Current Electricity

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Current Electricity
Continuous motion of charged particles through a
potential difference/voltage
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What will happen if two oppositely charged
parallel plates connected by wire?How long?
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Static transfers charge quickly. Current is
continuous.
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Potential Dif/Voltage induces charges to move.
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Conventional current positive charges in
motionReal/electron current electrons in
motion.
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Current continuous flow of charges. Amperes
(A) measures rate of charge flow in q/t. 1 A
1 C/s passing a point or cross section of wire.
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Ex 1 100 C pass a section of wire every 5 sec.
How much current flows in the wire?
I q 100C 20 C/s or 20 A. t 5 s
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Ex 2 The current in a light bulb is 0.835 A.
How long does it take for a total charge of 1.67
C to pass a point in the wire?
I Q/t t Q/I t 1.67C
2.00 s 0.835 C/s
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Ex 3 If, in 1 second 6.25 billion billion
electrons pass through a point in a wire, what is
the current?

• 1 A

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Static charges can discharge quickly (shock)
but no continuous flow.Static discharge can do
a little work.Continuous flow moving charge can
do continuous work.
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Capacitor is charged plates that discharge
quickly.
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Capacitors used for computer keys flash bulbs.
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To get continuous flow of charge we need 1.
voltage (p.d.) to push charges. Also called emf
electromotive force. 2. Charge Pump do work
on charge which gains PE. Pumps are batteries,
generators. 3. Closed Circuit continuous
pathway for charges to flow metal wire.
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Q then to fall accl thru p.d. They gain KE
as they fall.
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Voltages push the charge. The higher the
voltage the more energy each electron has.
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Two 1.5 V batteries. AA D cell. AA has less
charge to lift. It doesnt last as long.
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Outlet has p.d. 120V.Dont stick a fork in it!!
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How does p.d. make charge flow??
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Pot. Dif. Causes electric field to spread through
wire at near light speed. e- in wire respond by
moving in field colliding with neighboring e-
starting to flow.
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Electric field in wire caused by voltage source
induces e- to move through.
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Bulbs, toasters, etc. convert KE of e- to other
forms of E heat, light etc.These cause the e-
to lose KE. Devices are called resistors or
loads. They slow down the e- so they resist
current flow. (Like paddle wheel in river).
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Electrons flow through filament in bulb. Some KE
lost to heat light.
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A closed circuit required for current flow.
Circuit contains voltage source, charge pump,
wires, and resistors.Direct Current (DC)
circuits have current flow of e- in 1 direction
fr. neg to terminal of battery.
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Simple circuit with battery and bulb.
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Resistance to Current Flow
Occurs in wires as well as appliances. Certain
factors affect how much resistance a wire will
offer to current flow. Resistance measured in
ohms W.
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Factors affecting wire resistance.1. Length2.
Area3. Temperature4. Type of material
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Length longer wire offers more resistance. More
chances for friction in wire.
Less resistance
More resistance
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Cross Sectional AreaThick wires offer less
resistance.
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TemperatureHot offers more RCold offers less R
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At a given temperature,
R resistancer constant of resistivityl
length A cross sectional areaSee table
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Ex 1 A 9.5 cm length of copper wire has a cross
sectional area of 2.5 x 10-3 m2. What is the
resistance of the wire at 20oC?
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From table r 1.72 x 10-8 Wm.So R (1.72 x
10-8 Wm)(.095 m ) 2.5 x 10-3 m2 6.5 x 10-7
W.
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Ex 2 Find the resistance of a copper wire 10 m
long and 1.2 x 10-9 m2 in area.
143 W.
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Film Electric Current Amps, Alternating, DC
current outlets
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Hwk Read 19 1 19 - 2Do p 695 3-5 p 717
15, 205 6 wksht.
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Resistance, Current, Potential Difference
Equations
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Resistance for a conductor is pd across its ends
divided by current flowing through it.

• R V/I units V/A or W.

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Potential Drop

• If a current flows in a resistor, there must be a
  pd across the ends of the resistor. The resistor
  drops or lowers the PE of the charge.

V in Volts J/C I in Amps R in W
V IR
A
C
B
R
I
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Ohms Law

• When temperature across a metallic resistor is
  constant, the current is proportional to pd
  across it.
• V IR V/I R constant.
• V volts J/C
• I current A, C/s
• R total resistance
• ohms W.

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What is the graph of Ohms Law? Usu V on y-axis,
I on x-axis, R is slope of direct linear.
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V RI yield direct linear relationship.V on Y
axis. I on x axis. R is slope of straight line.
R constant.
Switch axis 1/R is slope.
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Ex 1 A 30-V battery maintains a current through
a 10- W resistor. Find the current in the circuit.
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V IRI V/R30 V/10 W 3.0 A
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Ex 2 What is the current flow through an 80-W
coil when it is connect to a generator supplying
120-V?
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V IR I V/R120V/80 W 1.5 A
Ex 3 What is the rate of electron flow through
the coil above?
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Need to convert coulombs of charge from current
equation to electrons so use equivalence of e- in
1 C1.5 C/s x (6.25 x 1018 e-/C) 9.4 x 1018
e-/s pass through a point in the coil.
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Light bulbheats up as current goes
through.Watch axis.
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Hwk read txt 19.2 do pg 703 prb 4 6pg 718
ques. 24 27.
Do page 707 in class.
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Two types of current.DC Direct current. Charges
flow in one direction only.
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AC (alternating current)Charges go back and
forth.
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Power in Resistors

• Resistors convert Elc E to other forms.
• P is rate E used or supplied J/s or Watts. E is
  converted or dissipated in resistor.
• Power rating of 500 W means elc energy converted
  to other kinds at rate 500 J/s.

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Derive some equations relating power, voltage,
resistance, and current.
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P W. Vq. VI t t

• P VI
• The power dissipated is thermal energy and work
  done in resistor.

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Other Power Equations

• For devices that obey Ohms Law we use R V/I to
  derive other equations for power.
• P RI2 V2.
• R

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Work done

• Since Power is Work/Time,
• Power x Time Work

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Ex 1. A toaster draws a current of 2.0 A from a
120-V source. What is its power rating?

• P VI
• P (120V)(2.0 A)
• 240 VA or 240 W.

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Ex B A resistor of 12 W has a current of 2.0 A
flowing through it. How much energy is generated
in the resistor in one minute?

• P RI2.
• (12W)(2A)2. 48 W or 48 J/s.

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Power Ratings for Appliances

• Devices are rated by the power they use. A bulb
  rated 60 W 220 V means
• the bulb will dissipate 60 W when attached to a
  220 p.d.
• If a different p.d. is used, then it wont
  dissipate 60 W.

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Fuses

• As current flows, wires heat up.
• Fuses designed to break circuit if current
  becomes to high for the wires.
• Fuse should be rated just above the ideal
  operating current for a circuit.

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Kilowatt hours kWh.

• Power is a rate of energy use.
• Electric sold in kWh which is Pt.
• 1 kWh is energy delivered to home in 1hour.
• 1 kWh (1000 W/kW)(60 min/h)(60s/min) 3.6 x 106
  Ws 3.6 x 106 J

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Hwk Read 19.3 Textdo p 710 1 4,p713 1 - 3
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