Transcript Ohm*s Law - Liceo “Gandhi”

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
Presentazione Fisica, Salzano Antonietta VD
This experience on the first Ohm's law shows the direct
proportionality between the potential difference and the
current intensity
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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:
VA  VB
I
R
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.
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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.
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The electrical resistance of a uniform conductor is given in
terms of resistivity by:
R
 l
A
where l is the length of the conductor in SI units of meters, a is the crosssectional 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.
ρt= ρ [1+α (t-20)]
where ρt indicates the resistivity at temperature t, p the resistivity at the
temperature of 20 ° C and a coefficient α of the material considered.
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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).
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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:
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. i2 + i3 = i1 + i4
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The algebraic sum of the potenzial differences that you meet
along a mesh is equal to zero.
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.
v1 + v 2 + v3 - v4 = 0
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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:
The SI unit of
electromotive force is
the volt, the same
that is used to
measure the
potential and
tension;
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
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 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:
-fem+ri+Ri=0
i=fem/R+r
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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.
LEGGE
JOULE:
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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.
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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.
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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.
Photograph of an
electrode during a
glow discharge in
argon.
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.
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