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Transcript Electricity > Did you know?
Electricity
M.D.
The Professional Development Service for Teachers is funded by the
Department of Education and Skills under the National Development Plan
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 lightning 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 )
This 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 JC-1
Volt = The P.d between two points
is one volt if one joule of work is
done bringing one coulomb from
one point to another.
Potential at a point = This is the
p.d between a point and the Earth,
where the Earth is at zero
potential.
Capacitance (C )
Discovered independently
in 1745 by von Kliest and
van Musschenbroek using
the Leyden Jar while
studying electrostatics.
http://micro.magnet.fsu.edu/electromag/java/lightning/
Capacitance (C)
This 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: This stores charge
Parallel Plate Capacitor:
C=A
d
A = area of overlap of plates
d = distance between plates
= permittivity of dielectric (insulator between plates)
Energy Stored in a Charged Capacitor
To charge a capacitor one plate is
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/R
C/app.htm
http://www.thephysicsteacher.ie/lcphysicscapa
citance.html
Capacitors
Allow a.c. to flow but
block 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
Electric Current (I)
This is the flow of electric
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.
I = Q
t
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. (This the defined direction of an electric current).
d.c. = direct current flows in one direction caused by a
power supply.
a.c. = alternating current is when the current reverses
direction every so often e.g. mains is 100 times per sec.
Current is the same at every point in a series circuit.
Sum of current flowing into a junction equals sum of
current flowing out of junction
Ammeter = used to measure current and is always
connected in series in the circuit.
e {Galvanometer = sensitive
+_
ammeter/microammeter}
e-
Electric Current Summary
Current is the same at every point in a series circuit.
Sum of current flowing into a junction equals sum of current
flowing out of junction
Ammeter = used to measure current and is always connected
in series in the circuit.
Galvanometer = 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 Divide both sides by t (time)
W = VQ
t
t
P = VI
(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 circuit to
be measured.
Voltages in Series and Parallel
Electromotive Force
(e.m.f.)
E.M.F. (E): a voltage
applied to a circuit.
Unit: Volt
Electric cell: 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
electrodes
Dilute sulfuric acid and copper
sulfate is the electrolyte
Plates chemically react with the
acid leaving the plates charged
Copper electrode is a positive
anode
Zinc electrode is a negative
cathode
This simple cell can’t be
recharged as the chemicals are
used up as a current flows
e.m.f. ≈ 1 V
Primary Cell
This type of cell can’t be recharged.
Also known as a dry cell because the
electrolyte is generally 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.
Resistance (R )
This is the ratio of the
p.d. across a conductor
to the current flowing
through it.
– i.e. R = V
I
Unit: ohm ()
http://micro.magnet.fsu.edu/elect
romag/java/filamentresistance/
Ohm’s Law
This states that for certain
conductors (mainly metals) the
current flowing through them
is directly proportional to the
p.d. across them at a constant
temperature.
– i.e.
V = IR
http://micro.magnet.fsu.edu/electromag/ja
va/ohmslaw/
Series Vs Parallel
+
_
+
_
Bulb
Resistors in
Series and Parallel
R1
In series the total resistance is:
R2
R = R1 + R2 + R3
R3
In parallel the total resistance is:
R1
R2
1 =1+ 1+ 1
R3 R
R1 R2 R 3
http://lectureonline.cl.msu.edu/~mmp/kap20/RR506a.htm
Factors affecting resistance of a conductor
Resistance depends on;
– Temperature
– Material of conductor
– Length
– Cross-sectional area
Temperature
The resistance of a metallic
conductor increases as the
temperature increases. e.g.
Copper.
The resistance of a
semiconductor/insulator
decreases as the temperature
increases. E.g. Thermistor.
Factors affecting Resistance
of a conductor
Length:
Resistance of a uniform
conductor is directly
proportional to its length.
i.e. R L
Cross-sectional area:
Resistance of a uniform
conductor is inversely
proportional to its crosssectional area.
i.e.
R1
A
Factors affecting
Resistance of a conductor
Material:
– The material also affects the resistance of a
conductor by a fixed amount for different
materials. This is known as resistivity ().
R = L
A
= Rd2
4L
= constant of proportionality
Unit: ohm meter (m)
(For a wire with circular cross-sectional area)
Wheatstone bridge
Uses:
– Temperature control
– Fail-safe device (automatic
switch circuit off)
– Measure an unknown resistance
– R1 = R3
B
(When it’s balanced)
A
I
C
R2
R4
Metre Bridge:
R1 = (|AB|)
R2
|BC|
D
http://www.magnet.fsu.edu/education/tutorials/j
ava/wheatstonebridge/index.html
http://www.electronics2000.co.uk/calc/calc
wstn.htm
Potential Divider
This is connected directly across
the voltage and divides voltage
into the ratio of the resistances.
E.g A rheostat (variable resistor,
two fixed resistors.
The greater voltage is across the
greater resistor.
The sum of the voltages is the
voltage supply.
If one of the resistances is
extremely large then the voltage
across it is almost the same as
the voltage supply.
Effects of an Electric Current
1. Heat
2. Chemical
3. Magnetic
Joule’s Law:
– States that the rate at which heat produced in
a conductor is directly proportional to the
square of the current provided its resistance is
constant i.e. P = I 2R
In order to prevent power lines from overheating, electricity is
transmitted at a very high voltage (EHT: Extra High Tension).
From Joule’s law the larger the current the more heat produced
hence a transformer is used to increase voltage and lower current
i.e. (P = VI).
Effects of an Electric Current
Electrolysis = the chemical effect
of an electric current.
Voltameter = electrodes,
electrolyte and container.
