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 ( T) Q =mc T
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 Q 1 + Q 2 + Q 3 … = 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 m s at T s is immersed in a mass m w of water at T w. What is the final temperature of the water? Q silver + Q water = 0 c s m s T + c w m w T = 0 c s m s (T f - T s ) + c w m w (T f - T w ) = 0 c s m s T f -c s m s T s + c w m w T f - c w m w T w = 0 c s m s T f + c w m w T f = c s m s T s + c w m w T w T f = (c s m s T s + c w m w T w )/(c s m s + c w m w )
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 A T1T1 T2T2 Q
Conductive Heat Transfer The rate of heat transfer via conduction is: where: T 1 is the temperature of the hot side and T 2 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 X W/m 2 K 4 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 P net = Ae T 4 -T 4 2 ) Where T 2 is the temperature of the surroundings Note that T must be in Kelvin
Next Time Read: Homework: CH 14, P: 13, 47, CH 13, P: 29, 48 Help sessions start next week Tuesday and Thursday 6-8pm Science 304