phy 113 c general physics i 11 am - 12:15 p m tr olin 101 plan for lecture 20:

PHY 113 C Fall 2013 -- Lecture 20 111/07/2013

PHY 113 C General Physics I11 AM - 12:15 PM TR Olin 101

Plan for Lecture 20:Chapter 19: The notion of temperature

1. Review of fluid physics2. Temperature equilibrium3. Temperature scales4. Temperature in ideal gases


The physics of fluids.•Fluids include liquids (usually “incompressible) and gases (highly “compressible”).•Fluids obey Newton’s equations of motion, but because they move within their containers, the application of Newton’s laws to fluids introduces some new forms.Pressure: P=force/area

1 (N/m2) = 1 Pascal

Density: r =mass/volume 1 kg/m3 = 0.001 gm/ml

Review:


)(ρ (constant) fluid, ibleincompressFor 00 yygPP r

Review of equations describing static fluids in terms of pressure P and density r:

gdydP ρ :surface sEarth'near fluids allFor

Note that for compressible fluids (such as air), the relationship between pressure and density is more complicated.

Buoyant force for fluid acting on a solid – net force due to volume Vdisplaced being displaced in fluid:

FB=rfluidVdisplacedg


Bernoulli’s equation:

22222

111

212

1 PgyvPgyv rrrr


Webassign questions on fluids (Assignment #17)

A large man sits on a four-legged chair with his feet off the floor. The combined mass of the man and chair is 95.0 kg. If the chair legs are circular and have a radius of 0.500 cm at the bottom, what pressure does each leg exert on the floor?

mgmg/4

P=F/A=(mg/4)/A



A swimming pool has dimensions 32.0 m ✕ 7.0 m and a flat bottom. The pool is filled to a depth of 2.50 m with fresh water. (a) What is the force exerted by the water on the bottom?

(b) What is the force exerted by the water on each end? (The ends are 7.0 m.)

(c) What is the force exerted by the water on each side? (The sides are 32.0 m.)



A swimming pool has dimensions 32.0 m ✕ 7.0 m and a flat bottom. The pool is filled to a depth of 2.50 m with fresh water. (a) What is the force exerted by the water on the bottom?

h=2.5m

Fbottom=PA=rghA



A swimming pool has dimensions 32.0 m ✕ 7.0 m and a flat bottom. The pool is filled to a depth of 2.50 m with fresh water.

(b) What is the force exerted by the water on each end? (The ends are 7.0 m.)

h=2.5m

w=7.0m

22

2

0 21

2 gwhhhgwwdyyhgAPF

i

h

iiside rrr


21

1

: thatNote

2

2

yAhA

yhgg

zyhg

zggP

HgOH

Hg

HgOH

rr

r

rr

22A

M

OHr

y

z


2

1

2

1

21

1

1

2

2

2

AA

h

AAghg

yAhA

yhgg

Hg

OH

HgOH

HgOH

r

r

rr

rr

m0049.02113600

1000m2.0

:2/ m, 0.2For 21

h

AA



The gravitational force exerted on a solid object is 5.30 N. When the object is suspended from a spring scale and submerged in water, the scale reads 3.50 N (figure). Find the density of the object.

fluid

object

B

B

Fmg

NmgNmgF

rr

3.55.3



A light balloon is filled with 373 m3 of helium at atmospheric pressure. (a) At 0°C, the balloon can lift a payload of what mass? Note: rair = 2.9 kg/m3 : rHe = 0.179 kg/m3

VmVgF

VgmmmgF

Heairload

airB

Heload

B

rrr

r

0



A hypodermic syringe contains a medicine with the density of water (see figure below). The barrel of the syringe has a cross-sectional area A = 2.40 10-5 m2, and the needle has a cross-sectional area a = 1.00 10-8 m2. In the absence of a force on the plunger, the pressure everywhere is 1.00 atm. A force of magnitude 2.65 N acts on the plunger, making medicine squirt horizontally from the needle. Determine the speed of the medicine as it leaves the needle's tip.

112

2121

22222

111

212

1

/ ;/ ; :case In this

vvaAvAFPPyy

PgyvPgyv

rrrr


Dictionary definition: temperature – a measure of the the warmth or coldness of an object or substance with reference to some standard value. The temperature of two systems is the same when the systems are in thermal equilibrium.

“Zeroth” law of thermodynamics:

If objects A and B are separately in thermal equilibrium with a third object C, then objects A and B are in thermal equilibrium with each other.

T1 T2 T3

Not equilibrium: Equilibrium:


Constant temperature “bath”

T

T

At equilibrium:


-100 -50 0 50 100 150 200-200

-150

-100

-50

0

50

100

150

200

250

300

350

400

450

500

T F

TC

Temperature scales TF=9/5 TC + 32

Kelvin scale:

T = TC + 273.15o

T 0


iclicker question:Suppose you find yourself in a hotel in Europe or Canada. Which Celsius temperature would you set the thermostat for comfort?

A. -20oCB. +20oCC. +40oCD. +60oCE. +80oC


There is a lowest temperature:

T0 = -273.15o C = 0 K

Kelvin (“absolute temperature”) scale

TC = -273.15 + TK

Example –

Room temperature = 68o F = 20o C = 293.15 K


Effects of temperature on matter

Solids and liquids

Li (equilibrium bond length at Ti)

Model of a solid composed of atoms and bonds

L

Thermal exansion:

L = a Li T


Typical expansion coefficients at TC = 20o C:

Linear expansion: L = a Li T

Steel: a = 11 x 10-6/ oC

Concrete: a = 12 x 10-6/ oC

Volume expansion:

V=L3 V = 3a Vi T = b Vi T

Alcohol: b = 1.12 x 10-4/ oC

Air: b = 3.41 x 10-3/ oC


iclicker question

On the last slide – we suggest that b=3a. Is this result

A. One of those mysteries of physics that has no explanation?

B. A result that we can derive?

L L+L

V=L3

V+V=(L+L)3@V(1+3(L/L))


Brass

Steel


Switch in thermostat

Modern thermostats use electrical circuits to detect temperature


Effects of temperature on materials – continued strange case of water:


Effects of temperature on materials – continued -- ideal gas “law” (thanks to Robert Boyle (1627-1691), Jacques Charles (1746-1823), and Gay-Lussac (1778-1850)

nRTPV

pressure in Pascals

volume in m3 # of moles

temperature in K

8.314 J/(mol K)

1 mole corresponds to 6.022 x 1023 molecules


nRTPV

P0 =12.6 atmT0 =27.5oCn0

P=?T=81.0oCn=n0/3

atm.

. atm. PTT

PP

RTnnRT

VPPV

/n nVVnRTPVRTnVP

9.4653003

15354612 3

3

000000

000000


Assuming that air behaves like an ideal gas, what is the density of air at T=0o C and P=1 atm?

V

Mn

Vm

RTP

Vn

nRTPV

iii

rdensity mass


Typical composition of air: url: http://www.engineeringtoolbox.com/molecular-mass-air-d_679.html

3kg/m 1.29 1000

density mass

RTPM

V

Mn

Vm avgi

ii

r

http://www.engineeringtoolbox.com/molecular-mass-air-d_679.html

phy 113 c general physics i 11 am - 12:15 p m tr olin 101 plan for lecture 20:

Documents

c fall

fluids assignment

compressible fluids

physics of fluids

static fluids

webassign questions

fresh water

c general physics i11