climate and weather
lym503829712
GEO 344 Weather and Climate Prof. Stuart Evans
Lecture 5 Energy Transfer
Announcements
• Homework #1 due 11:59pm Sunday (syllabus will be updated shortly)
• Reading Quiz #4 is available, due next Tuesday before class • It’s a little harder than usual because Chapter 5 is really important and I want to
give you some practice on something that only counts a little bit
• I will be posting the activity pages in Course Documents as a study resource
Homework #1
Save your answers as you go!
You can submit multiple attempts, but we will only grade the last one.
You must write your own answers to short-answer questions! Submitting an answer that is the same as someone else’s or from a webpage is plagiarism.
You’re better off trying to figure out the answers based on class materials (lectures, activities, the book, etc) than trying to Google the answers. I find a lot of wrong answers if I Google the questions.
“Anomaly” = difference from the average
From the National Weather Service yesterday:
What happened Normal Anomaly
What happened – Normal = Anomaly
The Ideal Gas Law
For gases (like the atmosphere), temperature, pressure, and volume are all related
pV= 𝑛𝑅∗𝑇
pressure volume number of molecules
temperatureconstant
If you change one of the variables, at least one of the other ones will have to change to compensate
The Ideal Gas Law
pV= 𝑛𝑅∗𝑇
If you change one of the variables, at least one of the other ones will have to change to compensate
Understanding the relationships: pretend everything = 1
pV= 𝑛𝑅∗𝑇 1*1 = 1*1*1
Now change something: T = 2
1*1 = 1*1*2
✓
✗
How could you balance this? Three ways:
1*1 = 0.5*1*21*2 = 1*1*22*1 = 1*1*2 p=2 V=2 n=0.5
✓ ✓ ✓
Energy! • There are numbers and equations here!
• You don’t have to memorize them. • I won’t make you do anything that requires a calculator on midterms.
• You will have to use them on homework. • You will have to understand how to use them, what they tell you,
what their inputs and outputs are.
• My hotplate outputs 900 W – almost as much as a 1 kW microwave
• We will run it on low. I guess that’s about 200 W coming out of the hot plate. The rock will get hot.
• I calculate 200 W into a 0.2 kg rock for 40 minutes = 3,000°C
• Will it get to 3,000°C?
Rock on a hot plate
What is energy?
Energy is that which allows work to be done
Make something move
Heat something up Make an object change shape
Make a chemical reaction happen
Make something melt or evaporate
Energy has to be added to make any of the happen
What is energy?
Energy comes in lots of forms!
Electricity
Gravity
Motion
Heat Light
Chemical
We’re going to focus on heat and
especially light
Check on the rock!
What is energy?
Energy comes in units of joules.
But we actually don’t care about joules!
In this class, we will study the rate at which energy is used or received.
The rate of energy use or flow is called power, and we measure it in watts (abbreviated W).
James Joule
James Watt
Common language: power = big vague concept Scientific language: power = energy per second
What is power?
The rate of energy use or flow is called power, and we measure it in watts (abbreviated W).
1 watt = 1 joule per second
Energy (Joules)
Po w
er (W
at ts
)In this picture → • if you think of the water as energy, • then the rate of the pour is the power. • The rate of water transfer to the glass is
like power, the rate of energy transfer from object to object.
Check on the rock!
How much is a watt?
1 watt each
60 watts (incandescent)
80 watts (resting
metabolism)
1000 watts = 1kilowatt
250 horsepower = 109,000 watts
= 109 kW
2,500,000 watts = 2.5 megawatts
18,000,000,000,000 watts = 18 terawatts (global energy use)
400,000,000,000,000,000,000,000,000 watts
4,900,000,000 watts = 4.9 gigawatts
Ways to transfer energy
Conduction Convection Radiation
They all happen in your kitchen!
Why the pan handle gets hot Why water boils all at once How a broiler works
Check on the rock!
Conduction – transferring heat energy by contact
Only the bottom of the pan is exposed to the flames, but all of the pan gets hot
Somehow the energy gets from here…
… to here Conduction is heat transferred by molecules in contact with each other. Basically the hot molecules bump against the cold ones and heat them up.
The longer things are in contact, the more heat is transferred
Conduction – transferring heat energy by contact
Some materials are good at conduction, like metal
You only have to touch the hot metal for a moment for enough
heat to be conducted to burn you
Some materials are bad at conduction, like air
You can reach into a 500° oven and the air doesn’t burn you
Check on the rock!
