Heat

Physics 102Professor Lee

CarknerLecture 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 (T)

Q =mcT

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

csmsT + cwmwT = 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?

Conduction Diagram

L

AT1

T2

Q

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 = 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 = AeT4-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