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Current Electricity 1 Copyright © Maire Duffy, Clonkeen College For non-commercial purposes only Enjoy! Please leave feedback on the Physics Homepage Forum
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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!!
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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
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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 d A = area of overlap of plates d = distance between plates = permittivity of dielectric (insulator between plates) Capacitor stores charge
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Capacitors Capacitor 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
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EnergyStored in a Charged Capacitor 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/RC/app.htm http://lectureonline.cl.msu.edu/~mmp/ kap23/RC/app.htm
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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 Q = It Unit Amp A or C s -1
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Electric Current Summary Electrons flow from – to + + _ e-e- e-e- 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 Conventional current flows from + to – i.e. flow of positive charge
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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 Summary Galvanometer is a sensitive ammeter/microammeter
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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: Note: W = VQ Divide both sides by t (time) W = VQ t t P = VI (P = W and I = Q ) tt
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Voltage (V) Voltages in series V = V 1 + V 2 + V 3 Voltages in parallel V 1 = V 2 = V 3 Voltmeter is used to measure voltage and is always connected in parallel with the part of the cct to be measured
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Voltages in Series and Parallel
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Electromotive Force (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 Cell Primary Cell Secondary Cell Thermocouple Mains
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Simple 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 + AnodeCathode e - e-e- Cu Zn 2+ Zn Cu 2+
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Primary Cell This type of cell can’t be recharged. Also known as dry cells as the electrolyte tends to be a chemical paste.
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Secondary Cell This is a cell that can be recharged Also known as an accumulator e.g. car battery is a lead-acid accumulator
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Credits Slide 2: Slide 2: Lightning Bolt Image http://www.msha.gov/Accident_Prevention/Tips/lightning.htm http://www.msha.gov/Accident_Prevention/Tips/lightning.htm Electric eel image ~ Amy Lebeau www.nfpa.org/riskwatch/teach_eslp_pkk_04.html 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 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.htmwww.mainlinegroup.co.uk/jacksonbrothers/5250.htm Slide 6: Slide 6: Capacitor image ~ Christopher Borg http://qarnita.tripod.com/comp.htm 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= 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 www.bbc.co.uk/science/robots/techlab/v_rollerbots.shtml Slide 8: Slide 8: None Slide 9: Slide 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 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 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 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 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 www.mpoweruk.com/cell_construction.htm
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