toc 1 physics 212 and 222 circuit elements and electric power resistors capacitors inductors
TRANSCRIPT
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Physics 212 and 222Circuit Elements and Electric Power
Resistors
Capacitors
Inductors
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Physics 212 and 222Circuit Elements and Electric Power
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3
Physics 212 and 222Circuit Elements and Electric Power
Resistors slow down the electrons
I
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Physics 212 and 222Circuit Elements and Electric Power
Resistors slow down the electrons
+-
+-
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Physics 212 and 222Circuit Elements and Electric Power
In most instances, the voltage across a piece of material is proportional to the current that flows through it
This is known as Ohm’s law and is usually written as
The constant of proportionality is called resistance, R, and has units of Ohms (Ω)
IRV
In a circuit diagram we draw resistance like this
R
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Physics 212 and 222Circuit Elements and Electric Power
For many materials, the resistance is proportional to the length of the resistor and inversely proportional to the cross-sectional area
Table 28.2 in the text gives resistivity for some materials
The constant of proportionality is called resistivity, ρ, and is different for different materials
A
LR
As length increases, the electrons are blocked for a longer time, slowing them down even more. Thus, the current is smaller and the resistance is greater
As area increases, more electrons pass through it. Thus, the current is greater and the resistance is smaller
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Physics 212 and 222Circuit Elements and Electric Power
Power is defined as the change in energy per unit time
In the case of electric potential energy, we can write
The power may be written in different forms using Ohm’s law
IVt
qV
t
UP
IVP
RIP 2
R
VP
2
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Physics 212 and 222Circuit Elements and Electric Power
E
Δs
dkV 2
kE 2
EdV
+σ
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Physics 212 and 222Circuit Elements and Electric Power
V=0
V=4πkσd
-σ
d
E
d
A
kd
A
dk
A
V
QC 044
+σ
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Physics 212 and 222Circuit Elements and Electric Power
•-σ
•E0
•Ed
•V=4πkσd0/κ
•d
•V=0
•d0
0
0
EEEE d
0EE
•+σ
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Physics 212 and 222Circuit Elements and Electric Power
Capacitors store the electrons
+-
+-
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Physics 212 and 222Circuit Elements and Electric Power
A capacitor is two conductors of equal but opposite charge separated by a dielectric.
A dielectric is a substance made up of stationary dipoles.
The capacitance of a capacitor is given by the equation.
V
QC
Q is magnitude of the charge of one of the conductors.
V is the magnitude of the difference in potential between them.
The capacitance depends only on the geometry (shape and size of the capacitor, not on Q or V because they are proportional to each other.
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Physics 212 and 222Circuit Elements and Electric Power
For any capacitor, the electric potential energy is given by
22
2
1
2
1
2
1CV
C
QQVU
But where is that energy found?
For a parallel plate capacitor…
dAECVU 20
2
2
1
2
1
202
1
VolumeE
Uue
This is the electrostatic field energy density.
The energy lies in the electric field!
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Physics 212 and 222Circuit Elements and Electric Power
Inductors create magnetic fields
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Physics 212 and 222Circuit Elements and Electric Power
Resistors
Capacitors
Inductors