PHY1039
Properties of Matter
State Variables
9 February, 2012
Lecture 2
Extrinsic State Variables
Pressure, P
A
F
F 𝐴𝑝𝑝𝑙𝑖𝑒𝑑 𝑓𝑜𝑟𝑐𝑒
P= =
𝐴
𝐴𝑟𝑒𝑎
Positive P is pressing inward.
P can also be a negative quantity.
Surface Tension, G
ℓ
A
Tension, F
F
Membrane
L
F
F
Γ=
2ℓ
At mechanical equilibrium,
the force acting along the
edge balances G.
Positive P is pulling outward.
Hydrostatic Pressure on a Solid
Piston
The surrounding fluid ensures that the
force per unit area is constant across
the entire surface of the solid.
Earth’s atmosphere exerts a hydrostatic
pressure of about 1.02 x 105 Pa (not a
constant)
High Pressure Diamond Anvil Cell
Used by Prof. Alf Adams’ group at
Surrey when studying electrical and
optical properties of semiconductors
for applications in lasers.
Figures from “Understanding Properties of Matter” by M. de Podesta
Measuring Surface Tension of Liquids
Meniscus
A force is required to
distort the meniscus (up
or down) – pulling
against molecular
attractive forces.
Vector g represents surface tension
Wilhelmy plate method
http://vydavatelstvi.vscht.cz/knihy/uid_es001/hesla/metody.vyvazovani_desky.html
http://fiesta.bren.ucsb.edu/~vbroje/oil_spill_research_laboratory.htm
Surface Tensions of Liquids
Image from:
http://wps.prenhall.com/wps/media/o
bjects/602/616516/Chapter_10.html
Table from “Understanding Properties of Matter” by M. de Podesta
Surface Tension is Temperature Dependent
At higher temperatures,
molecules are slightly farther
apart, and so their attraction to
each other is weaker – hence G
is lower.
K = C + 273.15
G usually does not vary much with
the area of a surface of liquid. Why?
Figures from “Understanding Properties of Matter” by M. de Podesta
Water