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 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  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

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

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  kap23/RC/app.htm 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 + + _ 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

 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

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

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

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

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+

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:  Slide 2: Lightning Bolt Image  Electric eel image ~ Amy Lebeau   Slide 3:  Slide 3: Animation ~ Irina Nelson and Johnny Erickson   Slide 4:  Slide 4: None  Slide 5:  Slide 5: First capacitor image    Slide 6:  Slide 6: Capacitor image ~ Christopher Borg   Slide 7:  Slide 7: Bulb and battery animation ~ David Chase Edventures.com  Electric Motor animation ~ UK Motion Gallery   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   Slide 12:  Slide 12: Voltages in series and parallel image ~ Graham Knot   Slide 13:  Slide 13: Lemon battery image and video link ~ Carol and Wayne Campbell   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   Slide 16:  Slide 16: Lead-acid battery image ~ EUROBAT The Association of European Storage Battery Manufacturers. 