Current Electricity 1 Copyright © Maire Duffy, Clonkeen College For non-commercial purposes only Enjoy! Please leave feedback on the Physics Homepage Forum

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 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 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 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- + _

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 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 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 e- 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 e- Zn - Cu + Anode Cathode Zn Cu Cu2+ Zn2+

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 Electric eel image ~ Amy Lebeau http://www.msha.gov/Accident_Prevention/Tips/lightning.htm Electric eel image ~ Amy Lebeau www.nfpa.org/riskwatch/teach_eslp_pkk_04.html Slide 3: Animation ~ Irina Nelson and Johnny Erickson www.slcc.edu/schools/hum_sci/physics/tutor/2220/e_potential Slide 4: None Slide 5: First capacitor image www.mainlinegroup.co.uk/jacksonbrothers/5250.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&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 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/page2.html Slide 16: Lead-acid battery image ~ EUROBAT The Association of European Storage Battery Manufacturers. www.mpoweruk.com/cell_construction.htm