Transcript Warm-up - Edublogs

Warm-up
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Suppose you want to connect your stereo to
remote speakers. If each wire must be 20m
long, what diameter copper wire (ρ =
1.68x10-8 Ωm) should you use to keep the
resistance less than .10Ω per wire?
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Take out your skills homework to hand in.
1/31/08
Objectives
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Describe origination of charge flow in a
circuit.
Draw and interpret schematic diagrams.
Explain operation of a capacitor.
Discuss voltage and apply to series and
parallel circuits.
Describe charge flow during transient and
steady-state processes.
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Agenda
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Finish Resistance
Castle Kits- Sections 2 and 3
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Section 2: 1, 2, 5, 7, 9, 10-12
Section 3: 5, 7, 13
Parallel and Series Discussion
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Resistance of Wires
R = rL/A
-------------------------------The longer the wire, the
larger its resistance.
The larger the area, the
smaller the resistance.
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For wires with a circular
cross section, A = pr2
R = rL/A
= rL/pr2
--------------------------------The thicker the wire,
the less its resistance
Example
You want to keep the resistance in your 20
m cooper (pg. 535) speaker wires to less
than 0.10 Ω per wire. What diameter wire
should you use?
A = ρ (L/R) = (1.68 x 10-8 Ω m)(20 m)/ (0.10
Ω)
= 3.4 x 10-6 m2
 Cross sectional area is related to diameter
by A = pd2/4
 Therefore d = 2.1 mm
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CASTLE Kit Section 2
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Light bulbs are resistors; both allow charge to
flow but at a lower rate than a conductor
Flow rate is NOT the same as speed
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Wires have resistance
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Symbology
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Pg. 530, 532, 534, 513
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Batteries
Wires
Resistors/bulbs
Capacitors
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Conventional Current
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Conventional
current flows in the
direction a positive
charge would
move, even though
this is opposite the
flow of electrons
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Charge
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Conventional meaning of + and –
+ is a more than normal charge
- is a less than normal charge
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True meaning of + and –
+ is an absence of charge
- is an excess of charge
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Circuit Diagrams
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Draw diagram of circuit with 3 batteries and 2
bulbs.
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Use arrow(s) to indicate the direction of
conventional charge flow.
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Flow of Charge
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Charge flows when there is a potential
difference (or difference in voltage)
between the ends of a conductor
 When there is no potential difference,
there is no current flow
The ampere is the unit of current flow
 An ampere is a current flow of one
coulomb of charge per second (6.24
billion billion electrons)
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Current is Flow of Charge
A steady current of 2.5 A flows in a wire for 4.0 min.
How much charge passed through any point in the
circuit? How many electrons was that?
2.5 A = 2.5 C/s (4.0 minutes = 240 s)
∆Q = I ∆t = (2.5 C/s) (240 s) = 600 C
 To count electrons
(600 C) / (1.6 x 10-19 C/electron) = 3.8 x 1023 electrons
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Current Flow and Resistance
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The current flowing in a circuit depends on the
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Voltage (sort of like the pressure in a hose)
Resistance to the flow of current (the diameter of the hose)
All elements in a circuit resist the flow of current to
some degree
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Things that use electricity to do work (lights, motors, etc.)
have relatively high resistance
 These are called electrical loads
Other parts of the circuit (wires, switches, etc.) have
relatively low resistance.
Parallel Resistors
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Parallel pair has less resistance than a single
bulb.
Lower resistance lets same pressure
differences drive more flow.
Circuit has lower resistance but equal
pressure differences (total) as earlier circuit.
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Ohm’s Law
voltage
current = resistance
or
I = V/R
and
V=IR
The unit of resistance is the ohm and its
symbol is Ω
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Ohm’s Law Example
A nine volt battery supplies power to a cordless
curling iron with a resistance of 18 ohms. How
much current is flowing through the curling iron?
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Ohm’s Law Examples
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CASTLE Kit Section 3
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Charge originates in all parts of circuit at
once.
Capacitors
Batteries have internal resistance.
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Where does charge originate?
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Activity 3.5: Set up circuit using ROUND
BULBS and BLUE CAPACITOR
What happens to the compass during the
charging of the capacitor? Discharging?
Draw schematic that shows flow.
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Capacitor
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2 layers of conducting material separated by
insulator
Capacitor plates
Terminals
Capacitance
Farad
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Closure
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Check Your Understanding:
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Explain how a capacitor charges and why it stops
taking charge.
Homework:
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Week 1 homework due Monday (should definitely
be “half-way” by now
“Ready” for Ohm’s Law Lab by Tuesday
Quiz Tuesday