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Comments (12)
johnlittlephysics said
at 8:58 am on Jul 28, 2013
312 Yu Kee:
1. For worksheet 4.4, I don't understand the difference among the three diagrams in qn 3
2. Does the unit cmHg refer to atmospheric pressure or the height of mercury in a liquid column?
johnlittlephysics said
at 9:21 am on Jul 28, 2013
2. cmHg or mmHg is just a unit of pressure expressed in terms of equivalent pressure exerted by a vertical column of mercury.
This can be used to express any pressure using P = h x density x g.
Example atmospheric pressure is P = 102 000 Pa, substitute into above equation with density = 13 600 kg/m^3 for mercury,
g = 10 m/^2 to obtain h = 0.75 m, which means the pressure is the same as that exerted by 75 cm of mercury.
So we may also say atmospheric pressure P = 75 cmHg or 750 mmHg or 0.75 mHg
johnlittlephysics said
at 9:22 am on Jul 28, 2013
1. WS 4.4 Question 3: Form equations for the pressure in each case.
For case (a), forces in equilibrium and F = P x Area
For (b) and (c), consider pressure at the lowest point, both just below and just above it.
See https://www.dropbox.com/s/pcw9krjixraqn2t/Photo%2028-7-13%208%2029%2029%20AM.jpg
johnlittlephysics said
at 8:51 pm on Jul 29, 2013
302 Li June: May I check how do we approach question 4(a) of AS 4.1 ? I do not really understand the answer key and explanation.
johnlittlephysics said
at 8:53 pm on Jul 29, 2013
See working at https://www.dropbox.com/s/8vpqql1scmv7x6j/Photo%2029-7-13%208%2049%2040%20PM.jpg
Also see Pressure mindmap above on approach to manometer problem.
johnlittlephysics said
at 7:59 am on Jul 31, 2013
312 Yu Kee:
1. For P = F/a, the a refers to surface area. So the base area of eg. a container of water would not affect the pressure? And so if the question involves such containers, would p = hdg be a better formula to apply?
2. For assignment 4.1, question 5(c), my answer was initially 60 and -10 initially, which of course is not logical, but I'm still not very clear. The question said "gas pressure increased by 20 cmHg", so I naturally thought the mercury level would increase by 20 as well, is it not so?
3. I'm quite confused by the phrase, "atmospheric pressure increases with height".
johnlittlephysics said
at 8:05 am on Jul 31, 2013
2. refer to Pressure mindmap on method to analyze manometer problem:
from the bottom of the manometer in mercury, B on the left arm has the same pressure as the 40 marking on the right arm (label this point as C).
Pressure at B = pressure at C
Hence, pressure of gas in cylinder = atmospheric pressure Patm (opening at the top) + pressure due to column of mercury AC.
Pgas = Patm + hd(40-10)
From this relation, if Pgas increases, the difference in levels of mercury (between A and B) will also increase.
This is because the mercury level at B will drop while the level at A will rise, so the increase in gas pressure of 20 comes from a drop of 10 at B and a rise of 10 at A, to give a total rise (difference of 20).
3. If height refers to altitude, the statement is incorrect.
Atmospheric pressure should decrease with altitude (e.g. height above sea level).
The higher you climb a mountain, the lower the surrounding atmospheric pressure.
johnlittlephysics said
at 8:06 am on Jul 31, 2013
1. by definition, pressure at the bottom of a container of water "due to water alone"
= weight of water / base area, so base area does affect the pressure!
However, when base area alone is increased, its weight also increases proportionately, such that P remains the same.
- starting with P=W/Area, we can derive P= hdg,
So pressure due to the column of water can also be calculated using its vertical height & density, so a change in vertical height or density of liquid would change its P at the bottom.
johnlittlephysics said
at 8:55 pm on Sep 28, 2013
302 Celeste: A hot air balloon filled with helium is released from the bottom of a tall building at sea level. At the bottom of the building, the balloon had a volume of 500cm3, while at the top, the volume was found 510cm3. The pressure at sea level is 100kPa. If the density of air is 1.29kgm-3, what's the height of the building ?
johnlittlephysics said
at 8:58 pm on Sep 28, 2013
Sketch diagram to visualize the problem (similar to height of mountain problems).
Write down relationships between variables.
See https://www.dropbox.com/s/x1i5l72gxddkmox/Photo%2028-9-13%208%2054%2008%20pm.jpg
Tam Li June said
at 4:51 pm on Oct 9, 2013
Hi Mr Ang, can you please explain the pressure question, question 8, from the physics revision pop quiz 1 that you have given us. Thanks.
johnlittlephysics said
at 10:44 pm on Oct 9, 2013
See steps in https://www.dropbox.com/s/qwcmx3cl5lu9ky7/Photo%209-10-13%2010%2042%2021%20pm.jpg
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