Inactive electrodes = electrodes
that don’t take part in the chemical
reaction e.g. platinum in H2SO4
Active electrodes = electrodes that
take part in the chemical reaction
e.g. copper in CuSO4
Effects of an Electric Current
Ion = an atom or molecule
that has lost or gained 1 or
more electrons.
Charge carriers = In an
electrolyte the charge
carriers are + and – ions
carriers.
Uses:
Electroplating to make metal
look better, prevent corrosion
Purifying metals
Making electrolytic capacitors
Relationship between V and I
for conductors
Metallic conductor:
Negative electrons are the
charge carriers
Filament bulb:
Negative electrons are the
charge carriers
Semiconductor:
Negative electrons and positive
holes are the charge carriers
I
V
I
V
I
V
Relationship between V and I
for conductors
Active electrodes:
Positive and negative ions are the charge
carriers
Inactive(Inert) electrodes:
Positive and negative ions are the charge
carriers
Gas:
Positive and negative ions and electrons are
the charge carriers
Vacuum:
Electrons are the charge carriers
I
V
I
V
I
V
I
V
Domestic electric circuits
Electricity entering the home is
supplied at 230V a.c.
2 wires enter the house from the
mains: Live + neutral and pass through
the meter box.
These 2 wires pass into a distribution
box with fuses.
Domestic Electric Circuits
Radial circuit: for appliances that take
a large current. Each circuit has their
own live + neutral wires and fuse e.g.
cooker, electric shower.
Ring circuit: for connections to
sockets. Live terminals are connected
together as are the neutral terminals.
Lights: connected in parallel and a
number of them are connected to
the same fuse.
Domestic Electric Circuits
Safety in house circuits:
– Switch: should always be connected in the live
wire.
Fuse: piece of wire that will melt when a current
of a certain size passes though it. Connected to
the live wire.
Domestic Electric Circuits
Safety in house circuits
MCBs: miniature circuit breakers are found in the
distribution box. They are bimetallic strips(for small
currents) and electromagnets (for large currents). Can
be reset when the switch trips, faster than fuse.
RCDs: residual current devices protect sockets and
people against electrocution by detecting a difference
between current in live and neutral wire (30mA).
Domestic Electric Circuits
Safety in house circuits:
– Bonding: All metal taps, pipes, water tanks etc
are connected to the earth
– Earthing: Earth wire prevents electrocution
from touching metal parts of appliances by
providing a path of least resistance when faults
occur.
E.S.B
Kilowatt-hour (kW h)
• This is the amount of
energy used by a 1000 W
appliance in one hour.
• The E.S.B charge bills
based on the no. of units
(kW h) used in the home.
Credits
Slide 2: Lightning Bolt Image
http://www.msha.gov/Accident_Prevention/Tips/l
ightning.htm
Electric eel image ~ Amy Lebeau
www.nfpa.org/riskwatch/teach_eslp_pkk_04.htm
l
Slide 3: Animation ~ Irina Nelson and Johnny Erickson
www.slcc.edu/schools/hum_sci/physics/tutor/22
20/e_potential
Slide 4: None
Slide 5: First capacitor image
www.mainlinegroup.co.uk/jacksonbrothers/5
250.htm
Slide 6: Capacitor image ~ Christopher Borg
http://qarnita.tripod.com/comp.htm
Slide 7: Bulb and battery animation ~ David Chase
Edventures.com
http://discover.edventures.com/functions/termlib.
php?action=&termid=153&alpha=c&searchs
tring=
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_as
sessment.php
Slide 12: Voltages in series and parallel image ~
Graham Knot
http://ourworld.compuserve.com/homepages/g_
knott/elect27.htm
Slide 13: Lemon battery image and video link ~ Carol
and Wayne Campbell
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
Slide 14: None
Slide 15: Battery image ~ EDF Energy
www.edfenergy.com/powerup/keystage3/in/page
2.html
Slide 16: Lead-acid battery image ~ EUROBAT The
Association of European Storage Battery
Manufacturers.
www.mpoweruk.com/cell_construction.htm
Credits
Slide 2: Resistors image
www.sffej.net/educational/resistor_Colour.ht
m
Resistor colour codes
www.radiodaze.com/rescarbcomp.htm
Slide 3: George Ohm image~
www.past.dk/artefacts/photos/53/photo1113908435-89551-5995.tkl?o
Slide 4: None (Note: Use P, for previous and N,
for next on key board to go back and forth
between photos if no remote control available.
Both circuits are connected to a 12V power supply
and can be compared in terms of how bright the 3
bulbs are)
Slide 5: None
Slide 6: Temperature and resistance animation ~
Science Joy Wagon (www.sciencejoywagon.com)
www.regentsprep.org/Regents/physics/phys
03/bresit/default.htm
Slide 7: Cross sectional area and resistance
animation ~ Science Joy Wagon
(www.sciencejoywagon.com)
www.regentsprep.org/Regents/physics/phys
03/bresit/default.htm
Slide 8: Resistors image
http://homepages.nildram.co.uk/~vwlowe
n/radio/alarm/how2.htm
Slide 9: Sir Charles Wheatstone image ~ from the
BT Connected Earth Collection.
See www.connected-earth.com
Slide 10: Sunset Power Lines
www.tonyboon.co.uk/imgs/pages/powerlines
.htm
Slide 11: Hoffman Voltameter image
www.dalefield.com/earth/hydrogen1.html
Slide 12: Electroplating image ~
www.finishing.com/faqs/howworks.html
Slide 13: None
Slide 14: None
Slide 15: Circuit Breaker image ~ Edfenergy
www.edfenergy.com/powerup/keystage3/in/
page2.html
Slide 16: Circuit Breaker image ~ Edfenergy as
above
Light Circuit image ~
www.buzzybee.org/diy/projects/electri
cal/lighting/wiring.html
Slide 17: None
Slide 18: None
Slide 19: None
Slide 20: None