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 1

Electromagnetism (electric and magnetic phenomena) 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) 2

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 1.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 3

1c. Electric charge and the structure of matter (atom) (very brief, qualitative description) The particles of the atom are the negative electron, the positive proton, and the uncharged neutron. Protons and neutrons make up the tiny dense nucleus which is surrounded by electrons The electric attraction between protons and electrons holds the atom together. A neutral atom has the same number of protons as electrons. A positive ion is an atom with one or more electrons removed. A negative ion has gained one or more electrons. 4

5 1d. Electric properties of different materials (classification) (very brief, qualitative description) Conductors - charges can move easily Insulators - charges cannot move easily Semiconductors - intermediate between good conductors and good insulators Superconductors - charges can move without resistance There are no perfect conductors or perfect insulators Examples Good conductors: most metals, solutions of salts (like tap water)… Good insulators: plastics, rubber, glass, wood, air, pure water…

6 Although the word comes from the Greek for "rubbing", τρίβω (τριβή: friction), the two materials only need to come into contact and then separate for electrons to be exchanged. After coming into contact, a chemical bond is formed between some parts of the two surfaces, called adhesion, and charges move from one material to the other to equalize their electrochemical potential. This is what creates the net charge imbalance between the objects. When separated, some of the bonded atoms have a tendency to keep extra electrons, and some a tendency to give them away. In addition, some materials may exchange ions of differing mobility, or exchange charged fragments of larger molecules. 1e. Charging by contact. Triboelectric effect + most positive Air Human skin Leather Rabbit's fur Glass Quartz Mica Human hair Nylon Wool Cat's fur Silk Aluminium Paper (Small positive charge) Cotton (No charge) 0 Steel (No charge) Acrylic Polystyrene Rubber balloon Brass, Silver Gold, Platinum Polyester Polyurethane Vinyl (PVC) Silicon Teflon Silicone rubber Ebonite − most negative

Stronger repulsion Charged rod (closer) Repulsion Charged rod Inducted charge disk Gold leaves (or vane) The electroscope (Charging by contact and by induction) Positive Negative Neutral 7

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. 8

2. Electrostatic forces (Coulomb’s law) Q1Q1 Q2Q2 r Units: Example: 9

Q1Q1 Q2Q2 r Q 1 and Q 2 have the same sign Q 1 Q 2 >0 Q1Q1 Q2Q2 r 2a. Coulomb’s law in vector form 2b. Principle of superposition: Q 1 and Q 2 have opposite signs Q 1 Q 2 <0 10

Example: Q1Q1 Q3Q3 r Q2Q2 r 11

Example: Q QQ Q a 12

Example: A 4.7μC and a -3.5μC charge are placed 18.5 cm apart. Where can a third charge be placed so that it experiences no net force? x – d 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. 13

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! 14

Example: electrostatic painting Induced positive charge on the metal object attracts the negatively charged paint droplets. 15 Example: laser printer makes use of forces between charged bodies