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Quantum Electrodynamics. The Cartoon Version. Placement: Late in the Year. 3 rd day of modern physics unit After units on waves, electricity, and magnetism Prerequisites: Electrical interactions: + and - Photons as quanta of energy Conservation of momentum. Feynman's Idea.

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Quantum Electrodynamics


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quantum electrodynamics

Quantum Electrodynamics

The Cartoon Version

placement late in the year
Placement: Late in the Year
  • 3rd day of modern physics unit
  • After units on waves, electricity, and magnetism
  • Prerequisites:
    • Electrical interactions: + and -
    • Photons as quanta of energy
    • Conservation of momentum
feynman s idea
Feynman's Idea
  • All electric charges love photons and can't get enough of them
  • "Electric charge" really means "a particle that collects photons"
  • Charges have different methods of collecting and storing their photons

I  photons!

electric charges collect photons like you and your friends collect dvd s
Electric charges collect photons like you and your friends collect DVD's.
  • You know you love them and can't get enough of them…
  • You're always on the hunt for more, and yet…
  • New ones are more valuable than old ones.
imagine yourself with your dvd s
Imagine yourself with your DVD's…
  • Would you like to get a new one for free? How likely is that?
  • Are you going to give yours up for nothing? Of course not!
  • But… might it be worth trading that second-season flashback episode that you've never really liked for a mint edition sweeps thriller?
model 1 like charges

Model 1: Like Charges

When photon sharing methods are compatible

an electric charge tooling along
An electric charge tooling along…

…and meets a like charge. With photons!

how does the interaction go
How does the interaction go?

Never liked this one anyway…

Can I have a photon?

Hmm... Okay!

No.

I'll trade you…

Never liked this one anyway…

wait a minute
Wait a minute…

By conservation of momentum,

The exchange should look more like…

the result two like charges repel
The result: Two Like Charges Repel

We do NOT see the photon exchange, which occurs too fast for any instrument to measure.

model 2 unlike charges

Model 2: Unlike Charges

When photon sharing methods are NOT compatible

so here s our electron strolling along
So here’s our electron, strolling along…

When he meets something as unlike as can be!

How???

But I have no arms to throw or catch.

Want to trade photons?

Why don't you come over here so we can both share?

Easy! I'll throw, you catch.

That is a problem. What do we do?

That's a problem!

That would be perfect! But neither of us have legs to walk with…

now let s think hard about that dvd analogy
Now let's think hard about that DVD analogy…
  • Imagine you've just met a fellow collector online.
  • She's smart, funny, and cool… and she's got all the episodes you've ever wanted.
  • She's even got a mutual friend to verify that she's not some 40-year-old pervert!
unfortunately
Unfortunately…
  • She lives in rural Alaska!
  • You cannot trade.
  • Your storing and trading mechanisms are incompatible.
good news
Good news!
  • Both you and your online friend have just gotten huge financial aid packages at your favorite college.
  • You'll both be going to school together next fall!
and remember
And remember…
  • She's smart, fun, and cool… and you two have the same tastes in everything.
  • And given that neither of you know anyone else on campus, wouldn't it be a great idea to…
now for the important part
Now for the important part!

What are you going to bring?

  • Clothes
  • Toiletries
  • The computer
  • Winter coat
  • Books, music…
  • Oh, yeah… and DVD’s?
are you really going to have room for your entire dvd collection
Are you really going to have room for your entire DVD collection?

What are you going to do?

that s right you re going to chuck anything that s
That's right, you're going to chuck anything that's…
  • Old
  • Scratched
  • Chipped
  • Not your favorite
  • Something she's bringing anyway
now back to those electric charges
Now, back to those electric charges…

Don’t need THIS anymore…

Don’t need THIS anymore…

what do we see
What do we see?

A spark!

Hey, look at that!

in conclusion
In conclusion…
  • Electric charges interact by trading or sharing photons.
  • Their apparent repulsion or attraction is the result of a "virtual" photon exchange!