Solar and Renewable Energies - University of Oregonpages.uoregon.edu/raghu/TeachingFiles/Winter2010Physics162/Phys16… · i.e. if I have 3000 J of energy, I can do 3000 J of work

Physics 162:

Solar and Renewable Energies

Prof. Raghuveer Parthasarathy

[email protected]

Winter 2010

Physics 162: Solar and Renewable Energies R. Parthasarathy Winter 2010

January 21, 2010

Lecture 6: Announcements

• Reading: Wolfson, Chapter 3; section 10.1

• Homework: Problem Set 3 due today.

• Homework: Problem Set 4 due in 2 weeks.

• Homework: Microwave Assignment Part 1 due Tuesday, Jan. 26.

• Quiz (30m) Tuesday Jan. 26 (see last lecture’s slides)


“Road Map”• We could (but won’t) do this:


Physics of energy

Energy conversion mechanisms

Thermodynamics

Heat Transfer

Then

Hydropower

Wind Power

Solar Power

Etc.

i.e. first go through all the physics, then the applications

This isn’t a bad approach, but it would be somewhat dry

“Road Map”• We certainly won’t do this:


Hydropower

Wind Power

Solar Power

Etc.

(w/ no Physics)

•i.e. Not discussing physics, but just (1) memorizing facts about how much we use each, etc. (2) discussing how warm and happy these make us feel.•As mentioned before, understanding what we (society) needs to do in the present / future requires understandingenergy at a deeper level than mindless (and pointless) memorization.•If we did this approach, what would you learn?

“Road Map”• We will do this:


Physics of energy

Energy conversion mechanisms

Thermodynamics

Heat Transfer

i.e. integrate our discussions of alternative energy sources with “basic physics.”

A bit convoluted, but it should keep things interesting.

Hydropower

Wind Power

Solar Power

Etc.

Recap: Force, Work, Energy

• Velocity is the same as speed, but with information on direction.– e.g. 3 mph up is the opposite velocity as 3 mph down

• Acceleration is the rate of change of velocity.– in SI units, we’d measure this in m/s2, i.e. how much our “meters per second” is changing “per second”

• Force is needed to accelerate an object; F = m × a (mass times acceleration)


Recap: Force, Work, Energy

• Force applied over distance (F × d) = WorkWork is force × distance, where the force and the motion are in the same direction

• Energy is the capacity to do work – it has the same units as work.i.e. if I have 3000 J of energy, I can do 3000 J of work

• Power is the rate of change of energy (i.e. Energy / Time)– SI Units: measured in Watts (W). 1 W = 1 J / s

• Remember these things.


Acceleration (#1)• You throw a ball up in the air. After you let go, but before the ball reaches its highest point, the ball’s...

A. velocity and acceleration are zero

B. velocity is nonzero, but its acceleration is zero

C. acceleration is nonzero, but its velocity is zero

D. velocity and acceleration are both nonzero


(it’s moving; velocity is nonzero. it’s slowing down, so the velocity is changing, so the acceleration is nonzero.)

Acceleration (#2)

• You throw a ball up in the air. At its highest point, the ball’s...

A. velocity and acceleration are zero

B. velocity is nonzero, but its acceleration is zero


D. velocity and acceleration are both nonzero


[Discuss]

Acceleration• You throw a ball up in the air. At its highest point, the ball’s...



At the top, zero speed, so zero velocity. But the velocity is changing – if we wait any interval of time, the velocity is increasing downward – so, there’s a nonzero acceleration.

Another way to look at it: gravity (a force) is acting on the ball, pulling downward. It’s always acting – that’s why the ball falls! There’s a force, so there must be an acceleration.

Force• Recall our fan cart. If we apply a constant force to our cart (e.g. with the fan on), neglecting friction...


A. The cart will stop

B. The cart will move steadily (constant speed)

C. The cart will accelerate

Friction

• Since several of you asked about friction, I’ll say a bit more about this later...


Forms of Energy• There are many forms of energy• Energy can be converted from one form to another

• By “forms” we don’t mean particular fuels, but more general physical forms...


Should you remember all the various forms of energy?

Yes. But since we’ll be discussing them a lot in the coming weeks, this will be easy to do.

Kinetic EnergyKinetic Energy

• Energy associated with motion (e.g. a moving car, a falling leaf...).

• More mass (M) →more kinetic energy

• Greater velocity (v) →more kinetic energy.

• Ekinetic = (½) Mv2 . (i.e. ½×M×v ×v)

Source: istockphoto.com


Potential Energy (1)

Potential Energy• If I hold a ball at some height, you know that it could gain kinetic energy if I let go. Until then, it has potential energy – specifically, gravitational potential energy.

• Potential energy is associated with work done against a force field – for gravitational potential energy, the Earth’s gravity.


Potential Energy (2)

• There are other forms of potential energy – e.g. elastic potential energy stored in a stretched rubber band.

