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Topic: Question about a textbook  (Read 352 times)

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Offline Orphanbike

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Question about a textbook
« on: February 07, 2020, 12:38:19 PM »
Sorry if this isn’t the right place to ask, but I wasn’t getting a response on the chemistry subreddit, so here’s my question:
I want to self teach myself chemistry and have started reading through the seventh edition of "Principles of Modern Chemistry" as listed in the suggested reading section of this subreddit. In section 1.4 they describe Thomson's Cathode Ray Tube experiment to find the charge to mass ratio of an electron. I'm confused as to how some of the equations are set up. For instance, they start with FE =eE=ma, but then they solve for a and get a = (em)E. Shouldn't it be divided by m? Later they use D = .5at^2 and substitute for a with the aforementioned equation as well as t with t = l*v (l for the length of the metal plates deflecting the cathode ray) but when they substitute these in they get D = .5(e/m)*E*(l/v)^2. So this time they get a = (e/m)*e correct but use l/v instead of l*v. Am I missing something here?

Offline Enthalpy

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Re: Question about a textbook
« Reply #1 on: February 10, 2020, 11:13:22 AM »
Welcome, Orphanbike! Your comments seem meaningful, but I'd prefer to see the original text.

Sidenotes:
- Here the deflection stops at the exit of the plates. Quite unusual.
- Relativistic corrections apply often with cathode ray tubes. At 15kV they make 3% difference already. It's one situation where Relativity is easy to observe.

What's subreddit? If everything is as inaccurate as the text you quote, you can't learn from it.

Offline Orphanbike

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Re: Question about a textbook
« Reply #2 on: February 10, 2020, 02:24:51 PM »
Welcome, Orphanbike! Your comments seem meaningful, but I'd prefer to see the original text.

Sidenotes:
- Here the deflection stops at the exit of the plates. Quite unusual.
- Relativistic corrections apply often with cathode ray tubes. At 15kV they make 3% difference already. It's one situation where Relativity is easy to observe.

What's subreddit? If everything is as inaccurate as the text you quote, you can't learn from it.

I've attached screenshots of the textbook, equations 1.1 - 1.4 are the equations in question. The subreddit is r/chemistry which I would hope is decent.

Offline mjc123

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Re: Question about a textbook
« Reply #3 on: February 11, 2020, 04:38:36 AM »
e/m and l/v are correct; looks like they've missed out some slashes in the body text.

You could check this for yourself by doing some dimensional analysis.

Offline Orphanbike

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Re: Question about a textbook
« Reply #4 on: February 11, 2020, 02:05:59 PM »
e/m and l/v are correct; looks like they've missed out some slashes in the body text.

You could check this for yourself by doing some dimensional analysis.

Yea that's what I thought, it seems weird to me that a seventh edition textbook would get the second listed equation wrong like that

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