Today 326

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Today 3/26

• Capacitors
• Circuits
• Lab questions?
• HW 3/26 Circuits 1 Due Friday 3/28

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Capacitors
Capacitance tells me how many coulombs of charge
are stored in a capacitor when it has 1 Volt
across it.
A 25?F capacitor will have 25?C of charge on it
when it has 1 volt across it. Think of
capacitance as coulombs per volt.
Units coulombs per volt or FaradsEquation C
q/V
3
Capacitors
Think of storing propane in a tank. The amount
of propane depends on the pressure just as the
amount of charge depends on voltage.
We will use parallel plate capacitors so that all
ideas from parallel plates apply.
V Ed E ?/?o (two plates) ? Q/A
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Capacitors
How much charge is on the capacitor?
Think 6x10-6 Coulombs per Volt and you wont
need the equation!
72x10-6 C or 72?C
12 V
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Capacitors
What is the E-field inside the capacitor if the
plates have an area of 1 m2?
Note that capacitance depends on the area and
separation distance
Q 72x10-6 C or 72?C
E ?/?o 8.2x106 N/C
What is the distance between the plates?
V Ed d 12/8.2x106
1.5x10-6 m
12 V
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Capacitors and Energy
How many Joules of energy are stored in the
capacitor?
Ask yourself, How much work must be done to
charge the plates?
12 V
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Capacitors and Energy
How many Joules of energy are stored in the
capacitor?
As the battery moves charge from one plate to
another, the potential difference increases from
zero to 12 V. The average is 6 V and the energy
stored is q times 6 V (q?Vave).
12 V
Energy q?Vave 1/2q?V 1/2C?V2
C q/?V
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Rank the bulbs from brightest to dimmest. Are
any equally bright?
D
A
C
B
1
3
2
E
A B C gt D E
Play Current Show powerpoint presentation
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Rank the bulbs from brightest to dimmest. Are
any equally bright?
D
A
C
B
1
3
2
E
Current is charge in motion, how many coulombs
per second pass by
A B C gt D E
Rank the bulbs for current.
A B C gt D E
Rank the batteries for current.
2 gt 1 gt 3
10
Rank the bulbs from brightest to dimmest. Are
any equally bright?
A
A gets all the current while B and C each only
get part so A is brightest.
At the junction the current divides 50/50 since
both branches have the same resistance.
C
B
Rank the bulbs for current.
A gt B C
Rank the bulbs for brightness.
More flow, more glow.
A gt B C
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Rank the bulbs from brightest to dimmest. Are
any equally bright?
Since branch D-E has mor R than branch F (extra
something added in series) the current does not
divide 50/50.
D
F
E
Current favors the path of least R. Beware of
the words takes the path as it implies none
goes through D-E.
Rank the bulbs for current.
F gt D E
Current for D current for E as all that goes
through D also goes through E. (See Current
Show.)
Rank the bulbs for brightness.
F gt D E
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Conceptual Circuits

• More flow-more glow
• What goes around, comes around
• Current divides at junctions
• Resistance inhibits current
• add something in series more R
• add something in parallel less R
• Think in terms of networks.

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Networks
A
D
F
C
B
E
Which circuit has the least resistance and hence
the greatest current through the battery?
Series
Parallel
More R, less current through battery
Less R, more current through battery
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Rank for resistance
A
B
C
D
E
Compared to A, B and C have an extra clog on an
existing path.
Most R Least R
E
B
C
A
D
Compared to A, D and E have an extra path.