Page **1** of **1**

### Example 1.5

Posted: **Sun Apr 15, 2018 10:11 pm**

by **Sonia Aronson 1B**

(a) of this example asks you to find the kinetic energy of the ejected electron. The problem gives you the speed of an electrom emitted from the surface of potassium by a photon as 668 km/s.

Using the equation E=mv^2, how do you know that m is 9.109 x 10^-31kg?

Do you use the molar mass of potassium?

I'm just not sure how the book got that particular number, any help is appreciated!

### Re: Example 1.5

Posted: **Sun Apr 15, 2018 10:16 pm**

by **Chem_Mod**

The formula for kinetic energy is E = mv^{2}/2. m is the mass of the moving object (electron is being emitted; so electron mass) and v is the velocity of that moving object. mass of electron is the one you have stated.

### Re: Example 1.5

Posted: **Sun Apr 15, 2018 10:19 pm**

by **Emely Reyna 1F**

You should look at the constants worksheet that value is given.

### Re: Example 1.5

Posted: **Sun Apr 15, 2018 10:41 pm**

by **Miya Lopez 1I**

Emely Reyna 1F wrote:You should look at the constants worksheet that value is given.

I was confused about where the book got this number too. So for all electrons, their mass will always be the same?

### Re: Example 1.5

Posted: **Tue Apr 17, 2018 5:31 pm**

by **Megan Potter 1G**

Chem_Mod wrote:The formula for kinetic energy is E = mv^{2}/2. m is the mass of the moving object (electron is being emitted; so electron mass) and v is the velocity of that moving object. mass of electron is the one you have stated.

I'm still confused on this too. so is 9.109x10^-31 always going to be the mass of an electron? or does it vary?

### Re: Example 1.5 [ENDORSED]

Posted: **Tue Apr 17, 2018 7:29 pm**

by **Daniel Cho Section 1H**

Most likely, the mass of the electron will never change because it is a discovered fact of the mass until someone eventually debunks or improves the value. For now, we would be using this value as our mass of the electron. The mass would never vary.

I hope this helps. Please correct me in places where I might be wrong.

### Re: Example 1.5

Posted: **Wed Apr 18, 2018 8:14 pm**

by **Chem_Mod**

Megan Potter 1G wrote:

I'm still confused on this too. so is 9.109x10^-31 always going to be the mass of an electron? or does it vary?

Yes, mass of electron is a constant as given on your constants and equations sheet.

### Re: Example 1.5

Posted: **Sun Apr 22, 2018 5:34 pm**

by **Alma Flores 1D**

I was also confused about 9.109x10^-31 being the mass of an electron. Did he briefly go over this during lecture because I don't remember this being mentioned.

### Re: Example 1.5

Posted: **Sun Apr 22, 2018 5:49 pm**

by **Chem_Mod**

I was also confused about 9.109x10^-31 being the mass of an electron. Did he briefly go over this during lecture because I don't remember this being mentioned.

He did mention it in lecture. You can also look it up in your book and other resources.