1. PHYSICS 112 Dr. Anatoli Frishman frishman@iastate.edu

2. Physics MechanicsThermal properties ElectromagnetismOpticsAtoms & particles Electrostatic Electric current Magnetism Geometrical optics Wave optics Condensed MaterHigh energyBiophysics Classical physics Quantum physics Relativistic physics Quantum relativistic physics

3. Electromagnetism (electric and magnetic phenomena) 1.Electric charge 1a. Qualitative description I. Electrostatics This is about: electric charges, electromagnetic forces, and electromagnetic fields This is about: non-moving electric charges, electrostatic forces, and electrostatic fields. (With very good approximation, electrostatics could be applied to slow moving charges. Slow means that the speed of the considered charge is much smaller then speed of light: v<<c) There are two and only two types of electric charges Charges of the same type repel, and charges of different type attract (this is already qualitative description of electrostatic forces) These two types are referred to as positive and negative (+q and –q) Charges can be treated algebraically, and the net amount of electric charges in an isolated system is conserved (is not changed in any process). This is the law of conservation of electric charges

4. 1c. Electric properties of different materials (very brief, qualitative description) Microscopic picture (atom) Insulators Conductors Semiconductors Superconductors 1d. “Games” with electric charges Charging by contact Charging by induction Electroscope Electrometer 1b. Elementary charge:

5. Positive Negative Neutral disk Gold leaves (or vane) The electroscope

6. Positive Negative Neutral repulsion Charged rod Inducted charge

7. Positive Negative Neutral Stronger repulsion Charged rod (closer)

8. Positive Negative Neutral No repulsion If we ground the electroscope while the rod is there, the charges in the electroscope that were “escaping” from the rod flow to the ground.

9. Positive Negative Neutral No repulsion Then we cut the grounding…

10. Positive Negative Neutral Repulsion And remove the rod… The electroscope is now charged. The charge spreads now all over the object. Electroscope charged by induction

11. 2. Electrostatic forces (Coulomb’s law) Q1Q1 Q2Q2 r Units: Example:

12. Q1Q1 Q2Q2 r Q 1 and Q 2 have the same sign Q 1 Q 2 >0 Q1Q1 Q2Q2 r Coulomb’s law in vector form: Principle of superposition: Q 1 and Q 2 have opposite signs Q 1 Q 2 <0

13. Example: Q1Q1 Q3Q3 r Q2Q2 r

14. Q QQ Q a

15. Example: Compare the gravitational attraction and the electric repulsion of two electrons Big! Example: One of your friends can resist a force of 100 lb (450 N) with his arms apart. You give him two charged balls with charges Q and  Q to hold on each hand. How large a charge Q can he hold outstretched? Q -Q r ~ 1.5 m Less than a cell in your body!