2. The Cartoon Version

3. 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

4. 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!

5. 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.

6. 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?

7. When photon sharing methods are compatible

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

9. How does the interaction go? No. Can I have a photon? I'll trade you… Never liked this one anyway… Hmm... Okay!

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

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

12. When photon sharing methods are NOT compatible

13. So here’s our electron, strolling along… When he meets something as unlike as can be! Want to trade photons? How??? Easy! I'll throw, you catch. But I have no arms to throw or catch. That's a problem! Why don't you come over here so we can both share? That would be perfect! But neither of us have legs to walk with… That is a problem. What do we do?

14. 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!

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

16. 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!

17. 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…

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

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

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

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

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

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