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HW1 return!
FERPA-approved military fashion!
Equipotential and field lines:
)cos(FdW E
Equipotential lines field lines:
Equipotential lines field lines:
Equipotential lines and the Electric field:Quantitative relation
V 100
V 50
V 10
V 100
V 50
V 10
?
Equipotential lines and the Electric field:Quantitative relation
V 100
V 50
V 10
V 50
Equipotential lines and the Electric field:Quantitative relation
V 100
V 50
V 10
V 50
V 9.94
V 50.1mm 1 d
Small enough scale, looks like parallel plate field: VEd
In direction of E
V 100
V 50
V 10
V 9.94
V 50.1mm 1 d
Small enough scale, looks like parallel plate field: VEd
(calculate)
In direction of E
d
Vd
0limE
V 9.94
V 50.1mm 1 d
V 50
Formally: Direction: downhill
Pretending that V(x) is like a topographic map:
A) The Electric field always points in the direction of steepest descent!
B) The magnitude of the electric field is given by the slope in that direction!
)lower towards(pointing ialequipotent VE
d
V
E
alternate units for E: V/m
Estimating Electric field strength:
Something Unrelated ….
Example 17.22 where’s V = 0?
μC3 μC2
5 cm
Two possibilities!
Example 17.22 where’s V = 0?
μC3 μC2
5 cmx
Possibility #1
Example 17.22 where’s V = 0?
μC3 μC2
x
Possibility #2
Conductors!
constant is V
VV 20
VV 20
VV 20
Electric fields just outside of a conductor are strong where the conductor is pointy:
Dielectric strength of a material = maximum electric field theMaterial can stand before breaking down.
Dielectric strength of air = 3 × 106 V/m
Example: Voltage of a 15 cm spark!
Capacitance:
Capacitance:
Q
Q
Must be at different potentials!
Capacitance:
Q
Q
V
QV CVQ
Capacitance: QV
CVQ
(F) FaradVolt
Coulomb
d
AC 0Parallel-plate capacitor:
22120 m N / C 1085.8
10 c
m
0.1 cmSquare plates
10 cm
Parallel-plate capacitor: Q = ?
V20V
d
AKC 0Parallel-plate capacitor:
22120 m N / C 1085.8
Dielectric constant!
0.1 cm
10 c
m
Dielectric!
Square plates
Quartz: k = 4.3
Leyden-jar demo
V de G demo again, big and little spheres – spark size
