ASRJC H2 D.C. Circuits notes

ANDERSON SERANGOON JUNIOR COLLEGE PHYSICS 9749 15-1 Additional Notes Topic 15: D.C. Circuits Content A Circuit symbols and diagrams B Series and parallel arrangements C Potential divider D Balanced potentials Learning Outcomes Candidates should be able to: (a) recall and use appropriate circuit symbols as set out in the ASE publication Signs, Symbols and Systematics (The ASE Companion to 16–19 Science, 2000) (b) draw and interpret circuit diagrams containing sources, switches, resistors, ammeters, voltmeters, and/or any other type of component referred to in the syllabus (c) solve problems using the formula for the combined resistance of two or more resistors in series (d) solve problems using the formula for the combined resistance of two or more resistors in parallel (e) solve problems involving series and parallel circuits for one source of e.m.f. (f) show an understanding of the use of a potential divider circuit as a source of variable p.d. (g) explain the use of thermistors and light-dependent resistors in potential divider circuits to provide a potential difference which is dependent on temperature and illumination respectively (h) recall and solve problems by using the principle of the potentiometer as a means of comparing potential differences Got a question to ask the lecturer, or doubts to clarify after attending the lecture? Key in your questions via the QR code above, or through https://tinyurl.com/j2 physicslectureq The lecturer will address your queries either in the following lecture, or through the link https://tinyurl.com/j2 physicslecturea , also accessible via the QR code below.

ANDERSON SERANGOON JUNIOR COLLEGE PHYSICS 9749 15-2 Additional Notes A Circuit symbols and diagrams A.1 Introduction Direct circuit  Direct current (in short d.c.) refers to current which flows in one direction only. In contrast, alternating current (in short a.c.) refers to current which changes direction periodically.  Direct current must be one-directional, not necessarily constant magnitude, i.e. it need not be a steady current In Fig. 15.1, Graph (a), Graph (b) and Graph (c) show examples of direct current whereas Graph (d) and Graph (e) show examples of alternating current Fig. 15.1 time current time current time current time current time current Graph (a) Graph (b) Graph (c) Graph (d) Graph (e)

ANDERSON SERANGOON JUNIOR COLLEGE PHYSICS 9749 15-3 Additional Notes Circuit symbols Symbol Description Symbol Description Switch Electric bell Fixed resistor Heater Variable resistor Thermistor Voltmeter Potential divider Ammeter Loudspeaker Galvanometer Lamp Cell Capacitor * Battery of cells Inductor * D.C. power supply Transformer with core Aerial * Microphone * Fuse Motor * Diode Light emitting diode (LED) Light Dependent Resistor (LDR) Earth Cross (no connection between wires) Junction connection (wires physically connected) Double junction connection Frame or chassis connection * *Less common in the H2 A-Level syllabus Fig. 15.2

ANDERSON SERANGOON JUNIOR COLLEGE PHYSICS 9749 15-4 Additional Notes Ammeter  An ammeter measures the rate at which electrons are flowing through a circuit at a given point.  It is connected in series with the part of the circuit through which the current is to be measured.  Ideal ammeters have zero resistance, so its inclusion in the circuit will not increase the effective resistance of the circuit. There will be no p.d. across an ideal ammeter. Voltmeter  A voltmeter measures the potential difference between any two points in the circuit.  It is connected parallel to the two points of the circuit, across which the p.d. is to be measured.  Ideal voltmeter have infinite resistance, so its inclusion in the circuit will not decrease the effective resistance of the circuit. There will be no current flowing through an ideal voltmeter. Thermistor  The resistance of a thermistor changes with temperature and it can be made of semiconductor material.  It acts like a variable resistor, except that the variation of the resistance is controlled by temperature. The resistance of a thermistor drops when temperature increases. Light-Dependent Resistor (LDR)  The resistance of LDR changes with light intensity.  It acts like a variable resistor, except that the variation of the resistance is controlled by light. The resistance of a LDR drops when light intensity increases. The term “thermistor” usually refers to the Negative Temperature Coefficient (NTC) thermistor.

ANDERSON SERANGOON JUNIOR COLLEGE PHYSICS 9749 15-5 Additional Notes A.2 Kirchhoff’s Current Law  Kirchhoff’s current law (or junction rule) states that the algebraic sum of currents entering a junction must be equal to the algebraic sum of currents leaving that junction.  It is based on the conservation of charge which means that the total charges entering a junction must be equal to the total charges leaving that junction.  Kirchhoff’s current law (or junction rule) is a consequence of the conservation of charge. No charge can accumulate at a junction, so the total charge entering the junction per unit time must equal the total charge leaving per unit time.  Since charge per unit time is current, so the total current entering the junction must equal to the total current leaving the junction. Fig. 15.3  Hence, for the junction in Fig. 15.3 above, I1 + I2 = I3 + I4. Check your understanding 1 The diagram shows a part of a circuit where four wires are connected at a junction. The current in 3 of the wires are shown in the diagram. Which of the following shows the current in last wire? A B C D 2.5 A 4.0 A 1.0 A Kirchhoff’s current law is also referred to as Kirchhoff’s 1st Law. There is also Kirchhoff’s voltage law, which is also referred to as Kirchhoff’s 2nd Law. It states that the algebraic sum of e.m.f. in a loop must equal to the algebraic sum of p.d. in the loop. It is a consequence of the Principle of Conservation of Energy. (Not in syllabus) 2.5 A 2.5 A 10 A 0.5 A

ANDERSON SERANGOON JUNIOR COLLEGE PHYSICS 9749 15-6 Additional Notes A.3 Potential versus Potential difference  Consider the circuit as shown in Fig. 15.4 below: Fig. 15.4  For the e.m.f. source, the positive terminal is always at a higher potential than the negative terminal. i.e. potential at P > potential at Q VP > VQ (In order to determine the exact value of VP and VQ, a reference point, e.g. earth, must be specified in the circuit. Do not always assume the negative terminal of the cell to be 0 V.)  The potential difference across the e.m.f. source would give the terminal p.d. of the source. i.e. terminal p.d. = VP - VQ  Since the point P and the point A in the circuit are connected by just a wire with no component in between, the potential at P must be the same as the potential at A. i.e. potential at P, VP = potential at A, VA  Similarly, the potential at Q must be the same as the potential at C as they are connected by a wire with no component in between them (switch is assumed to have no resistance). i.e. potential at Q, VQ = potential at C, VC  In this closed circuit, there is a drop in potential each time the current flows through a resistor. i.e. potential at A > potential at B > potential at C VA > VB > VC  The difference between the potential values at each end of the resistor is known as the potential difference. e.g. potential difference across R1, VAB = VA – VB potential difference across R2, VBC = VB – VC