lectures 8,9 (ch. 25) electric current 1. drift velocity 2. ohms law 3. volt-amper characteristics...
DESCRIPTION
Drift velocity Drift velocity does depend on a sign of charges + ions in plasmas or electrolytes, holes in semiconductorsElectrons in metals, - ions in plasmas, etc.TRANSCRIPT
Lectures 8,9 (Ch. 25)
Electric Current1. Drift velocity2. Ohm’s law3. Volt-Amper characteristics4. Thermal dependence of resistance5. Resistors in series and parallel6. Electric power7.emf, battery8.Simple circuits
CautionSo far we studied electrostatics (equilibrium) Now we start to study electric current
(nonequilibrium state)The following statements are not correct in the
presence of electric current:• 1. E inside conductors=0• Electric charges reside on the outer surface of
conductor• 2. Inside conductors V=const=Vsurface
Drift velocity
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Drift velocity does depend on a sign of charges+ ions in plasmas or electrolytes, holes in semiconductors Electrons in metals, - ions in plasmas, etc.
Electric current is a flow of charges (charge transferred per unite time via a given cross section)
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If different types of carriers present:
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Ohm’s Law:RVI
Georg Ohm (1787 - 1854).
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Resistance
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SI unite of R: [R]=[V]/[I]=1V/1A=1Ω (Ohm)
ῤ is called a resistivity, [ῤ]=Ωm
σ=1/ῤ is called a conductivity
Volt-Amper characteristics
Ohm’s Law:RVI
R is constant (characteristic of the conductor)It is valid for many conductors in a wide range of conditions, but not always!
Semiconductor diode is a junction of two semiconductors withpositive (p) and negative (n) carriers
→p+
n-I
+Change of a polarity of the battery results in zero current.It can be used for rectification of the current.
Thermal dependence of R
In metals
)](1[
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lowTatinvalid
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ALR Hence if L(T),A(T) are negligible then )](1[ 00
TTRR Measuring R allows to find T (termistors) In semicoductors n~T→ῤ~1/T
Superconductors
10 Nobel prizes were given for studies of SC ;The last one in 2003 to theorists: Alexei Abrikosov, Vitaly Ginzburg, Anthony J. Legget
1911, Hg, Tc~4.2K , H.Kamerlingh Onnes , Nobel Prize in Physics in 1913 Up to 1986 Tc<20K1986 , Tc~40K Karl Müller and Johannes Bednorz, Nobel Prize in Physics in 1987 cuprate-perovskite ceramic materials, such as bismuth strontium calcium copper oxide (BSCCO) and yttrium barium copper oxide (YBCO); 1987, Tc~90K,….1993 Tc~135K still a record2008 Tc~55K, Fe-based superconductors
H.Kamerlingh Onnes,1853-1926
VitalyGinzburg,1916-2009
Applications: electromagnets, motors, generators, transformers, etc.Open problems: 1.Mechanism of HTS? Why it’s possible?2.How to sustain large current (high magnetic field)3.Fragility of the materials
Levitation
Resistors in series
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Resistors in parallel
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I1
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NB:Opposite to capacitors!
C=Q/VR=V/I
Example1.
Example 2
Electric Power
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Alternative current (ac current)
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How to get more light with two bulbs?
Bulb B
or ?
Thomas Edison (1847-1931)1882
How to get more light with two bulbs?
Bulb B
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less light! more light!
emf, batteryClosed loop
0Rloss of energy→need a source of emf (ε), a battery
I+
emf (ε) is a work per unite chargeby external (nonelectric force).Ideal case (neglecting losses in the battery): abV
IVP about
a bε
a b
εr
Terminal voltage and power output of the battery
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ab
2
Terminal voltage is the voltage between the electrods of the battery connected to an external circuit, i.e. it is a voltage supplied by the battery to an external circuit. Real battery includes internal resistance, r. If the current through the battery is from – to + then the terminal voltage is smaller then emf:
Terminal voltage and power input into the battery
If the current through the battery is from + to - then the terminal voltage is larger then emf:
rIIP
IVPIrV
out
about
ab
2
a b
The rate at which the battery is charged The rate at which the
battery is heated
Alternator (the battery with larger emf delivers the energy to the battery with smaller emf
Ammeter measures the current. It should be placed in series with the element of circuit where it measures the current. Ideal ammeter has resistance=0 in order do not disturb in the current it measures. Voltmeter measure V. It should be placed in parallel with the element across which it measures the voltage. Ideal voltmeter has resistance=∞ in order do not disturb the voltage it measures.
I
IV=0
I
Simple resistors circuits1.Open circuit. What ideal ammeter and voltmeter measure?
A
V
I=0 (infinite resistance )
V=ε, P=0rε
It’s dangerous to touch the ends!
V=120V, R(wet body)=1kΩ→I~0.1A→ fibrillations (chaotic beatings of the heart)Defibrillator: I~1A complete stop