: When to Use De Broglie

You only use E=hc/lambda for finding the energy of photons, since this equation doesn't have mass. Something helpful for me to remember is that whenever there is just an E in our equation sheet (not Ek or En), the equation is for photons. To find the energy of an electron or something else with mass, you would have to use Ek=1/2mv^2.

Emerald Wong 1B
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### Re: When to Use De Broglie

You would use De Broglie for electrons, since electrons have mass, while photons do not. You are correct in that E=hv is for photons.

Jessica Manriquez 1H
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### Re: When to Use De Broglie

I associate De Broglie with velocity, so if I see a question asking for the wavelength of an electron going at a certain speed, I immediately think of De Broglie; Hope this helped :)

Sam_Marasigan_3D
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### Re: When to Use De Broglie

You use the de Broglie wave equation when the problem involves mass (m) and/or velocity (v).

Kiara Phillips 3L
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### Re: When to Use De Broglie

If you are dealing with mass or velocity this formula is the best.

Jordan_OBrien_2k
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Joined: Wed Sep 30, 2020 9:43 pm

### Re: When to Use De Broglie

Usually when I see a question that indicates a velocity or speed of an electron, I know to use the de broglie equation. It is also helpful to know that electrons have a mass and photons do not, so you know that you can use m*v to find the momentum and apply this to the de broglie equation.

John_Tran_3J
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Joined: Wed Sep 30, 2020 9:58 pm

### Re: When to Use De Broglie

There are a couple ways of using DeBroigle's euqation, first if you are given a wavelength and are asked to calculate the velocity of an ejected electron. Second if you are given the velocity and asked to calculate the wavelength of an ejected electron.

Tanner Bartyczak 1K
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### Re: When to Use De Broglie

Like many have said before, to figure out either the wavelength, speed, or mass of a photon when given two of the other values.

Yijia_Yang_3A
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Joined: Wed Sep 30, 2020 9:39 pm

### Re: When to Use De Broglie

When you are trying to figure out wavelength, mass or speed, De Broglie is a useful option

Ethan Goode 2H
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### Re: When to Use De Broglie

De Broglie is good to get mass, velocity, momentum, or wavelength when any of those are missing for a photon or wave.

Zainab Jamali 1H
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Joined: Wed Sep 30, 2020 9:54 pm

### Re: When to Use De Broglie

The simplest way to think about it is that E = hv and c = v(lambda) applies to electromagnetic radiation and deBroglie's equation applies to particles with mass, including all the subatomic particles.

305572629
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### Re: When to Use De Broglie

Make sure to be familiar with the De Broglie wavelength equation and the other equations for frequency and wavelength using the speed of light and Planck's constant so that you can interchange them easily.

Jack Kettering 3D
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### Re: When to Use De Broglie

Yes when the question is referring to an electron you use the De Broglie Equation because the electron has mass and is not traveling at the speed of light like a photon would be doing so you cannot use the equation with the constant c.