Matter and Energy
Drill
A manometer filled with water is used to
measure the pressure of a gas in a tank. The
water (ρ = 1000 kg/m3) is displaced by a
height of 60 centimeter. What is the gage
pressure of the gas in the tank? What is the
absolute pressure?
Matter and Energy
Thermodynamics - “the study of systems and energy transfer”
Heat (Q) – energy transfer caused by a temperature
difference
•If we place a flame beneath a system, we would
be adding heat to it.
•We could also remove heat by surrounding the
system with an ice bath.
Matter and Energy
Heat (Q) – energy transfer caused by a temperature difference
So far, we have defined heat transfer and talked
about its sign convention…
Q in is (+)
Q out is (-)
…but we haven’t talked about how heat is
transferred.
Matter and Energy
Heat (Q) – energy transfer caused by a temperature difference
Now let’s talk about the modes of heat transfer (the
ways in which heat goes in/out of a system)…
Q in is (+)
Q out is (-)
1. Conduction
2. Convection
3. Radiation
Matter and Energy
Heat (Q) – energy transfer caused by a temperature difference
This man is holding one end of a metal rod inside of
a furnace with his bare hands. Do you think that he
can do that for very long?
Why or why not?
Conduction!!!
Over time the rod heats up…
Matter and Energy
Heat (Q) – energy transfer caused by a temperature difference
Conduction Heat Transfer – energy transfer
between molecules due to a temperature difference

T

The
parts 
of -the
rod inside the furnace
Q cond
kA
heat up first.

x

Q, heat transfer rate (W)
Over time, those molecules pass
W
k,
conductivi
ty
of
a
substance
(
that heat along to the molecules )
m-K
near them that are outside
the
2
A,
cross
sectional
area
(m
)
furnace.
T, temperature difference (K)
Eventually
x, distancethe
(m)whole rod heats up!
Matter and Energy
Energy is transferred from a warm room at 20C inside a house to the outside
air at -10C through a single-pane window. The glass is 5mm thick with an
area of 0.5 m2 and a conductivity of 1.4 W/m K. What is the rate of heat
transfer through the glass?
Known: Heat flows through a window (A=0.5m2, ∆x = 5 mm, k = 1.4 W/m K)

Q
Find:
cond , [W]

Q
cond
Sketch:
Troom = 20C
= 293 K
Solution:
∆x
Assumptions: Closed System
Toutside = -10C
= 263 K
T

Q cond  - kA
x
2 293K  263K

Q

(1.4
W/m
K)(0.5
m
)
cond
1m
5mm( 1000
)
mm

Q
cond  - 4200 W
Matter and Energy
Heat (Q) – energy transfer caused by a temperature difference
The insides of a computer get very hot as it
operates. How do we keep them from overheating?
Computers use fans to
cool down…just like us!
This type of heat transfer
is called Convection.
Matter and Energy
Heat (Q) – energy transfer caused by a temperature difference
Convection Heat Transfer – energy transfer due
to the bulk motion of a fluid
As
and
chips
inside the PC
 the boards
Q

hA

T
heatconv
up, the fan turns on.

Q, heat trans fer rate (W)
The
fan pulls in cool air from
W
h,
convection
heat
trans
fer
coefficien
t
(
)
outside the PC which picksmup- Kthe
heat
from
the area
boards
and
A, cross
- sectional
of surface
(m chips.
)
2
2
T, temperatu re difference between surface and fluid (K)
The now hot air is forced out of
the PC, cooling it down.
Matter and Energy
Cold air at -10C blows over a warm window-pane with a surface
temperature of 12C. The glass has a surface area of 0.5 m2 and the
convective heat transfer coefficient is h =100 W/m2 K. What is the rate of
heat transfer through the glass?
Known: Heat flows through a window pane (A=0.5m2, 100 W/m2 K)
 , [W]
Q
Find:
conv
Assumptions: Closed System
Toutside = -10C
Sketch:
= 263 K

Q
conv
Tglass = 12C
= 285 K
Solution:

