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# Pressure/Temperature Problem

Author Topic:   Pressure/Temperature Problem
Percy
Member
Posts: 21258
From: New Hampshire
Joined: 12-23-2000
Member Rating: 2.4

 Message 1 of 5 (540833) 12-29-2009 12:37 PM

The formatting made it hard to tell what you were trying to say in your email, but when calculating the effect of temperature on pressure you can use Charles Law:
$\color{white} {P_1\over{P_2}} = {T_1\over{T_2}} and {V_1\over{V_2}} = {T_1\over{T_2}}$
You gave these values:
$\color{white} P_1 = 3500\ psi\newline\indent T_1 = 30^{\circ}C = 86^{\circ}F = 546^{\circ}F\ above\ absolute\ zero\newline\indent T_2 = 4^{\circ}C = 39.2^{\circ}F = 499.2^{\circ}F\ above\ absolute\ zero$
Plugging in the values:
$\color{white} {3500\over{P_2}} = {546\over{499.2}}$
Multiplying both sides of the equation by P2 and switching sides around the equals sign:
$\color{white} {546{P_2}\over{499.2}} = 3500$
Now we can multiply both sides by 499.2:
$\color{white} {546{P_2}} = (3500)(499.2)$
And now we can divide both sides by 546:
$\color{white} {P_2} = {(3500)(499.2)\over{546}}$
And doing the math gives us:
$\color{white} {P_2} = {3200}$
Which is roughly the same answer you got, except that you were a little off because you rounded 39.2 into 39, so it looks like your method was correct.

 Replies to this message: Message 2 by Taz, posted 12-29-2009 2:55 PM Percy has replied

Taz
Member (Idle past 2720 days)
Posts: 5069
From: Zerus
Joined: 07-18-2006

 Message 2 of 5 (540861) 12-29-2009 2:55 PM Reply to: Message 1 by Percy12-29-2009 12:37 PM

Percy writes:
Which is roughly the same answer you got, except that you were a little off because you rounded 39.2 into 39, so it looks like your method was correct.
No, the correct answer is 39. 3500 only has 2 sig figs.

 This message is a reply to: Message 1 by Percy, posted 12-29-2009 12:37 PM Percy has replied

 Replies to this message: Message 3 by Percy, posted 12-30-2009 6:56 AM Taz has not replied Message 4 by Dr Jack, posted 12-30-2009 1:13 PM Taz has replied

Percy
Member
Posts: 21258
From: New Hampshire
Joined: 12-23-2000
Member Rating: 2.4

 Message 3 of 5 (540934) 12-30-2009 6:56 AM Reply to: Message 2 by Taz12-29-2009 2:55 PM

You may be right, I don't know. These are multiple choice tests, and the answers tend to be in round numbers, but I don't have a copy of this test.
--Percy

 This message is a reply to: Message 2 by Taz, posted 12-29-2009 2:55 PM Taz has not replied

Dr Jack
Member (Idle past 1533 days)
Posts: 3507
From: Leicester, England
Joined: 07-14-2003

 Message 4 of 5 (540984) 12-30-2009 1:13 PM Reply to: Message 2 by Taz12-29-2009 2:55 PM

You should round only when giving your answer; you should always use the most precise number available to you during intermediate calculations.

 This message is a reply to: Message 2 by Taz, posted 12-29-2009 2:55 PM Taz has replied

 Replies to this message: Message 5 by Taz, posted 12-30-2009 1:39 PM Dr Jack has not replied

Taz
Member (Idle past 2720 days)
Posts: 5069
From: Zerus
Joined: 07-18-2006

 Message 5 of 5 (540992) 12-30-2009 1:39 PM Reply to: Message 4 by Dr Jack12-30-2009 1:13 PM

Haha, I suppose you belong to the old school of thought when this was still even an issue. I belong to the newer school of thought when you work out all the variables first and then insert all the values into the final equation to crank out the answer all at once. In other words, there's no such thing as an intermediate calculation, just like there's no such thing as an intermediate evolutionary step. Hey, I just realized something. Are you a creationist trapped inside an evilutionist body?
Edit.
By the way, you're right. I just glanced through it and thought he was talking about the final answer. Yes, if it was an intermediate calculation, he should have used 39.2. I guess there's always a first time I could be wrong
Edit again.
Everyone should be required to learn how to use a graphing calculator nowadays.
Edited by Taz, : No reason given.
Edited by Taz, : No reason given.

 This message is a reply to: Message 4 by Dr Jack, posted 12-30-2009 1:13 PM Dr Jack has not replied

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