Unit 3 Section 1 Electrostatics
Electrostatics Electrostatics is the study of stationary electric charges. Only two types of electric charge have been discovered, positive and negative. proton (1.673 x 10 27 kg) electron (9.11 x 10 31 kg) neutron (1.657 x 10 27 kg) The SI unit for measuring the magnitude of electric charge is the coulomb (C). q = Ne q > magnitude of electric charge (C) N > number of charges e > elementary charge (C) e = 1.60 x 10 19 C represents only the magnitude of the charge on a proton or electron Example: How many electrons are there in 1.00 C of negative charge? (6.25 x 10 18 ) NOTE: 1 electron = 1.60 x 10 19 C 1 proton = 1.60 x 10 19 C 1.00 C = 6.25 x 10 18 electrons
Transfer of Charge Electric charge can be transferred from one object to another. Usually electrons are transferred. Add two electons Remove two electons 2 +2 Separation of charge often occurs when two unlike materials are rubbed together electrification by friction. 22:15 https://www.youtube.com/watch?v=x1 SibwIPM4&list=PLyQSN7X0ro2314mKyUiOILaOC2hk6Pc3j&index=2
Static Electricity Series fur acetate glass wool silk cotton vinyl rubber amber (tree resin) A material will lose electrons when rubbed with a material lower in the list. Examples: (i) rubber and silk (ii) silk and glass (iii) glass and cotton (iv) wool and vinyl
Charging by Conduction The process of giving one object a net electric charge by placing it in contact with another object that is already charged is known as charging by conduction. When a negatively charged rod touches a metal sphere, some of the excess electrons from the rod are transferred to the sphere. Once the electrons on are the sphere, they repel one another and spread out over the sphere's surface. The insulated stand prevents the electrons from flowing to the earth, where they would spread out even more. When the rod is removed, the sphere is left with a negative charge distributed over its surface. negatively charged rod metal sphere metal sphere insulated stand insulated stand What if a positively charged rod was used instead? positively charged rod metal sphere metal sphere insulated stand insulated stand Electrons from the sphere would be transferred to the rod leaving the sphere with a positive charge.
Charging by Induction The process of giving one object a net electric charge without touching the object to a second charged object is called charging by induction. negatively charged rod negatively charged rod metal sphere grounding wire insulated stand connection to ground
Law of Conservation of Electric Charge Charge can't be created or destroyed. The net electric charge of an isolated system is constant. Electric charges are involved in chemical reactions, electric circuits and radioactive decay. Three spheres are made of the same substance. total charge 4q 6q 2q
Electrostatic Force A fundamental characteristic of charge is that like charges repel and unlike charges attract. Demo Tape The force that charges exert on one another is called an electric force or electrostatic force. FF F q 1 q 2 r F FF q 1 q 2 r
Coulomb's Law French physicist Charles Augustin Coulomb (1736 1806) carried out a number of experiments to determine how the electric force that one point charge applies to another depends on the amount of each charge and the separation between them. F α q 1 q 2 F α 1 r 2 F = k q 1 q 2 r 2 F > magnitude of the electrostatic force (N) q 1 > magnitude of point charge 1 (C) q 2 > magnitude of point charge 2 (C) r > distance between point charges (m) k > proportionality constant k = 9.0 x 10 9 Nm 2 NOTE: Charges are often given in units of μc (1μC = 10 6 C). C 2 The electrostatic force is attractive if the charges have unlike signs and repulsive if the charges have like signs.
Think About: Two charges, q 1 and q 2, are separated by distance, r, and exert a force, F, on each other. What new force will exist if: a) q 1 is tripled? b) r is doubled? c) r is halved? d) q 2 is doubled and r is tripled?
Try: Two charges, q 1 and q 2, are separated by distance, r, and exert a force, F, on each other. What new force will exist if: a) q 1 is tripled? b) r is doubled? c) r is halved? d) q 2 is doubled and r is tripled?
Electric Fields A charge creates an electric field around itself in all directions. If a second charge is placed at some point in the field, the second charge interacts with the field at that point. How can an electric field be detected and measured? Imagine that an electric field is produced by a source charge, q. To describe the field around charge q, a test charge*, q t, is placed at point in the field generated by q. *test charge a point charge with a magnitude so much smaller than the source charge that any field generated by the test charge itself is negligible in relation to the field generated by the source charge. and
An electric field can be illustrated using imaginary electric field lines (sometimes called force lines). *The direction of the electric field at any point is indicated by the arrow on the electric field line. *The strength of the electric field is indicated by the spacing of the field lines. (Use only positive test charges.) positive source charge: negative source charge: a positive source charge and a negative source charge:
Strength (Intensity) of an Electric Field Coulomb's Law can be written to describe the force between q and q t. Divide both sides by q t. F = kqq t r 2 F = kqq t q t r 2 q t F = kq q t r 2 F r q q t q E = F q t E = kq r 2 E magnitude of electric field intensity (N/C) F magnitude of electric force (N) q t magnitude of test charge (C) E magnitude of electric field intensity (N/C) k Coulomb's constant q magnitude of source charge (C) r distance between q and q t (m)
NOTE: To determine the directions of E and F and/or the type of charge (+ or ) of the source or test charge, use a diagram. q q t E and F point in the same direction. q q t E and F point in opposite directions.
Review Gravitational Potential Energy ΔE g = mgδh gravitational potential energy When released, the box accelerates downward. Δh F net = F g zero gravitational potential energy floor reference level New Situation Electric Potential Energy + + r (+) ( ) two oppositely charged plates When released, the charged particle accelerates downward. F net = F e
Formula for Electric Potential Energy Remember: So: W = Fd W = Fr F = kqq t r 2 W > work (J) F > magnitude of electric force (N) r > distance (m) W = Fr W = kqq t r r 2 W = kqq t r E Q = kqq t r E Q > electric potential energy (J) k > 9.0 x 10 9 Nm 2 C 2 q > source charge (C) q t > test charge (C) r > distance between charges (m)
Electric Potential Difference Electric potential difference (voltage) is defined as the work done against an electric field in moving a positive test charge from one point to another in an electric field. + +q t A B V = W q t V > potential difference/voltage W > work q t > test charge V = W q t V = E Q qt V = kqq t rq t V = kq r V > electric potential difference or voltage (J/C = V) k > 9.0 x 10 9 Nm 2 C 2 q > source charge (C) r > distance from source charge to point in field (m) V = Ed d + _ V > electric potential difference or voltage (J/C = V) E > magnitude of electric field (N/C) d > distance between charged plates (m)
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