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Quantum Electrodynamics

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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  1. Quantum Electrodynamics The Cartoon Version

  2. 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

  3. 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!

  4. 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.

  5. 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?

  6. Model 1: Like Charges When photon sharing methods are compatible

  7. An electric charge tooling along… …and meets a like charge. With photons!

  8. 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…

  9. Wait a minute… By conservation of momentum, The exchange should look more like…

  10. The result: Two Like Charges Repel We do NOT see the photon exchange, which occurs too fast for any instrument to measure.

  11. Model 2: Unlike Charges When photon sharing methods are NOT compatible

  12. 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…

  13. 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!

  14. Unfortunately… • She lives in rural Alaska! • You cannot trade. • Your storing and trading mechanisms are incompatible.

  15. 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!

  16. 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…

  17. Now for the important part! What are you going to bring? • Clothes • Toiletries • The computer • Winter coat • Books, music… • Oh, yeah… and DVD’s?

  18. Are you really going to have room for your entire DVD collection? What are you going to do?

  19. That's right, you're going to chuck anything that's… • Old • Scratched • Chipped • Not your favorite • Something she's bringing anyway

  20. Now, back to those electric charges… Don’t need THIS anymore… Don’t need THIS anymore…

  21. What do we see? A spark! Hey, look at that!

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

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