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Chapter 1 Lecture
Biological PhysicsNelson
Updated 1st Edition
Slide 1-1
What the ancients knew
Slide 1-2
Announcements
• Study methods
– Read each chapter BEFORE class
– Do the homework exercises (hand in)
• Grading: Participation 30%
– Midterm report/presentation 35%
– Final report/presentation 35%
• One more irregular class on April 30 or May 6 …
• Those of you taking Advanced Physics 1 (AP1)
will be encouraged to take AP2, sorry.
Slide 1-3
Intro: Entropy and 2nd Law
• The second law of thermodynamics can be stated in terms of
entropy: No process is possible in which the total entropy of an
isolated system decreases.
• In Figure below, the entropy (disorder) of the ink-water system
increases as the ink mixes with the water. Spontaneous unmixing
of the ink and water is never observed.
Slide 1-4
However.
• Have you heard of the Belousov–Zhabotinskii
reaction?
• Watch this SHOW STOPPER:
http://www.youtube.com/watch?v=E-ZIXGgt8sI
• What is happening here? This is related to how
life can somehow defy the 2nd Law of
Thermodynamics
Slide 1-5
The Energy Analogy
• Energy is an abstract concept a bit like money
• Money is a nice analogy:
– KE = Cash; PE = Savings
– Work > 0 is earning money
– Work < 0 is paying bills
• Power is how much and over how long
– Earning a $1,000,000 over 100 years is worse
than earning 1000000/2400 = $416/month
• Conservation of energy more fundamental than
Newton’s laws and applies to things like
quantum mechanics
Slide 1-6
Conservative and Nonconservative Forces
• Examples of conservative forces include
• Gravity
• The static electric force
• The force of an ideal spring
• Nonconservative forces include
• Friction/Heat
• The electric force in the presence of changing
magnetism
Slide 1-7
Conservative and Nonconservative Forces
• A conservative force stores any work done against it, and
can “give back” the stored work as kinetic energy.
• For a conservative force, the work done in moving between
two points is independent of the path:
• A nonconservative force does not store work done
against it, the work done may depend on path, and the work
done going around a closed path need not be zero.
Slide 1-8
The cycle of life?
Slide 1-9
Conservation of Mechanical Energy
• By the work-energy theorem, the change in an object’s
kinetic energy equals the net work done on the object:
∆K = Wnet= Wext
• When only conservative forces act, the net work is the
negative of the potential-energy change: Wnet = –∆U
• Therefore when only conservative forces act, any change in
potential energy is compensated by an opposite change in
kinetic energy:
∆K + ∆U = 0
• Equivalently,
K + U = constant = K0 + U0
• Both these equations are statements of the law of
conservation of mechanical energy.
Slide 1-10
Conservation of energy with heat?
• Non-conservative forces do not store potential
energy, but they do change the internal energy of a
system.
• The law of the conservation of energy means that
energy is never created or destroyed; it only
changes form.
• This law can be expressed by including a change
in internal energy Uint = Etherm this is also a
change in thermal energy hence
K + U + Etherm = Wext
Slide 1-11
First law of thermodynamics
• Now consider a pan of water over a flame,
clearly Etherm > 0 but no external work is being
done.
• So we must include something else:
K + U + Etherm = Wext + Q
• Generally these systems are stationary:
K=U = 0 and this implies:
Etherm = Wext + Q
• This is the first law of thermodynamics for work
done on the system. If work done by => –Wext
Etherm= Q - Wext
Slide 1-12
Thermodynamics systems
• A thermodynamic system is any
collection of objects that may
exchange energy with its
surroundings.
• In a thermodynamic process,
changes occur in the state of the
system.
• Careful of signs! Q is positive
when heat flows into a system. W
is the work done by the system,
so it is positive for expansion.
(See the figure on the right.)
Slide 1-13
First law of thermodynamics
• First law of thermodynamics: The change in the internal energy U of a system is equal to the heat added minus the work done by the system: U = Q – W. (See figure on right.)
• The first law of thermodynamics is just a generalization of the conservation of energy.
• Both Q and W depend on the path chosen between states, but U is independent of the path.
• If the changes are infinitesimal, we write the first law as dU = dQ – dW.
Slide 1-14
Note: Work done during volume changes*
• Figures below show how gas molecules do work when the gas volume changes.
Slide 1-16
High to low quality energy
Slide 1-17
Free energy transducer
Slide 1-18
Approaches to understanding
Slide 1-19
Dimensional analysis can guess laws
• Consider the “viscous friction coefficient” ζ = F/v
with dimensions M/T and the “diffusion constant”
D with dimensions L2/T.
• Both ζ and D depend in very complicated ways
on the temperature, the shape and size of the
object, and the nature of the fluid.
• However now assume the product ζD actually
turns out to be simple.
• What is the relation?
Slide 1-20
Dimensional analysis can guess laws
• Consider the “viscous friction coefficient” ζ = F/v
with dimensions M/T and the “diffusion constant”
D with dimensions L2/T.
• Both ζ and D depend in very complicated ways
on the temperature, the shape and size of the
object, and the nature of the fluid.
• However now assume the product ζD actually
turns out to be simple.
• Using dimensional analysis the relation depends
on energy: ζD=Etherm
• Einstein used this to prove atoms are real!
Slide 1-21
Parity
Slide 1-22
The BIG picture
This chapter’s Focus Question.
Section 1.2 discussed the idea that the flow of energy, together with its
degradation from mechanical to thermal energy, could create order. We
saw this principle at work in a humble process (reverse osmosis,
Section 1.2.2 on page 12), then claimed that life, too, exploits this
loophole in the Second Law of thermodynamics to create—or rather,
capture—order. Our job in the following chapters will be to work out the
details of how this works. For example, Chapter 5 will describe how tiny
organisms, even single bacteria, carry out purposeful motion in search
of food, enhancing their survival, despite the randomizing effect of their
surroundings. We will need to expand and formalize our ideas in
Chapters 6 and 8. Then we’ll be ready to understand the self-assembly
of complicated structures in Chapter 8. Finally, Chapters 11–12 will see
how one paragon of orderly behavior, namely how nerve impulses
emerge from the disorderly world of single-molecule dynamics.
Slide 1-23
Useful equations
Slide 1-24
Homework
• Read over Chapter 2 (on the cell) and watch the
student videos.
• Do Problem 1.4 (Earth’s temperature) & 1.5
(Franklin’s experiment)
• Next week we start Chapter 3, so also start
reading that (some probability is involved as
well)