Physics 115
“Physics for FUTURE LEADERS”
Prof. Paul Steinhardt
Princeton in the Nation’s Service
Woodrow Wilson, 1896
PLEASE DO NOT SIT IN THE LAST 3 ROWS
THREE KEY FACTS ABOUT ENERGY:
There are many forms of energy
and you can convert one form of energy
into another
Total Energy is conserved
In any conversion,
some stored/kinetic energy
is turned into heat
… but you cannot revert back 100%
About how much energy per second
do you consume on average?
(A) 10 W
(B) 100 W
(C) 1000 W
(D) 10,000 W
About how much energy per second
do you expend on average?
(A) 10 W
(B) 100 W
(C) 1000 W
(D) 10,000 W
You consume energy at about the rate of 100 W.
You produce energy at the rate of about 100 W.
As a citizen of the 21st century,
at what rate do you use energy?
(A) 10 W
(B) 100 W
(C) 1000 W
(D) 10,000 W
You consume food energy at about the rate of 100 W.
You produce energy at the rate of about 100 W.
As a citizen of the 21st century,
at what rate do you use energy of all types?
need to include all the energy that is used for you to support your
life – the energy that runs your lights, your computer, that powers
the cars and trucks that supply your food; the tractors and other
vehicles needed to farm your food; the pesticides and fertilizers;
the energy to cook that food; to refrigerate it. Some of you fly or
drive to come to campus at the beginning of the year; you are
responsible for some share of all the buildings on campus here to
support your lifestyle; the lawnmowers that cut the grass and all
that
You consume energy at about the rate of 100 W.
You produce energy at the rate of about 100 W.
As a citizen of the 21st century,
at what rate do you use energy?
(A) 10 W
(B) 100 W
(C) 1000 W
(D) 10,000 W
From R. Wolfson, Energy Environmet and Climate (Norton Pub, 2012)
2. Atoms & Heat
Recall:
Energy
Ability to do work
Anything that can be turned into heat
à HEAT IS A FORM OF ENERGY
What do you think?
Are HEAT and TEMPERATURE the same thing?
A) Yes
B) No C) I don’t know
Let’s ask ourselves some questions:
If I increase the temperature of an object,
do I increase its heat energy?
If I double the temperature of an object,
do I double its heat (energy)?
Do two objects at the same temperature have
the same amount of heat energy?
Do HEAT and TEMPERATURE have the same units?
F = C + 32
9
5
K = C + 273
K =0 ⇔
absolute zero
0th Law of Thermodynamics:
Objects in contact reach same T
1st Law of Thermodynamics
• Heat is a form of energy
(1 Calorie ~ 4 kilojoules)
• The amount of energy is proportional
to the temperature
• Energy (including heat) is conserved
Heat Capacity & Specific Heat
temperature
in oK
mass
Q = m Cp T
heat energy
specific heat:
the value of Cp depends
on the substance.
If I increase the temperature of an object, do I increase its heat energy?
If I double the temperature of an object, do I double its heat (energy)?
Do two objects at the same temperature have the same amount of heat energy?
Do HEAT and TEMPERATURE have the same units?
Heat Capacity & Specific Heat
temperature
in oK
mass
Q = m Cp T
heat energy
specific heat:
the value of Cp depends on
the substance.
Example:
water: Cp = 1 Cal /( kg °K)
copper: Cp = 0.09 Cal/(kg °K)
Which requires more energy to raise 1 degree?