PHYSICS 5B Lecture 3: Physics 3-4
First Law of Thermodynamics
OE = Work + heat = W + Q
Using Energy :Energy can be Transfer from one kind to another .
e± : use electricity to light a light bulb
Elastic Energy →Light Energy +heat energy
e± :eat food to perform food
Efficiency : e = whaty.cn#
what you had to pay
EI :light bulb .A 15W fluorescent bulb and a75W incandescent bulb . They both produce
3W
"visible light
"
P= Elt (power )
3W
.t.
Fluorescent e=
-
= 0.2 = 20%
15W . t
3t
Incandescent e= It =
0.04 = 4%
Here energy was transferred Into heat into The environment .Energy was not conserved .
Ex :food
.A weight litter curls a30kg bar ,raising it each
time at a distance of 0.6 m. How many times must he repeat this exercise to burn off energy In one slice of pizza .
food → molecules (ex .glucose)→cells in the body → chemical enery
digestion ✓ in metabolism
blood with
02
system .
1cal = 4.18 J
Epizzq =300 Cal = 300 × 103cal × 4. 18 J = 1260 × 103 J= 1260 KJ
Enter = M .
g. h=30 kg × 9.8 × 0.6 = 176J
←repitition
Epizza
. C = n .
mgh
t
typical = 25% :
Epizza
. C 1260×103 . G. 25
n =mgh-
=-176
=
2000 times
1st Law of Thermodynamics .
OE .
.wt Q
Heat engine CCarnot engine)
.
T"
Hot
Resevior
//QH .
<7
work
-
out ←Engine T,+
>To
It Qc
*
old
resevior
car Engine Example
q
→Foam
Movable Tv
system gas
cylinder
stage 2
Stage I
,
isolated from the resevior
mmm
qcalled adiabotk expansion
Mamas
. .
Capture heat Q,from hot Res .
Tt ,Hot Res .isothermal expansion
Stage 3stage 4
t-
=Give heat away to cold resevior ¥adiabatic Compression
-
Called Isothermal Compression
Tc (old Res .
*1
yz 1
Document Summary
Transfer from one kind to another e use electricity to light a light bulb. Efficiency : e whaty. cn# what you had to pay. A weight litter curls molecules ( ex . glucose) digestion a time at chemical raising. 30kg body metabolism enery cells each the bar it in in blood with 02 a distance of. How many times must he repeat this exercise to burn off energy in one slice of pizza . system. Mamas stage 2 mmm isolated from the resevior called adiabotk expansion. Workout heat in e efficiency of an automobile engine function at. , exhausting to spit fuel at about1100 c e =/ one. Is impossible to create a heat engine that work s 100% efficiency whatever process you do. How from not body to cold body , but not. = et no violation at conservation of energy. Clausius proposed a measure of this hidden quantity : entropy s.
