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Chapter 2: Electricity

Form 5. Chapter 2: Electricity. Physics. Next >. The study of matter. 1. < Back. Next >. Physics: Chapter 2.

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Chapter 2: Electricity

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  1. Form 5 Chapter 2: Electricity Physics Next > The study of matter 1

  2. < Back Next > Physics: Chapter 2 Objectives: (what you will learn)1) electric fields & charge flow2) electric current & potential difference3) series & parallel circuits4) electromotive force & internal resistance5) electrical energy & power 2

  3. < Back Next > electric field pattern – + Positive point charge Negative point charge Electric Fields Electric field: region where a charged body experiences a forceIt is shown by a field pattern that are lines of forces.line of force = path of a test charge in the fielddirection = motion of a free positive charge 3

  4. < Back Next > Electric Fields Electric lines of force Between a positive and a negative point charge Between two positive point charges 4

  5. Electric field between two parallel metal plates that are oppositely charged. < Back Next > Electric field between two opposite charges. Electric Fields 5

  6. + + + – – – < Back Next > Positive ions Negative ions Candle flame spreading sideways between 2 plates due to attraction between oppositely charged ions and metal plates. Electric Fields Experiments to show existence of electric fields. Ball coated with conductor hangs vertically in the centre because it is neutral. Ball oscillating between 2 plates, after it touches one side causing a force, F to repel the ball due to like charges. – + F F 6

  7. < Back Next > Current, I electrons + - Electric Fields Electric fields cause charges to move. Net movement of charges = electric current In the late 1700s scientists chose the direction of electric current to be the direction in which positive charges move in an electric field. They did not know that electrons and protons were the negative and positive charge particles, and that the electron moved much more easily. In a copper wire, the outer electrons of the copper atom move relative to the nucleus of the atom. 7 So, the charge carriers (electrons) move in the opposite direction to the current.

  8. < Back Electric charge, Q = Itunits Q in Coulomb, I in Ampere, t in second Next > Q I= , t=time t Electric Charge Basic unit of electric charge = Coulomb (C) Charge of a proton or electron =  ± 1.60 10-19 C A Coulomb of charge is a lot, at 6.25 x 1018 electrons – most objects have charges in the µC (10-6 C) range. C = A s Electric current = Rate of flow of electric charge 8

  9. W Work done < Back V= = Q Charge Next > Potential difference between 2 points Moving 1 coulomb of charge A B J Volt (V) = = J C-1 C Potential Difference Potential difference (V) between 2 points in an electric field = work done (W) in moving 1 coulomb of charge (Q) between the 2 points. Unit of potential difference: 9

  10. V < Back = constant I Next > Switch I I Rheostat A Conductor V 0 V Electric Current Ohm’s LawThe current (I) in a conductor is directly proportional to the potential difference (V) across the conductor if the temperature is constant. Ohmic conductorA conductor that obeys Ohm’s Law. Circuit used to find the relationship between current I and potential difference V for a conductor. 10

  11. I I I V V V 0 0 0 < Back Next > Examples Dilute sulphuric acid Filament lamp Junction diode Electric Current Non-ohmic conductorA conductor that does not obey Ohm’s Law. A circuit element is non-ohmic if the graph of current versus voltage is nonlinear. A filament lamp is a non-ohmic conductor since its resistivity, like most materials, varies with temperature. As the filament gets hot, the resistance increases quickly. 11

  12. V < Back Resistance, R = I Next > conductor I I V Potential difference, V = IR Resistance The resistance, R of a conductor is defined as the ratio of the potential difference V across the conductor to the current I in the conductor. The unit of resistance is the ohm (Ω). 12

  13. pl < Back Resistance, R = A Next > R R T/oC 0 0 T/oC Metal Semi-conductor Resistance Factors that affect the resistance of a conductor: a. length of wire, lb. cross-sectional area, Ac. type of material with resistivity, pd. temperature, T Based on a constant temperature: 13

  14. I R1 R2 R3 V1 V2 V3 V1 = IR1 V2 = IR2 V3 = IR3 < Back V Next > • R = R1 + R2 + R3 Series Circuit • When resistors are connected in series: • Same current I is in all the resistors • Potential difference, • c. V = V1 + V2 + V3 • d. Effective resistance, 14

  15. V V V I3 = I1 = I2 = R1 R3 R2 < Back Next > I R3 R2 R1 I1 I3 I2 1 1 1 1 V + = + R2 R1 R R3 Parallel Circuit When resistors are connected in parallel: a. Same potential differences across all resistors, V • b. Current in the resistors, • c. I = I1 + I2 + I3 • d. Effective resistance, 15

  16. < Back E = 1.5 V Next > r V I V I R R + r = 1 + = R E r V R Electromotive Force Electromotive force (e.m.f.), E Work done to drive a unit charge (1 C) around circuit – where the unit is volt, V = J C-1 Using a high resistance voltmeter Potential difference V < e.m.f. E because work is done to drive a charge through a cell with internal resistance, r. E = V + Ir = I(R + r) 16

  17. Energy dissipated, E < Back From volt, V = J C-1 = Charge, Q Next > substitutions Energy dissipated, E = QV Q = It = IVt V = IR = I2Rt I = V/R V2t E = R Electrical Energy The potential difference V across a conductor is the work done in moving a charge of 1 C across the conductor. The work done is transformed into heat which is dissipated from the conductor. 17

  18. Energy dissipated Electrical power, P = Time, t V2 V2 = IV V = IR substitutions P R < Back = I2R I = V/R Next > P = Resistance of the appliance, R = Electrical Power E = IVt Power rating of an electrical appliance is the power consumed by it when the stated voltage is applied. 1 unit of electrical energy consumed = 1 kW h = (1000 Js-1)(3600 s) = 3.6 x 106 J 18 Cost of electrical energy = units x cost per unit

  19. Summary What you have learned: • Electric fields & charge flow 2. Electric current & potential difference < Back 3. Series & parallel circuits 4. Electromotive force & internal resistance 5. Electrical energy & power 19 Thank You

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