• Again, this “stored energy” can be converted to kinetic energy (e.g. by releasing the rubber band) or to other forms.


Gravitational Potential Energy• Gravitational potential energy is proportional to the mass of the object (M), its height (h), and the “acceleration due to gravity” (g) – a measure of the gravitational field.

• Egrav = M g h• In Problem Set 3, you’ve seen how this follows from “Work = Force × distance”.

• At the Earth’s surface, g = 9.8 m/s2.

• SI units:M in kg, g in m/s2, h in m for E to come out in Joules.


Potential and Kinetic Energies

• ...gravitational potential energy ...

• ...being converted into kinetic energy...

• ... and then back into gravitational potential energy ...

• ... and then back into kinetic energy, etc...?

Can you think of an example of


Potential and Kinetic Energies

• Consider a pendulum. (A bob of mass Mon a string.)

• Recall that kinetic energy is associated with motion: Ekinetic = ½M v2.

• Recall that gravitational potential energy is associated with position in a gravitational field (i.e. height): Egrav = M g h.


PendulumWhich of the following statements is true?

When the bob is at the top of its arc...

A. ... its kinetic energy is maximal and its potential energy is minimal.

B. ... its potential energy is maximal and its kinetic energy is minimal.

C. ... its potential and kinetic energies are both maximal.

D. its potential and kinetic energies are both minimal.


PendulumB. ... its potential energy is maximal and its kinetic energy is minimal.

• At the top of the arc, the height (h) is maximal, and so the gravitational potential energy is maximal: Egrav = M g h.

• At the top of the arc, the velocity is zero, so the kinetic energy is zero: Ekinetic = ½M v2.


Potential and Kinetic Energies• Hydroelectric power generation.

• Converting the gravitational potential energy of high‐altitude water into kinetic energy (to drive turbines).

The Niagara Power Project, one of the world's largest hydroelectric power plants (Source: UN Atlas of the Oceans)


• We’ll examine this quantitatively soon.

Forms of Energy• Kinetic energy

• Potential energy, esp. gravitational

• Chemical Energy: Energy stored in chemical bonds – released by chemical reactions (e.g. burning gasoline or coal). Also: chemical batteries, food (energy released in the digestive system).


(both)


By the way: A candy bar has an energy density of ≈ 20 MJ/kg. This is typical of biofuels – food is a good thing!

Forms of Energy

• Kinetic energy


• Chemical Energy

• Electrical Energy: Electricity!– Associated with charged objects (electrons)

– A particularly useful form of energy, for reasons we’ll see later.

– Chemical energy is fundamentally electrical: rearrangements of electrons in molecules





• Chemical Energy

• Electric Energy

• Electromagnetic Radiation


Light!


Electromagnetic radiation• Light! An electromagnetic wave.

Red: Electric Field

Blue: Magnetic Field

Speed: The speed of light! (3 × 108 m/s), or 186,000 miles per second

Source: Leiden University, Molecular Physics Group


Electromagnetic radiation• Light! An electromagnetic wave.

Source: Olympus / Florida State University

λ = wavelength – the distance between crests


Electromagnetic radiation

• Light! An electromagnetic wave.

• λ = wavelength, distance between crests– Visible light: λ = 400 to 750 × 10‐9 m

– Microwaves (microwave oven): λ ≈ 10 cm.

– Radio waves (FM): λ = few meters.

• Electromagnetic waves carry no mass, just energy. We harness this for...

• ...Photosynthesis, solar energy, (everything)




• Chemical Energy

• Electric Energy


• Mass Energy: Energy and mass are equivalent: E=mc2.



Mass Energy

• Until the 20th century, it was thought that mass, energy are separate (& separately conserved)

• Deeper insights into physics: Relativity. Consequence: Mass & Energy can convert into one another. (Einstein, 1905). Mass “contains” energy E=mc2, where c = the speed of light. Harnessed in nuclear power (fission), the sun (fusion).


Forms of Energy

• Kinetic energy


• Chemical Energy

• Electric Energy


• Mass Energy

• Thermal Energy: Energy of random molecular motion. (related to Heat)



Thermal Energy

• Associated with random molecular motion

• Closely related to temperature

Glass beads in water, radius 1 ×10‐6 m

Time: 5s

Source: Parthasarathy Lab Play movie


Thermal Energy

• Associated with random molecular motion

• Closely related to temperature

• Intuitively, seems like a hot object has more energy than a cool one; this is true

• Higher temperature →more random motion

• Higher temperature →more thermal energy


Thermal Energy“This thermal energy seems just like another category we listed!” Do you agree; if so, what is it?

A. Chemical energy

B. Mass Energy

C. Kinetic Energy

D. Disagree – it’s different.


Thermal energy is kinetic energy, but it’s categorized separately because the randomness of the velocities is very important – we’ll see why later.

