Transcript Energy Density

Energy Density
Leyden Jar
 A Leyden jar has metal foil on both the inside and
outside surface. The surfaces are connected each to a
voltage source. The electrical energy is stored
•
•
•
•
A) nowhere since there the charges are static.
B) on the metal foil.
C) in the glass of the jar.
D) in the space inside the jar.
Work Between
 Moving a charge from one
+
-
plate of a capacitor to another
requires work.
• Move positives from negative
plate
+
 As more charge moves the
field increases
• So does work needed
+
 The average work needed is
equal to energy stored.
W
1
1
CV 2  Q 2 / C
2
2
-
W
1
QV
2
Total Energy
Q  C1V + C2V
 The energy stored in a system
C1
+
-
Q1  C1V
of capacitors can be
determined from the energy
stored in each
E1  12 C1V 2
E2  12 C2V 2
C2
Q2  C2V
E  12 C1V 2 + 12 C2V 2
E
+
-
1
2
C1 + C2 V 2
Open Circuit
 A charged capacitor will hold
the charge unless there is a
path for the charge to move
freely.
 If the wires are disconnected
this is an open circuit.
+
-
Closed Circuit
 A wire connecting the plates
forms a conducting path.
 This is a closed circuit.
• Also called a short circuit
 A short circuit releases the
energy stored in the capacitor.
• Sudden release can cause
unwanted surges of charge
+
-