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GEO344Lecture5-EnergyTransfer.pdf

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