AP PHYSICS

Tuesday, 1/13/09 AP PHYSICS

“We teach people how to treat us.”

Cesar MillanThe Dog Whisperer

(*rev and rev hw 43note: water’s high C p 355

and things it causes*)

Tuesday, 1/13/09 AP Physics

The last of Ch. 11The three ways to transfer heat:conduction –

convection –

radiation –

direct contact with an object having higher temperature (*emphasize direction of flow*)

movement of a heated fluid (gas or liquid) electromagnetic waves (infrared)

ex. camp fireconduction – convection – radiation –

grab the burning wood put hands over the fire (hot air rises)sit beside the fire (radiated heat only)

Heat transfer rate (heat current)(*for conduction*) H ≡ Q/Δtunits: J/s = W

H = Q/Δt A(T2 – T1)/L

H = Q/Δt = kA(T2 – T1)/L

k – constant, p. 367 “thermal conductivity”A – cross-sectional areaT2 – higher temperatureT1 – lower temperatureL – length (or thickness of slab)

(*ice demo*)

p. 382 #38A glass windowpane in a home is 0.62cm thick and has dimensions of 1.0m x 2.0m. On a certain day, the indoor temperature is 25°C and the outdoor temperature is 0°C.(a) What is the rate at which energy is transferred by heat through the glass?(b) How much energy is lost through the window in one day, assuming the temperature inside and outside remain constant.

(*multiple layers*)

Ch. 12 The Laws of Thermodynamicsdefn. Internal energy (U) –

Most books also call this “thermal” energy.Note: Your book differentiates between them.It calls thermal energy, the part of internal energy that changes when the temperature changes.When we talk about the state of a system we’ll talk about pressure, volume, temperature, and internal energy.For simple systems, the pressure and volume will be the most important information.

all of the energy belonging to all the molecules of a system while it is stationary.

Here is a very simple system: A gas expanding at constant pressure.

W = FΔy (work done BY the gas)

= (PA)Δy = PΔV

P

W = PΔV

PV graphs are used to study these systems.

The work done BY the system equals the area under the PV curve!NOTE: We can and will have work done ON the system! (*backwards*)(*calculus – W = ∫ P dV*)

The work done by a gas from an initial state to a final state depends on the path taken.

1 2 3

W2 > W3 > W1

HW 44: ws. Ch. 11 problems 30, 33-35

Ch. 12 problems 2 - 6

hw 4

4

30)

a) 2

23W

b) 0

.013

2W

c)

0.0

53W

33)

37X

mor

e35

) 0.

0899

m3)

11,

300J

5) a

) 81

0J

b)

507J

c

) 20

3J