Current Current Electricity 1Electricity 1

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ElectricityElectricity> Did you know?> 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 (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 (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

Parallel Plate Capacitor C = A

dA = area of overlap of platesd = distance between plates = permittivity of dielectric (insulator between

plates)

Capacitor stores charge

CapacitorsCapacitorsCapacitor uses

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

EnergyEnergy Stored in a Charged Stored in a Charged CapacitorCapacitor

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/RC/app.htm

Electric Current Electric Current (I)(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

Q = It Unit Amp A or C s-1

Electric Current Electric Current SummarySummary

Electrons flow from – to +

+_

e-

e-

d.c. direct current flows in one direction in a closed circuit, it is caused by a power supplya.c. alternating current is when the current constantly reverses direction e.g. mains current alternates 100 times per second

Conventional current flows from + to – i.e. flow of positive charge

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

Electric Current SummaryElectric Current Summary

Galvanometer is a sensitive ammeter/microammeter

Potential Difference Potential Difference (V)(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:Note: W = VQ Divide both sides by t (time)

W = VQ t tP = VI (P = W and I = Q )

t t

Voltage (V)Voltage (V) Voltages in seriesV = 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 ParallelVoltages in Series and Parallel

Electromotive ForceElectromotive Force (e.m.f.)(e.m.f.)

e.m.f. (E) is a voltage applied to a cct.

Unit Volt

http://video.google.com/videoplay?docid=-6226504780579469841

Electric cell is a device that converts chemical energy into electrical energy and is a source of e.m.f.Sources

Simple CellPrimary CellSecondary CellThermocoupleMains

Simple CellSimple Cell Copper and zinc plates are

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

Zn

-

Cu

+

Anode Cathode

e-e-

Cu

Zn2+

Zn

Cu2+

Primary CellPrimary Cell

This type of cell can’t be recharged. Also known as dry cells as the electrolyte

tends to be a chemical paste.

Secondary CellSecondary Cell

This is a cell that can be recharged Also known as an accumulator e.g. car battery is a lead-acid

accumulator

CreditsCredits Slide 2: Slide 2: Lightning Bolt Image

http://www.msha.gov/Accident_Prevention/Tips/lightning.htmElectric eel image ~ Amy Lebeau

www.nfpa.org/riskwatch/teach_eslp_pkk_04.html Slide 3:Slide 3: Animation ~ Irina Nelson and Johnny Erickson

www.slcc.edu/schools/hum_sci/physics/tutor/2220/e_potential Slide 4:Slide 4: None Slide 5:Slide 5: First capacitor image

www.mainlinegroup.co.uk/jacksonbrothers/5250.htm Slide 6:Slide 6: Capacitor image ~ Christopher Borg

http://qarnita.tripod.com/comp.htm Slide 7:Slide 7: Bulb and battery animation ~ David Chase Edventures.com

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:Slide 8: None Slide Slide 9: 9: None Slide 10:Slide 10:None Slide 11:Slide 11: Voltages in series image ~ Andrew Turner Primary School Science

www.primaryschoolscience.com/about/about_assessment.php Slide 12:Slide 12: Voltages in series and parallel image ~ Graham Knot

http://ourworld.compuserve.com/homepages/g_knott/elect27.htm Slide 13: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:Slide 14: None Slide 15:Slide 15: Battery image ~ EDF Energy

www.edfenergy.com/powerup/keystage3/in/page2.html Slide 16:Slide 16: Lead-acid battery image ~ EUROBAT The Association of European Storage Battery Manufacturers.

www.mpoweruk.com/cell_construction.htm