s er ap eP m e tr .X w w w om .c Scheme of work – Cambridge International AS Level Physical Science (8780) Ph5 PHYSICS Unit 5: Electricity and magnetism Recommended prior knowledge Students should be aware of the two types of charge and charging by friction and by induction. They should be able to distinguish between conductors and insulators using a simple electron model. Context Current electricity is an important topic in the general education of any person in this modern world. An understanding of static charge and current electricity in this Unit is an essential prerequisite to further, more advanced studies in physics or physical science. Outline Students will become familiar with the basic definitions and equations used in the study of current electricity. The I-V relationships for different circuit components are investigated. Components are then linked together so that practical circuits may be studied. Electric fields and the motion of free charged particles are also studied as further examples of charges in motion. Syllabus ref Learning objectives Suggested teaching activities Learning resources Candidates should be able to There is a limited supply of past papers for the 8780 examination, therefore all examples of examination questions are taken from Physics 9702 AS papers. These reflect the type of question that candidates are likely to meet in the 8780 examinations. 10(a) 10(b) v1 2Y05 recall the two types of charge recall the laws of electrostatics Discussion and revision of basic ideas, simple demonstrations / class experiments showing the laws of electrostatics Cambridge International AS Level Physical Science (8780) Polythene and acetate rods, cloth and tissue to charge rods, means of suspending the rods. 1 Syllabus ref Learning objectives Suggested teaching activities 10(c) distinguish between conductors and insulators using a simple electron model Discussion of conductors and insulators, non-metals are generally conductors – whereas metals are conductors. Relate to simple models from chemistry section of the syllabus. 10(d) show an understanding of the concept of an electric field as an example of a field of force and define electric field strength as force per unit positive charge acting on a stationary point charge. Discussion: what is a field of force? electric field of force definition of electric field strength H.T. supply, various shapes of electrode, olive oil, semolina, petri dish, OHP (if available) 10(e) represent an electric field by means of field lines. Discussion: representation of an electric field – electric field lines Demonstration: electric field lines Examples including: May/June 2010, 9702 Paper 21, question 5 Oct/Nov 2007, 9702 Paper 1, question 27 Properties of field lines including spherical charge approximating to a point charge (Although the field shape of point charge is not formally on the syllabus it is useful for students to be aware of this common type of field) 10(f) recall and use E = V/d to calculate the field strength of the uniform field between charged parallel plates in terms of potential difference and separation. Field due to parallel plates, mention of edge effect. Field strength = V/d in centre region 10(g) calculate the forces on charges in uniform electric fields. Discussion: force = Eq is constant for constant E Discussion: force on particle gives rise to acceleration 10(h) apply the effect of a uniform electric field on the motion of Demonstration of deflection using ‘Teltron’ displacement tube Discussion: acceleration of electrons in a c.r.o. and the role of the x and y-palates. v1 2Y05 Cambridge AS Level Physical Science (8780) Learning resources Examples including: Oct/Nov 2009, 9702 Paper 12, question 26 Oct/Nov 2009, 9702 Paper 12, question 28 Oct/Nov 2007, 9702 Paper 1, question 26 Examples including: Oct/Nov 2010 9702 Paper 22 question 4 May/June 2009, 9702 Paper 21, question 6 Oct/Nov 2008, 9702 Paper 2, 2 Syllabus ref Learning objectives Suggested teaching activities Learning resources question 4 Oct/Nov 2009, 9702 Paper 12, question 27 May/June 2011 9702 paper 12 question 31 charged particles and apply to the c.r.o. 12(a) recall and use appropriate circuit symbols as set out in the ASE publication Signs, Symbols and Systematics. Discussion and revision: – why use circuit symbols? – symbols already met Handout of symbols based on Signs, Symbols and Systematics 12(b) draw and interpret circuit diagrams containing sources, switches, resistors, ammeters, voltmeters, and/or any other type of component referred to in the syllabus. Drawing circuits: meaning of ‘series’ and ‘parallel’ Experiment: interpreting and setting up circuits Circuit diagrams and corresponding components, leads 11(a) show an understanding that electric current is the flow of charged particles. Discussion: what is electric current? Evidence provided by (i) electrolysis (ii) migration of ions Direction of movement of charge – movement of electrons in metals – conventional current copper voltameter, d.c. supply h.t. supply, leads and croc clips, filter paper, ammonia solution, potassium permanganate crystals Revision of base units – unit of current (amp) is a base unit – unit of time (second) is a base unit 11(b) define the coulomb 11(c) recall and solve problems using Q = It v1 2Y05 Definition of charge and the coulomb in terms of base units. Q = It Cambridge AS Level Physical Science (8780) Examples sheet including May/June 2011 9702 Paper 11 question 31 Oct/Nov 2009, 9702 Paper 12, question 33 Oct/Nov 2007, 9702 Paper 1, question 30 3 Syllabus ref Learning objectives Suggested teaching activities 11(d) define potential difference and the volt. Discussion: charges in motion – where has the energy come from? Potential difference (p.d.) as energy per unit charge transferred from electrical to some other form. V = W/Q and / or as the work done per unit charge, revision of term work. Volt defined as joule per coulomb. 