1. Assessment for this course
Final exam: %
Mid-semester exam: %
Quiz x 3: %
Homework: %
If you do the homework, the rest should be easy!

2. In this course, we learn about four fundamental principles.

3. (3) Angular momentum principle 角动量原理 (4) Fields 场
(2) Energy principle
能原理
(3) Angular momentum principle
角动量原理
(4) Fields 场
For the rest of the semester:
Electric fields (电场)
Magnetic fields (磁场)

4. Electric forces

5. Two kinds of charge (电荷)
One type from rubbing
glass玻璃
One type from rubbing
amber 琥珀
(Greek word for amber: elektron)

6. Particles with the same charge repel each other (相斥)

7. Particles with the same charge repel each other (相斥)

8. Particles with opposite charges attract each other (相吸)

9. - + Benjamin Franklin (1706-1790)
Believed that electricity was a fluid (流体) that can flow between objects.
Too much fluid = “positive” charge
Too little fluid = “negative” charge - +
Physics would be less confusing if he named them the other way around!

10. Unit of charge: Coulomb (C)
Particle
Mass (kg)
Charge (C)
electron
9.11 x 10-31
-e = 1.60 x 10-19
positron +e
proton
1.67 x 10-27
anti-proton -e
muon
1.88 x 10-28
+e or –e (antimuon)
pion
2.48 x 10-28
+e or 0 or -e
Unit of charge: Coulomb (C)

11. Charge is quantized (量子化).
Everything is charged in multiples of e, the elementary charge.
Except…

12. Quarks (夸克) have a charge less than e, but are never seen alone.
Proton
Neutron
Pion

13. Nuclei and atoms 核 原子 电子 质子 中子

14. Electrostatic force 静电力 Coulomb’s Law: ; ε0 is called the
“permittivity of free space”

15. Example
How strong is the repulsive electrostatic force between two protons in a nucleus?

16. Example
How strong is the attractive gravitational force between two protons in a nucleus?
The strong nuclear force is what keeps nuclei from blowing themselves apart.

17. Electrostatic forces are strong.
We usually don’t notice because most things – including us – have the same amounts of positive and negative charge.
We are electrically neutral (中性).

18. +

19. + + +

20. + We can divide the world into two parts:
The source charges, which are causing the force
The test charge, which feels the force
This is a bit like the system/surroundings idea in mechanics.

21. What if we remove the test charge?+ + +
The “influence” of the source charges is still there… we call it the electric field.

22. Electric field
is a vector field. It has a value and a direction at every position in space and every moment in time:

23. Electric field Two steps when thinking about electric forces:
Calculate the field caused by the source charges.
Calculate the effect the field has on other charges.

24. Electric field of one point chargeq