Conduction – transferring heat energy by contact
Warm ground heats the atmosphere from below (like the pan on the stove heats food)
Convection– transferring heat energy by circulation
Heating from below
Circulation is created
A boiling pot is the classic example
Check on the rock!
Conduction + Convection = heated atmosphere
Warm ground heats the atmosphere from below (like the pot on the stove), creating a circulation that moves heat upward (like the water in the pot)
Convection is important to rainfall. We’ll talk more about it in the coming weeks.
Light energy
Everything with a temperature gives off light. à everything has a temperature à everything gives off light
We call this blackbody radiation or thermal radiation
The hotter something is, the more light it gives off
hot hotter hottestnot hot
Check on the rock!
Light energy
Question: If everything gives off light, how come everything doesn’t glow in the dark?
Answer: Unless things are really hot (1000 0F or more) they give off light at wavelengths our eyes can’t detect.
there’s light we can’t see?
The electromagnetic spectrum
Light comes in a huge range of wavelengths
Our eyes can detect this part
Short wavelength = high energy
à ultraviolet (UV) gives us sunburns
à X-rays damage our cells
Long wavelength = low energy
à Radio waves don’t cook us
Check on the rock!
Color Bluer Redder
Wavelength Short wavelengths Long wavelengths
Energy High energy Low energy
Names Ultraviolet, X-Ray,
Gamma Infrared,
Microwave, Radio
The electromagnetic spectrum
How I remember that there’s more energy in blue light: “ultraviolet” sounds like “super purple”, so it must be beyond the blue end of the spectrum, and “ultra” sounds like it has lots of energy. I also know that UV has so much energy it burns me.
A brief aside
Light is also called electromagnetic radiation.
Scientists refer to sunlight as “solar radiation” or “insolation”.
Radiation is not radioactivity!
≠
Check on the rock!
m or
e lig
ht e
m itt
ed
0 0.5 1.0 1.5 wavelength (microns)
Objects giving off light
So what wavelengths does an object emit light at? It depends on the temperature of the object
Hotter things à more total power peaks at shorter (bluer) wavelengths
Notice the temps are in Kelvin What’s Kelvin???
visible range
Each curve represents an object at a different temperature
Temperature scales
Julien Emile-Geay USC, 2013
The three temperature scales
Kelvin Absolute, logical
Celsius Relative, logical
Fahrenheit Relative, illogical
William Thomson, 1st Baron Kelvin (1824 - 1907)
There’s a third temperature system!
K stands for Kelvin.
A degree of Kelvin is the same as a degree of Celsius. 0 K is absolute zero. Nothing can ever be colder than this.
Kelvin = Celsius + 273.15
William Thomson (Lord Kelvin) derived absolute zero in 1848.
Check on the rock!
m or
e lig
ht e
m itt
ed
0 0.5 1.0 1.5 wavelength (microns)
Wien’s Law
Hotter things à more total power peaks at shorter (bluer) wavelengths
visible range
Wilhelm Wien (derived law in 1893)
Question: which part of the lava is hottest?
A. B. C.
Check on the rock!
Question: which star is hotter?
A
B
There’s an equation for this (for interest only) wavelengthmax = 2898 / T
Surface temperature of the sun: ~5800 K
2898 / 5800 = 0.5 microns
Wien’s Law
Hotter things à more total power peaks at shorter (bluer) wavelengths
Green!
Check on the rock!
m or
e lig
ht e
m itt
ed
0 0.5 1.0 1.5 wavelength (microns)
Stefan-Boltzmann Law
Hotter things à more total power peaks at shorter (bluer) wavelengths
visible range
Ludwig Boltzmann and Josef Stefan, derived law in 1884 and 1879
E = power (W/m2) σ = 5.67 x 10-8 T = temperature (°K)
Total energy emitted per second per square meter
E = σ T4
An infrared animal for everyone
More energy (in this case infrared) is being emitted by the hot parts of the animal
Dog noses are cold.
Check on the rock!
Stefan-Boltzman Law
Hotter things à more total power peaks at shorter (bluer) wavelengths
Power emitted goes up with the 4th power of temperature.
If you double the temperature, the power emitted becomes 16 times larger!
Practice with Google
Let’s practice with the rock!
Calculate how much energy the rock was giving off ! (Final temp = 84.9 °C)
Power into rock from
hot plate
Power out of rock through radiation
Hot plate
1 micron0.1 microns 10 microns 100 microns