Dr. Anatoli Frishman frishman@iastate.edu PHYSICS 112 Dr. Anatoli Frishman frishman@iastate.edu Web Page: http://course.physastro.iastate.edu/phys112

Physics Electromagnetism Optics Atoms & particles Mechanics Thermal properties Geometrical optics Wave optics Electrostatic Electric current Magnetism Condensed Mater High energy Biophysics Classical physics Quantum physics Relativistic physics Quantum relativistic physics

(electric and magnetic phenomena) Electromagnetism (electric and magnetic phenomena) This is about: electric charges, electromagnetic forces, and electromagnetic fields I. Electrostatics 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)

1a. Qualitative description Electric charge 1a. Qualitative description 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 1b. Elementary charge (smallest isolated charge found in nature) Electron has negative charge: -e Electric charge is quantized in units of e Quarks have 1/3e or 2/3e but cannot be isolated 1c. Electric properties of different materials (very brief, qualitative description) Microscopic picture (atom) Insulators, conductors, semiconductors, superconductors

(Charging by contact and by induction) The electroscope (Charging by contact and by induction) Positive Negative Neutral Repulsion Charged rod Inducted charge Stronger repulsion Charged rod (closer) disk Gold leaves (or vane)

No repulsion Repulsion 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. No repulsion Then we cut the grounding… Repulsion Electroscope charged by induction And remove the rod… The electroscope is now charged. The charge spreads now all over the object. No repulsion

Q1 Q2 r 2. Electrostatic forces (Coulomb’s law) Units: (coulomb) Example:

2a. Direction of Coulomb’s force Q1 Q2 Q1 Q2 r r Q1 and Q2 have the same sign Q1Q2 >0 Q1 and Q2 have opposite signs Q1Q2 <0 2b. Principle of superposition:

Example: Q1 Q2 Q3 r r

Example: Q Q a Q Q

Example: A 4. 9μC and a -3. 6μC charge are placed 16. 0 cm apart Example: A 4.9μC and a -3.6μC charge are placed 16.0 cm apart. Where can a third charge be placed so that it experiences no net force? – d x To experience no net force, the third charge Q must be closer to the smaller magnitude charge (the negative charge). The third charge cannot be between the charges, because it would experience a force from each charge in the same direction, and so the net force could not be zero. And the third charge must be on the line joining the other two charges, so that the two forces on the third charge are along the same line. Equate the magnitudes of the two forces on the third charge, and solve for x > 0.

Example: Compare the gravitational attraction and the electric repulsion of two electrons Big! Example: A student 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!