Download specific heat

Heat
Physics 102
Professor Lee Carkner
Lecture 3
“If you can’t stand the
heat, get out of the
kitchen.”
-Harry S. Truman
PAL #2 Galileo Thermometer
How does it work?

Limitations

Heat
What is heat?

Same temperature, no heat

Heat used to be thought of a fluid (caloric)
that could flow to change temperature
Heat is represented by the letter Q
Measuring Heat

Common unit of heat is the calorie:
Amount of heat necessary to increase the
temperature of 1 gram of water by 1 C

In nutrition the Calorie is used

Case sensitive!
For rates of heat transfer (Q/t), unit is the
Watt (W)

Heat and Temperature

If you heat a metal spoon and a
wooden spoon for the same time, which
will have a higher T?

The specific heat
Specific Heat
The specific heat is defined as:
c has units of J/kg C

Need to know the mass of the stuff (m) and
the change in temperature (DT)

Q =mcDT
Today’s PAL
A certain amount of heat Q will warm 1
g of material A by 3 degrees C and 1 g
of material B by 4 degrees C. Which
material has the greater specific heat?
Explain.
Calorimetry

Insulated container that prevents heat transfer
from outside

Since calorimeter is insulated, negative heat lost
cancels out positive heat gained
Q1 + Q2 + Q3 … = 0

Heat gained always positive, heat lost always
negative

Make sure units for T and m match units for c
Example: Quenching a Dagger
Suppose a silver dagger of mass ms at Ts is
immersed in a mass mw of water at Tw. What
is the final temperature of the water?
Qsilver + Qwater = 0
csmsDT + cwmwDT = 0
csms(Tf - Ts) + cwmw(Tf- Tw) = 0
csmsTf -csms Ts + cwmwTf - cwmw Tw = 0
csmsTf + cwmwTf = csms Ts + cwmw Tw
Tf = (csms Ts + cwmwTw)/(csms+ cwmw)
How Does Heat Move?
Heat (like information) is transferred in
different ways
Conduction

Radiation

Convection

Conduction

Why?

They interact and collide with other atoms
and electrons and pass the energy on

Conduction Rate Factors
Free electrons

Density

Cross sectional area
Large window loses more heat than small
Temperature difference

Thickness
Heat takes less time to move through thinner material
Radiation
How does the energy from the Sun get to
Earth?
How can energy be transported with no
physical contact?

Photons are emitted by the Sun and absorbed
by you

All objects emit photons

Radiation Rate Factors
Surface area

Emissivity


Radiation is strongly dependant on T
The Surface of the Sun
Convection

Hot air is less dense than the cooler air above it

After cooling the air may fall back down

Examples: baseboard heating, boiling water,
Earth’s atmosphere
Convection Rate Factors
Fluidity

Energy exchange with environment

?
How rapidly will the material lose heat?

Small temperature difference, not enough density
difference to move
Today’s PAL
A hot piece of metal is at the bottom of a
canister that can be completely filled with:
solid iron
liquid water
air
a vacuum
Consider the heat flow from the bottom to the
top.
In which situation(s) would there be no
conduction?
In which situation(s) would there be no
convection?
In which situation(s) would there be no radiation?
A
T1
T2
Q
L
Conduction Diagram
Conductive Heat Transfer
The rate of heat transfer via conduction is:
where:
T1 is the temperature of the hot side and T2 is the
temperature of the cold side
A is the cross sectional area
L is the thickness
k is the thermal conductivity

High k = large heat transfer
Low k = small heat transfer
Radiative Heat Transfer
The amount of heat radiated out from an object is
called the power (P):
where
s = the Stefan-Boltzmann constant
5.6696 X 10-8 W/m2 K4
A is the surface area
e is the emissivity (number between 0 and 1)
0 =
perfect reflector
1 =
perfect absorber or black body
Radiation Exchange
All objects emit and absorb radiation

Pnet = sAe(T4-T42)
Where T2 is the temperature of the
surroundings
Note that T must be in Kelvin

Next Time
Read: 13.6-13.11
Homework: CH 14, P: 13, 47, CH 13, P:
29, 48
Help sessions start next week
Tuesday and Thursday 6-8pm Science 304