Electricity Revision ELECTRICITY REVISION KEY CONCEPTS TERMINOLOGY & DEFINITION. Physical Sciences X-Sheets

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1 Electicity Revision KEY CONCEPTS In this session we will focus on the following: Stating and apply Coulomb s Law. Defining electical field stength and applying the deived equations. Dawing electical field lines aound chaged objects. Defining and explaining potential diffeence. Undestanding capacitance and its applications in a capacito. TERMINOLOGY & DEFINITION Chage: the oveall excess o deficiency of electons on an object. All electons ae identical and cay the same tiny quantity of chage Electostatics: the study of chages at est Elementay chage: 1,6 x C is the smallest known chage Electic field: a space within which a paticle beaing an electic chage expeiences a foce. SI unit is V.m 1 Electic field stength: the foce expeienced by a unit positive chage when placed at any point in the electic field. It is a vecto quantity Electic potential: the enegy associated with a chage at a paticula point within an electic field Electostatic foce: the electic foce between chaged paticles at est. Like all foces it is a vecto quantity Lightning: the sudden flow of electicity fom a thundecloud which has become chaged by the ubbing togethe of ai and wate molecules Potential diffeence: the diffeence in potential between two chaged points. Electons flow fom a low potential to a high potential fom the negative teminal to a positive teminal. It is a scala quantity. SI unit is volt (V) Quantised: chage consists of multiples of the basic chage, namely 1,6 x C Physical Sciences XSheets 9

2 Physical Sciences LAWS Coulomb s Law states: The electostatic foce between two point chages is diectly popotional to the poduct of the chages, and invesely popotional to the squae of the distance between them. Q 1 F 2 _ F 2 Q 1 = F 2 [Newton 3] Electic Field An electic field extends outwad fom evey chage and pemeates all of space. Thus when a second chage is placed nea the fist chage, the second chage expeiences a foce because of the electic field that is pesent. The diection of the electical field (E) is defined as the diection of the foce expeienced by a positive test chage (q) being placed in the field. By a test chage we mean a chage so small that the foce it exets does not intefee with the chage that is being measued. SYMBOLS, UNITS & EQUATIONS 10Physical Sciences F = k Q Q whee F is the foce between two points with chages Q 1 and is the distance between the chages. The popotionality constant is k = 9 x 10 9 N.m 2 C 2 The diection of the electic foce is always on the line joining the two objects. Like chages epel, and unlike chages attact. The type of foce must always accompany the magnitude in an answe. Accoding to Newton s Thid Law, the foce that one chage exets on anothe chage, is equal, but opposite to that exeted by the second chage on the fist chage. Both Coulomb s Law and Newton s Law of Gavitation ae invese squae laws. The diffeence between the two laws is that gavity is always an attactive foce, wheeas the electic foce can eithe be attactive o epulsive. When calculating with Coulomb s Laws, we usually ignoe the signs of the chages and detemine the diection based on whethe the foce is attactive o epulsive. All values must fist be conveted to the SI units of coulombs and metes befoe substitution in the equation. The chage on an electon is negative( e) and the chage on a poton is positive (e). The magnitude of the chage is the same. Since an object cannot gain o lose a faction of an electon, we say that electic chage is quantised. Physical Sciences XSheets

3 XAMPLE 1: CONVERTING UNITS Convet the following to SI units: 1.1 5nC 1.2 1,5uC 1.3 2pF 1.4 0,2mV XAMPLE 2 Two small identical sphees, suspended by long silk theads, ae fee to move as shown in the diagam. 100mm Q 1 = 2μC _ = 6μC a) Calculate the initial electostatic foce between the sphees. b) Descibe and explain the movement of the sphees. c) Calculate the chage on each sphee afte they have touched. d) Calculate the numbe of electons tansfeed when the sphees ae in contact. XAMPLE 3 A chage of 5 μc expeiences a foce of 0,012 N at a specific point in an electic field. Calculate the magnitude of the electic field at that point. XAMPLE 4 What is the electic field stength at a distance of 15mm away fom a chage of 5μC? XAMPLE 5 Two oppositely chaged paallel plates A and B ae 100 mm apat in a vacuum and have a potential diffeence of V acoss them. A small paticle q with a chage of 3 ηc moves feely fom plate A to B unde the influence of the electic field. A B 100 mm q V Physical Sciences XSheets 11

4 Physical Sciences a) Which is the positive plate? Explain you answe. b) Explain whethe o not the chage will move with unifom acceleation between A and B. (Conside only the effect of the electic field.) c) Calculate the magnitude and diection of the electostatic foce expeienced by the chage as it moves between the plates. XERCISE 1 Calculate the magnitude and diection of the electical field at a point P which is 30 cm to the ight of a point chage. Q = 3 x 10 6 C XERCISE 2 Thee objects A, B and C ae placed in a line and given diffeent chages. The chages ae: QA = 2 μc; QB = 5 μc and QC = 4 μc. a) Calculate FA on B and FC on B. b) Daw a foce diagam of the foces on B. 1,0m 0,5m c) Calculate the esultant foce on B. A B C 12Physical Sciences XERCISE 3 In a Millikantype expeiment, a chaged oil doplet of mass 1 x 1012 g hangs motionless between two oppositely chaged paallel plates. The plates ae 20 mm apat and the potential diffeence between them is 250 V. a) What is the sign of the chage of the oil doplet? b) Identify the foces acting on the oil doplet. Daw a fully labelled foce diagam. c) Calculate the magnitude of the chage on the oil doplet. d) What potential diffeence is equied between the plates in ode to hold a second dop, that has twice the mass and onetenth the chage of the fist dop, motionless? e) Give a eason why we would not expect an oil doplet to cay a chage of 3,2 x 1020 C. EXERCISE 4 A foce of epulsion of 400 N is expeienced by two point chages of magnitude 5 μc and 8 μc. 5.1 Calculate the distance between the chages. oil doplet 5.2 What does the fact that the foce is epulsive tell you about the chages? Physical Sciences XSheets

5 ANSWERS 1. E= 3 x 10 5 N.C 1 towads Q 2. a) F A on B = 0,09N towads C; F C on B = 0,72N towads A b) c) F esultant = 0,63N towads A 3. a) negative b) F E =EQ F g =mg c) 8 x C d) 5000 V e) Not a whole numbe multiple of the chage on one electon. Chage is quantised. 4. a) 0,03m b) Chages ae the same, eithe both positive o both negative. Physical Sciences XSheets 13