1. Georg Simon Ohm Alfred (Erlangen, March 16, 1789 - Monaco, Germany, July 6, 1854) was a German physicist and mathematician. As a high school teacher, Ohm began his research with the new electrochemical cell, invented by Italian scientist Alessandro Volta. Using equipment of his own creation, Ohm formulated two laws. Presentazione Fisica, Salzano Antonietta VD

3. This experience on the first Ohm's law shows the direct proportionality between the potential difference and the current intensity

4. Ohm's law states that the current through a conductor between two points is directly proportional to the voltage across the two points. Introducing the constant of proportionality, the resistance, one arrives at the usual mathematical equation that describes this relationship: Presentazione Fisica, Salzano Antonietta VD where I is the current through the conductor in units of amperes, V is the voltage measured across the conductor in units of volts, and R is the resistance of the conductor in units of ohms. More specifically, Ohm's law states that the R in this relation is constant, independent of the current.

5. Material that obeys Ohm's Law is called "ohmic" or "linear" because the potential difference across it varies linearly with the current. All conductors whose characteristic curve is a straight line are called ohmic conductors. Presentazione Fisica, Salzano Antonietta VD

6. The electrical resistance of a uniform conductor is given in terms of resistivity by: where l is the length of the conductor in SI units of meters, a is the cross- sectional area (for a round wire a = πr2 if r is radius) in units of meters squared. The resistance is measured in ohms ( Ω ). ρ is the resistivity in units of ohm·meters. ρ t= ρ [1+ α (t-20)] where ρ t indicates the resistivity at temperature t, p the resistivity at the temperature of 20 ° C and a coefficient α of the material considered. Presentazione Fisica, Salzano Antonietta VD

8. Georg Gustav Robert Kirchhoff (Königsberg, March 12, 1824 - Berlin, 17 October 1887) was a German physicist and mathematician. He is occupied especially spectroscopy, electrical engineering and thermodynamics. Kirchhoff formulated the two famous laws for the calculation of the distribution of electric currents in a network of wires (1845 and 1847). Presentazione Fisica, Salzano Antonietta VD

9. The principle of conservation of electric charge implies that: At any node (junction) in an electrical circuit, the sum of currents flowing into that node is equal to the sum of currents flowing out of that node or equivalently: The algebraic sum of currents in a network of conductors meeting at a point is zero. This principle can be stated as: Presentazione Fisica, Salzano Antonietta VD n is the total number of branches with currents flowing towards or away from the node The current entering any junction is equal to the current leaving that junction. i 2 + i 3 = i 1 + i 4

10. The algebraic sum of the potenzial differences that you meet along a mesh is equal to zero. Presentazione Fisica, Salzano Antonietta VD Here, n is the total number of voltages measured This law is based on the conservation of energy.The principle of conservation of energy implies that The directed sum of the electrical potential differences (voltage) around any closed network is zero. The sum of all the voltages around a loop is equal to zero. v 1 + v 2 + v 3 - v 4 = 0

11. The f.e.m of a generator, equal to the d.d.p. between its poles open circuit, it is the ratio of the work done to carry a positive charge q from the negative pole to the positive pole and the charge itself: Presentazione Fisica, Salzano Antonietta VD In a ideal generator not have internal resistance that hinders the motion of charge carriers from one pole to the other, whereby the d.d.p. at the terminals of the generator it is exactly equal to the f.e.m The SI unit of electromotive force is the volt, the same that is used to measure the potential and tension;

12. In a real generator there is always a resistance to the motion of charged, for which, if no current is flowing in the circuit, the d.d.p. at the terminals of the generator is exactly equal to the f.e.m., but if the current in the circuit circulates d.d.p. across the generator it is different (lower) from f.e.m. A real generator can be outlined using a resistor r in series to the generator itself. We have a circuit with a mesh then apply the second law of Kirchoff and has: Presentazione Fisica, Salzano Antonietta VD -fem+ri+Ri=0 i=fem/R+r

13. A metallic conductor traversed by current heats up because the electrons collide with the ions of the crystal lattice and give them the kinetic energy that they received the electric field which accelerates them. The transformation of kinetic energy of the electrons into internal energy of the metallic conductor is known as the Joule effect. Presentazione Fisica, Salzano Antonietta VD LEGGE JOULE:

14. Presentazione Fisica, Salzano Antonietta VD electrolyte = liquid which can conduct an electric current (must contain ions = acids, bases or salts dissolved or melted) electrolysis = process when the current passes through the electrolyte and substances are liberated, deposited or dissolved. The electric current flowing through the electrolytic solution is due to the ordered motion of the positive ions moving toward the cathode and to the motion of negative ions moving toward the anode.

15. Presentazione Fisica, Salzano Antonietta VD A gas in normal conditions is made from neutral molecules and therefore can not conduct electricity. A gas can become a conductor when subjected to ionizing agents, that produce positive ions, negative and electrons. The electric conduction in a gas can be studied by closing the gas in a tube with two electrodes, and by varying the voltage applied.The current in the tube will be produced by the motion of the positive ions, negative ions and free electrons.The characteristic of conduction in gas, is an increase of the current if the potential difference between the electrodes exceeds a value called the ignition voltage.

16. For difference of potential greater than the ignition voltage, the ions are so accelerated by the electric field between the electrodes, that bumping neutral molecules form new ions in a process called glow discharge. Presentazione Fisica, Salzano Antonietta VD The glow discharge is always accompanied by emission of light. This is because the ions accelerated by the electric field in addition to ionize the molecules, can excite them. After a short time the electrons return to their initial configuration and the energy previously absorbed is emitted as light. Photograph of an electrode during a glow discharge in argon.