DHS Current of Electricity

1 | Page ___________________________________________________________________________ Content • Electric current • Potential difference • Resistance and resistivity • Sources of electromotive force Learning Outcomes Candidates should be able to: (a) show an understanding that electric current is the rate of flow of charged particles. (b) define charge and the coulomb. (c) recall and solve problems using the equation Q = It. (d) define potential difference and the volt. (e) recall and solve problems using V = W/Q. (f) recall and solve problems using P = VI, P = I2R. (g) define resistance and the ohm. (h) recall and solve problems using V = IR. (i) sketch and explain the I-V characteristics of a metallic conductor at constant temperature, a semiconductor diode and a filament lamp. (j) sketch the temperature characteristic of a thermistor. (k) recall and solve problems using R =ρl/A. (l) define e.m.f. in terms of the energy transferred by a source in driving unit charge round a complete circuit. (m) distinguish between e.m.f. and p.d in terms of energy considerations. (n) show an understanding of the effects of the internal resistance of a source of e.m.f. on the terminal potential difference and output power. Current of Electricity (For ALL H1 and H2 Physics Students) Home Based Learning Please log-in to AskNLearn individually H1/H2 PHYSICS (2013) Instructions for HBL Learning Outcomes covered for H1 Physics includes (a) – (e) and (j) – (m) and H2 Physics includes the whole topic from (a) – (n). The requirement for Current of Electricity is the same for H1 and H2 Physics hence the notes is the same. Participation in online forum, quiz is compulsory. H1 Students This HBL is a revision from your Junior High Physics hence you should spend less than an hour on the notes to read up to page 11 and another 45 min to complete the tutorial, online discussion and online assessment (part 1) H2 Students Learning outcome (n), internal resistance of a source of e.m.f., is new content not learned in Junior High, hence you should spend more time going through page 12 -14 of your notes and go through the rest of the materials on AskNLearn. Suggested Time Allocation H1 Students 1.5 hr – Lecture notes and tutorial 0.5 hr – Online forum and assignment H2 Students 3.0 hr – Lecture notes and tutorial 1.0 hr – Online forum and assignment Optional – Self-explore Yenka Physics

Topic 2 | Page CONTENT Page Concept Map 3 1.0 Flow of Charged Particles 4 1.1 Charge and the Coulomb 4 1.2 Electric Current 4 2.0 Potential Difference and Electromotive Force 6 2.1 Electric Potential Difference 7 2.2 Electromotive Force 7 2.3 Differences between Electromotive Force and Potential Difference 8 2.4 The Volt 8 3.0 Resistance 8 3.1 The Ohm 9 3.2 Ohm’s Law 9 3.3 I-V Characteristics 9 3.3.1 Metallic Conductor at Constant Temperature 10 3.3.2 Filament Lamp 10 3.3.3 Semi-Conductor Diode 10 3.3.4 Negative Coefficient Thermistor 11 3.4 Resistivity 11 4.0 Electrical Power 12 4.1 Power Rating 13 4.2 Effects of Internal Resistance of a Source 14 Definitions List 16 Tutorial 18 H1 students, your HBL is only 2 hours, you just need to learn up to 3.4. The rest will be discussed in class next year. H2 students are to complete this whole set of notes

Topic 3 | Page Concept Map

4 | Page Learning Objective: Candidates should be able to: (a) show an understanding that electric current is the rate of flow of charged particles. (b) define charge and the coulomb. (c) recall and solve problems using the equation Q = It. 1.0 Flow of Charged Particles 1.1 Charge and the Coulomb Charge is a property of some elementary particles (particles of matter which cannot be subdivided into smaller matter) that gives rise to interactions between them and consequently to the host of material phenomena described as electrical. Charge occurs in 2 forms, conventionally described as positive and negative. The natural unit of negative charge e (1.60 x 10 -19 C) is the charge on an electron, which is equal but opposite in effect to the positive charge on the proton. The SI unit of charge is the coulomb (C) which is defined as the quantity of electric charge transported through a cross -section of a conductor in one second by an electric current of one ampere. 1.2 Electric Current An electric current , which is a SI base quantity measured in the SI base unit of ampere (A), consists of charges in motion from one region to another. When this motion takes place within a conducting path that forms a closed loop, the path is called an electric circuit . In metals, current is due to the flow of electrons which carry negative charges. In semiconductors, gases and electrolytes, current can be attributed to both positive and negative charges. It should be noted, however, that the direction of the current is, by convention, the direction that positive charges move. At room-temperature, ordinary metals such as copper or iron have electrons which are free to move within the physical constraints of the material. These free electrons move randomly in all directions, somewhat like the molecules of a gas but with much greater speeds (of the order of 10 6 m s-1). Considering a cross-section of the material, the rate at which electrons pass the cross -section in one direction is equal to the rate at which they cross in the other direction. Hence, the net flow of charge through the cross-section is zero. + + + + + _ _ _ _ _ Direction of electric current

5 | Page If the ends of the conducting wire are connected to form a loop, all points on the loop are at the same electric potential and hence the electric field is zero within and at the surface of the conductor. Because the electric field is zero, there is no net transport of charge through the wire and therefore there is no current. However if the ends of the conducting wire are connected to a battery, all the points on the loop are not at the same potential. The battery sets up a potential difference between the ends of the loop, creating an electric field within the wire. The electric field exerts forces on the conduction electrons in the wire, causing them to move in the wire, thus creating a net movement of electric charges. Electric current flowing through a cross-section of a conductor is defined as the rate of flow of charges through it. Hence, the i nstantaneous current passing through a cross section of the conductor at a given time is given by 𝐼 = 𝑑𝑄 𝑑𝑡 Hence, if the electric current I is steady, then the total amount of charge Q which flows past a point in a time duration t would be given by 𝑸 = 𝑰𝒕 Worked Example 1 The diagram on the right shows a model of an atom in which two electrons move round a nucleus in a circular orbit. The electrons complete one full orbit in 1.0 x 10 -10 s. What is the current caused by the motion of the electrons in the orbit? 𝑰 = 𝑸 𝒕 = 𝟐�𝟏. 𝟔 × 𝟏𝟎−𝟏𝟗� 𝟏. 𝟎 × 𝟏𝟎−𝟏𝟎 = 𝟑. 𝟐 × 𝟏𝟎−𝟗𝐀 nucleus electron electron

6 | Page Worked Example 2 A high potential is applied between the electrodes of a discharge tube so that the gas is ionized. Electrons and positive ions move towards the positive and negative electrodes respectively. In each second, 5 x 10 18 electrons and 2 x 1018 singly charged positive ions pass a cross -section of the tube. Determine the current flowing in the discharge tube. The movement of both electrons and positive ions contribute to electric current. 𝑰 = 𝑸 𝒕 = �𝟓 × 𝟏𝟎𝟏𝟖 + 𝟐 × 𝟏𝟎𝟏𝟖��𝟏. 𝟔 × 𝟏𝟎−𝟏𝟗� 𝟏 = 𝟏. 𝟏𝟐𝐀 Learning Objective: Candidates should be able to: (d) define potential difference and the volt. (e) recall and solve problems using V = W/Q. (l) define e.m.f. in terms of the energy trans