Home End HolisticTuition CashPlants Chapter 2: Electricity Form 5 1 Physics Next > The study of matter

Home End HolisticTuition CashPlants Objectives: (what you will learn) Objectives: (what you will learn) 1) electric fields & charge flow 2) electric current & potential difference 3) series & parallel circuits 4) electromotive force & internal resistance 5) electrical energy & power Physics: Chapter 2 2 < Back Next >

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

Home End HolisticTuition CashPlants 4 < Back Next > Electric Fields Between a positive and a negative point charge Between two positive point charges Electric lines of force

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

Home End HolisticTuition CashPlants 6 < Back Next > Electric Fields +– +– + – FF 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. +– Negative ions Positive ions Candle flame spreading sideways between 2 plates due to attraction between oppositely charged ions and metal plates. Experiments to show existence of electric fields.

Home End HolisticTuition CashPlants 7 < Back Next > Electric Fields 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. +- Current, I electrons So, the charge carriers (electrons) move in the opposite direction to the current.

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

Home End HolisticTuition CashPlants 9 < Back Next > Potential Difference V = V = W Q Work done Charge = 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: Volt (V) = = J C -1 J C AB Moving 1 coulomb of charge Potential difference between 2 points

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

Home End HolisticTuition CashPlants 11 < Back Next > Electric Current Non-ohmic conductor Non-ohmic conductor A conductor that does not obey Ohm’s Law. I V 0 I V 0 I V 0 Dilute sulphuric acidFilament lampJunction diode Examples 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.

Home End HolisticTuition CashPlants 12 < Back Next >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. V I Resistance, R = The unit of resistance is the ohm (Ω). conductor V II Potential difference, V = IR

Home End HolisticTuition CashPlants 13 < Back Next >Resistance Factors that affect the resistance of a conductor: a.length of wire, l b.cross-sectional area, A c.type of material with resistivity, p d.temperature, T pl A Resistance, R = Based on a constant temperature: R T/ o C 0 MetalSemi-conductor R 0 T/ o C

Home End HolisticTuition CashPlants 14 < Back Next > Series Circuit I R1R1 R2R2 R3R3 V1V1 V2V2 V3V3 V V 1 = IR 1 V 2 = IR 2 V 3 = IR 3 When resistors are connected in series: a.Same current I is in all the resistors b.Potential difference, c. V = V 1 + V 2 + V 3 d.Effective resistance, R = R 1 + R 2 + R 3

Home End HolisticTuition CashPlants 15 < Back Next > Parallel Circuit I R1R1 I1I1 R2R2 I2I2 R3R3 I3I3 V When resistors are connected in parallel: a.Same potential differences across all resistors, V 1 R 1 R1R1 1 R3R3 1 R2R2 =+ + c. I = I 1 + I 2 + I 3 d.Effective resistance, b.Current in the resistors, V R1R1 I 1 = V R2R2 I 2 = V R3R3 I 3 =

Home End HolisticTuition CashPlants 16 < Back Next > 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 = 1.5 V V I r V R I E = V + Ir = I ( R + r ) E V R + r R r R = = 1 +

Home End HolisticTuition CashPlants 17 < Back Next > 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. From volt, V = J C -1 = Energy dissipated, E Charge, Q Energy dissipated, E = QV Q = It = IVtV = IR = I 2 Rt I = V/R V2tV2t R E = substitutions

Home End HolisticTuition CashPlants 18 < Back Next > Electrical Power Electrical power, P = Energy dissipated Time, t V2V2 R P = = I 2 RI = V/R = IVV = IR substitutions E = IVt Power rating of an electrical appliance is the power consumed by it when the stated voltage is applied. V2V2 P Resistance of the appliance, R = 1 unit of electrical energy consumed = 1 kW h = (1000 Js -1 )(3600 s) = 3.6 x 10 6 J Cost of electrical energy = units x cost per unit

Home End HolisticTuition CashPlants 19 Summary < Back What you have learned: 1.Electric fields & charge flow Thank You 2.Electric current & potential difference 3.Series & parallel circuits 4. Electromotive force & internal resistance 5.Electrical energy & power