Forms of Energy

• Kinetic energy


• Chemical Energy

• Electrical Energy


• Mass Energy

• Thermal Energy

• & More


Conservation of Energy

• Energy can be converted from one form to another but cannot be created or destroyed

• “Conservation of Energy”

• A fundamental law of nature!– I.e. no one has ever found a situation that violates this

• Not to be confused with “conserving energy” –i.e. trying to “use” less energy. We’ll return to this point soon.


Conservation of Energy

• Energy can be converted from one form to another but cannot be created or destroyed

• [e.g. pendulum] Kinetic ↔ Potential Energy• “But wait!,” you say. Eventually, the pendulum stops – it’s lost the gravitational potential energy it had, and it has no kinetic energy – so clearly energy isn’t conserved; we’ve lost it!

• ??


Friction

• Friction! Our pendulum isn’t just converting itspotential energy to kinetic energy & v.v.

• It’s also transferring energy to the air, my fingers, etc. – largely in the form of thermal energy and other random processes

• If we account for all this energy, it’s conserved –i.e. always the same.


Friction• But, you might say, that energy “lost” due to friction is useless to us...

• ... and you’re (mostly) right.

• We need to consider the efficiency of any energy conversion mechanism and

• We need to consider fundamental limitations on the efficiencies of physical processes.

• E.g. we use a solar collector to heat water; how much of this thermal energy can we extract to create electricity? Why? (This will make more sense in a week or so.)


Friction (a brief detour)• If we apply a constant force to an object with no friction it accelerates.

• Experience tells us that “really,” it will not accelerate forever. Why? Because there’s another force acting on it – friction – due to contact with surfaces, air resistance, etc.

• In general, the dependence on frictional forces on speed, etc., can be complicated (depending, e.g. on the shape of an object). But some simple general statements are possible:


Friction (a brief detour)

• The frictional force due to air resistanceincreases with speed.

• Suppose we apply a force (F) to an object at rest. It accelerates, and at some instant its velocity gives rise to a force from air resistance (Fdrag) that is less than F. What will happen to the object’s velocity? [Ask]


A. Agree (velocity will decrease; object will slow down)

B. Disagree

Note (added after class): The force we’re applying is constant (i.e. steady).

Friction (a brief detour)• The net force is nonzero and the object will accelerate (its speed will increase)...

• ... and Fdrag will increase...

• ...until the speed rises to such a value that Fdrag is equal to our applied force F.

• Now, the net force is zero, and the object moves with zero acceleration (constant velocity).

• This final velocity is called the terminal velocity.


F Fdrag

Friction (a brief detour)• This final velocity is called the terminal velocity.

• Graphically...


F Fdrag Now back to energy...

we apply:

Converting Energy

• Ignoring issues of efficiency for now...

• Converting energy from one form to another is crucial to our use of energy. E.g.– How do we convert the chemical energy of a chemical bond into the kinetic energy of a car?

– How to we take electromagnetic radiation from the sun (i.e. sunlight) and convert it to electricity?


Converting Energy• We could take any forms and ask how we can convert energy between them:


Kine

tic ene

rgy

Gravitational potential energyChemical EnergyEle

ctrica

l Ene

rgy

Electromagnetic Radiation

Mass Energy Ther

mal E

nerg

y

Etc.

(Some conversions are more interesting / relevant than others!)

Converting Energy

• We’ll look at various conversion mechanisms and their relevance to various alternative energy sources.

• Goal for today: How much power can we get from a hydroelectric dam?


Grav. Potl. → Kinetic Energy• Converting gravitational potential energy to kinetic energy (and then to electrical energy) is the principle behind hydroelectric power.

• High, slow water → lower, faster water; turns turbines (that generate electricity by means we’ll see later).


The Niagara Power Project, one of the world's largest hydroelectric power plants

(Source: UN Atlas of the Oceans)

Grav. Potl. → Kinetic Energy

• Converting gravitational potential to kinetic energy: very familiar (e.g. ball on a ramp).

• Demo (last time): hanging mass pulling cart.


Grav. Potl. → Kinetic Energy• Hanging mass pulling cart (last time).


Start Large grav. potl. energy (mgh); Zero kinetic energy (½Mv2)

End Smaller grav. potl. energy (mgh); Nonzero kinetic energy (½Mv2)

The amount by which the grav. potential energy decreases is equal to the amount by which the kinetic energy increases. This is conservation of energy.

Pendulum Revisited

• Consider a pendulum (Mass M).

• Suppose its maximum height is h2, and minimum height is h1.

• What is the maximum speed of the bob, in terms of h1 and h2? (Call it vmax)


h2h1

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Solar and Renewable Energies - University of Oregonpages.uoregon.edu/raghu/TeachingFiles/Winter2010Physics162/Phys16… · i.e. if I have 3000 J of energy, I can do 3000 J of work