Current Electricity 1
Copyright © Maire Duffy, Clonkeen College
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Electricity
> Did you know?
 7 percent of power generated at large central
stations is lost during transmission to the user
over high-voltage lines!
 1 lightening bolt has enough power to service
200 000 homes!!
 An electric eel can produce a voltage of up to
650 Volts!!
 20 mA of current running through your body
can stop your heart!!
Potential Difference (V)
 Potential difference is the work
done per unit charge to transfer a
charge from one point to another
(also Voltage)
i.e
V=W
Q
 Unit Volt V or J C-1
 Volt is the p.d. between two points
if one joule of work is done bringing
one coulomb from one point to the
other
 Potential at a point is the p.d.
between a point and the Earth,
where the Earth is at zero potential
Capacitance (C)
 Capacitance is the ratio of the charge on a conductor
to its p.d.
i.e. C = Q
V
 Unit Farad F or C V-1
Capacitor stores charge
Parallel Plate Capacitor
C =A
d
A = area of overlap of plates
d = distance between plates
 = permittivity of dielectric (insulator between plates)
Capacitors
Capacitor uses
 Allow a.c. to flow but block
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d.c.
Tune in radio stations
(variable capacitor)
Smooth out variations in
d.c.
Camera flash
Filtering: allow certain
frequencies of an
alternating signal to pass
but block others
Energy Stored in a Charged Capacitor
 To charge a capacitor one plate is
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connected to + terminal and the other
to – terminal and the power supply is
turned on
An equal – charge builds up on one
plate and a + charge on the other
This charge remains even when
disconnected from the power supply
It can be discharged by connecting it
to a conductor
W = ½ CV 2
http://lectureonline.cl.msu.edu/~mmp/
kap23/RC/app.htm
Electric Current (I)
 This is the flow of electric
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
charge
{In a metal conductor it is
the flow of electrons}
Size of current in a conductor
is the amount of charge
passing any point of that
conductor per second
Q = It
Unit Amp A or C s-1
Electric Current Summary
 Electrons flow from – to +
Conventional current flows from + to – i.e. flow of
positive charge
d.c. direct current flows in one direction in a
closed circuit, it is caused by a power supply
a.c. alternating current is when the current
constantly reverses direction e.g. mains current
alternates 100 times per second

e-
+_
e-
Electric Current Summary
Current is the same at every point in a series closed circuit cct
Sum of current flowing into a junction equals sum of current
flowing out of junction
Ammeter is used to measure current and is always connected
in series in the cct
Galvanometer
is a sensitive ammeter/microammeter
Potential Difference (V)
 This can also be said to be the energy lost by 1
coulomb as it moves between 2 points in a circuit
i.e.
V=W
Q
Note:
W = VQ
W = VQ
t
t
P = VI
Divide both sides by t (time)
(P = W and I = Q )
t
t
Voltage (V)
 Voltages in series
V = V1 + V2 + V3
 Voltages in parallel
V1 = V2 = V3
Voltmeter is used to measure voltage and is always
connected in parallel with the part of the cct to be
measured
Voltages in Series and Parallel
Electromotive Force
(e.m.f.)
 e.m.f. (E) is a voltage
applied to a cct.
 Unit Volt
Electric cell is a device that
converts chemical energy into
electrical energy and is a
source of e.m.f.
Sources
Simple
Cell
Primary Cell
Secondary Cell
Thermocouple
Mains
http://video.google.com/videoplay?docid=-6226504780579469841
Simple Cell
 Copper and zinc plates are
e-
e-

Anode
Cathode
Zn
Cu
-
+
Zn
Zn2+


Cu

Cu2+

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electrodes
Dilute sulphuric acid and
copper sulphate is the
electrolyte
Plates chemically react with the
acid leaving the plates charged
Copper electrode is a positive
Cathode
Zinc electrode is a negative
anode
This simple cell can’t be
recharged as the chemicals are
used up as a current flows
e.m.f. generated is 1V
Primary Cell
 This type of cell can’t be recharged.
 Also known as dry cells as the electrolyte tends to
be a chemical paste.
Secondary Cell
 This is a cell that can be recharged
 Also known as an accumulator
 e.g. car battery is a lead-acid accumulator
Credits

Slide 2: Lightning Bolt Image

http://www.msha.gov/Accident_Prevention/Tips/lightning.htm
Electric eel image ~ Amy Lebeau

www.nfpa.org/riskwatch/teach_eslp_pkk_04.html
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Slide 3: Animation ~ Irina Nelson and Johnny Erickson
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Slide 4: None
Slide 5: First capacitor image
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www.slcc.edu/schools/hum_sci/physics/tutor/2220/e_potential
www.mainlinegroup.co.uk/jacksonbrothers/5250.htm
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Slide 6: Capacitor image ~ Christopher Borg
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Slide 7: Bulb and battery animation ~ David Chase Edventures.com
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http://qarnita.tripod.com/comp.htm
http://discover.edventures.com/functions/termlib.php?action=&termid=153&alpha=c&searchstring=
Electric Motor animation ~ UK Motion Gallery
 www.bbc.co.uk/science/robots/techlab/v_rollerbots.shtml
Slide 8: None
Slide 9: None
Slide 10:None
Slide 11: Voltages in series image ~ Andrew Turner Primary School Science

www.primaryschoolscience.com/about/about_assessment.php
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Slide 12: Voltages in series and parallel image ~ Graham Knot
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Slide 13: Lemon battery image and video link ~ Carol and Wayne Campbell
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http://ourworld.compuserve.com/homepages/g_knott/elect27.htm
www.hilaroad.com/camp/projects/lemon/lemon_battery.html
Note: google video player needs to be downloaded from the web page to play video clip
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Slide 14: None
Slide 15: Battery image ~ EDF Energy
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Slide 16: Lead-acid battery image ~ EUROBAT The Association of European Storage Battery Manufacturers.


www.edfenergy.com/powerup/keystage3/in/page2.html
www.mpoweruk.com/cell_construction.htm