Q
conv  hA T
2
2

Q

(100
W/m
K)(0.5
m
)( 285K  263K )
conv

Q
conv  1100 W
Matter and Energy
Heat (Q) – energy transfer caused by a temperature difference
The temperature outside changes over time; it’s cool
at night and warm throughout the day. Why?
Radiant energy leaves
thesun
earth
during
night,
from the
warms
thethe
earth
cooling
theday
earth
down
sun is “down”).
during the
(when
the(when
sun isthe
“up”).
Matter and Energy
Heat (Q) – energy transfer caused by a temperature difference
The temperature outside changes over time; it’s cool
at night and warm throughout the day. Why?
4

Q rad  AT
 , heat trans fer rate (W)
Q
 , emissivity
 , Stefan - Boltzmann constant (5.67x10 -8
A, cross - sectional area of surface (m 2 )
T, surface temperatu re (K)
W
m 2 -K 4
)
Matter and Energy
A small light bulb with a surface area of 0.025m2 and an emissivity of 0.6
fluoresces at a temperature of 100C. What is the rate of heat transfer from
the light bulb?
Known: Heat radiates from a bulb (A=0.025m2, Tbulb = 100 C,  = 0.6)
 , [W]
Q
Find:
rad
Assumptions: Closed System
Sketch:

Q
rad
Tbulb = 100 C
= 373 K
Solution:
  AT 4
Q
rad
  (0.6)(5.67x10 -8
Q
rad
  16.5 W
Q
rad
W
m 2 -K 4
)(0.025 m 2 )(373 K ) 4
Matter and Energy
Thermodynamics - “the study of systems and energy transfer”
Heat (Q) – energy transfer caused by a temperature
difference
Conduction
Convection
 t
QQ
Radiation
(J, Joules)
Matter and Energy
A piston cylinder containing a gas was compressed over a period of 20 seconds.
The change in energy of the system was 300 kJ and the average rate of heat
transfer from the cylinder was 12 kW. Find the work done on the gas in kJ.
Known: A gas is compressed inside of a piston-cylinder
Find: W, [kJ]
Sketch:
Assumptions: Closed System

Q
Solution:
E1-2  Q1-2 - W1-2
W1-2  - E1-2  Q1-2
 t - E
W1-2  Q
W1-2  - 300 kJ  (-12 kJ/s)(20s)
W1-2  - 540 kJ
Matter and Energy
Identify the type of heat transfer:
On a relatively cool, but very sunny day, you notice that the surface temperature of
the sidewalk is above that of the surrounding air.
A fan inside of an oven forces hot air to circulate around a turkey to cook it faster.
One half of a metal rod is in the sun and the other half in the shade. You touch the
shaded end and notice that it is hot.
Matter and Energy
The temperature inside of a room with insulated walls is 70C while the outside
temperature is 95C. Heat enters the room through a 0.5 m2 window with a
thickness and thermal conductivity of 0.05m and 0.3 W/m-K. A portion, 0.4%, of
the radiant energy from the sun ( = 0.9, T = 5200 K) enters the room as well.
A student pulls a cart 10m across the floor using a force of 30N; the handle of the
cart is at an angle of 60 degrees from the horizontal. How much work is done by
the student? How much is done by force of gravity?
Matter and Energy
A vertical piston-cylinder device (D = 5 cm) contains a trapped mixture of gases.
The walls of the cylinder are insulated, but heat is allowed to enter through the
floor of the cylinder at a rate of 2.75 W over 20 seconds. The absolute gas
pressure is 101.3 kPa, initially. If the change in energy of the system is 41 J, how far
does the piston rise (in cm) during this process?
Known: A gas mixture is heated and expands inside of a piston-cylinder
Find:
rise of piston, h [cm]
Sketch:
Assumptions: Closed System ,
Vertical-piston cylinder -> Isobaric
Solution:
V1-2  r 2 h
h
πr 2V1-2
h
2
0.00013
8 m3
πr
h
π(0.025 m) 2
h  0.07 m  7 cm
W1-2  P W
(V  V )
V1-2  12-2 1
P 1-2
W1-2  PV
14J
W

V

1
2
V11--22  101.3
P kPa
V1-2  0.000138 m 3

E
W11--22  Q11--22 --W
E1-22 Q  Qt 
Q(2.75
Q
)(t20 s)
Q

W
 55J 41J
WW
1- 21- 2 Q1- 2 - E1- 2
W1-2  14J
Q  (55 J)