11(e) recall and solve problems using V = W/Q 11(g) define resistance and the ohm. 11(h) recall and solve problems using V = IR. Discussion: of resistance and Ohm’s law including its limitations. Formal definitions. Experiment: I-V characteristic of a wire (Note that the experiment is not intended to be rigorous. Rather, it provides practice at circuit building and meter reading). 11(f) recall and solve problems using P = VI, P = I2R. Revision of term power from Unit 2 Derivation of power = VI = I2R using V = W/Q, P = W/t, Q = It and V = IR Learning resources variable d.c. supply or battery and variable resistance, switch, length of resistance wire, croc clips, leads, ammeter, voltmeter (digital or analogue). Examples sheet including May/June 2010 9702 paper 22 question 6(a) May/June 2010 9702 paper 23 question 6 Oct/Nov 9702 paper 21 question 6 Examples sheet including May/June 2010, 9702 Paper 22, question 6(a) Oct/Nov 2009, 9702 Paper 12, question 30 May/June 2008, 9702 paper 2, question 6 Oct/Nov 2007, 9702 Paper 1, question 29 11(i) v1 2Y05 Sketch and explain the I-V characteristics of: (i) a metallic Experiment: I-V characteristic of a wire. Plot graph for forward and reverse voltage. Cambridge AS Level Physical Science (8780) Variable d.c. supply or battery and variable resistance, switch, length of 4 Syllabus ref Learning objectives Suggested teaching activities Learning resources conductor at constant temperature, Discussion: idea of ‘resistance’ to current flow Resistance and ohm defined - ratio V/I, not the gradient of a graph. R = V/I. Note: forward and reverse voltages should be plotted for all devices. Explanation in terms of constant resistance enamelled constantan wire on a former, croc clips, leads, ammeter, voltmeter, (digital or analogue), means of temperature control e.g. water bath and thermometer. Data logger etc (if available) (ii) a semiconductor diode Experiment: I-V characteristics of a semiconductor diode. Note: forward and reverse voltages. Explanation in terms of different resistance values The ideal diode and its I-V characteristic. Variable d.c. supply or battery and variable resistance, switch, semiconductor diode with protective resistor, croc clips, leads, ammeter, voltmeter, (digital or analogue) Data logger etc (if available) (iii) a filament lamp. Experiment: I-V characteristics of a filament lamp Note: forward and reverse voltages Explanation in terms of increase of resistance of a metal with temperature. Variable d.c. supply or battery and variable resistance, switch, filament lamp (e.g. 12 V, 36 W), croc clips, leads, ammeter, voltmeter, (digital or analogue) Data logger etc (if available) (iv) a thermistor (thermistors will assumed to be of the negative coefficient type) Experiment: temperature characteristic of a thermistor Explanation of graph in terms of large decrease of resistance (c.f. metal) with temperature rise variable d.c. supply or battery and variable resistance, switch, bead thermistor on insulated leads, croc clips, leads, milliammeter, voltmeter, (digital or analogue), means of temperature control e.g. water bath and thermometer, warm water Examples including: May/June 2011 9702 paper 23 question 5 May/June 2010, 9702 Paper 23, question 6 Oct/Nov 2009, 9702 Paper 22, question 6 v1 2Y05 Cambridge AS Level Physical Science (8780) 5 Syllabus ref Learning objectives Suggested teaching activities Learning resources 12(c) recall Kirchhoff’s first law and appreciate the link to conservation of charge. Discussion: charge conservation leading to statement of Kirchhoff’s first law 12(d) recall Kirchhoff’s second law and appreciate the link to conservation of energy. Discussion: energy conservation leading to statement of Kirchhoff’s second law 12(e) derive, using Kirchhoff’s laws, a formula for the combined resistance of two or more resistors in series. Derivation of R = R1 + R2 + …… Expt: resistors in series 12(f) derive, using Kirchhoff’s laws, a formula for the combined resistance of two or more resistors in parallel. Derivation of 1/R = 1/R1 + 1/R2 + …… Expt: resistors in parallel 12(g) solve problems using the formula for the combined resistance of two or more resistors in series. Worked examples 12(h) solve problems using the formula for the combined resistance of two or more resistors in parallel. Worked examples 12(i) apply Kirchhoff’s laws to solve simple circuit problems. Worked examples Examples May/June 2011 9702 paper 22 question 5(b)*(c) Oct/Nov 2011 9702 paper 22 question 5** Oct/Nov 2011 9702 paper 23 question 5*** May/June 2011, 9702 Paper 21, question 5 May/June 2011, 9702 Paper 23, question 5 *Redraw circuit with dotted boxes around batteries removed, relabel resistors R1, R2 and R3. Reword the stem to read ’3 resistors, R1, R2 and R3.’ ** Redraw circuit with dotted boxes around batteries removed. Reword stem (first 3½ lines) to read ‘A cell of e.m.f. E1 is connected in series with two resistors of resistance r1 and R1 and a uniform metal wire of total resistanceR2. A second cell of e.m.f. E2 is connected in series with a resistor of resistance r2 and a sensitive ammeter and is then connected across the wire at BJ.’ *** Redraw circuit with dotted boxes around batteries removed. Reword the stem to read ’3 resistors, of resistance 2.3 Ω, 5.5 Ω and 1.8 Ω ’ 12(j) show an understanding of the use of a potential divider as a source of variable p.d. Discussion: ‘sharing’ p.d. between two resistors in series. Experiment: showing the use of a long wire as a potential divider dissection of a small potentiometer with circular carbon track. Power supply, 1 metre resistance wire taped to a metre ruler, voltmeter, jockey, leads Discussion: p.d. along a current-carrying uniform wire, V ∝ l, with conditions v1 2Y05 Cambridge AS Level Physical Science (8780) 6