N4 Electrotechnics 1

LECTURER: MR PC CHABALALA SUBJECT: ELECTROTECHNICS N4 CAMPUS: ITEMOHELENG MALUTI TVET COLLEGE Downloaded from hlayiso.com

LEARNING OUTCOMES: Explain how the structures of the atom affects electrons and current flow Explain EMF, resistance and power Calculate resistivity, temperature coefficient of resistance Compute and recall Kirchhoff’s laws Downloaded from hlayiso.com

THE ATOM AND ITS STRUCTURE (page1-2) The atom is made up of electrons, protons and neutrons. The neutron and protons are located in the nucleus of the atom whilst the electrons are in the electron shells as shown figure 1.1 of the text-book. At the centre of the atom is the nucleus consisting of protons and neutrons bound by extremely powerful nuclear forces. Rotating in orbits around the nucleus are electrons. Downloaded from hlayiso.com

MOVEMENT OF ELECTRONS IN A CONDUCTOR (page3) When electrons drift in a specific direction is known as current. The electrons flow from “negative” to “positive” and this is known as electron flow. This electron flow is used when referring to “light current” or “electronics”. Downloaded from hlayiso.com

ELECTROMOTIVE FORCE (EMF) (page 4) Electromotive force is that force which tends to produce an electric current in a circuit. The unit of emf is the volt. Downloaded from hlayiso.com

RESISTANCE (page 5) The unit of resistance is that resistance in which a current of 1 ampere flowing for 1 second generates 1 joule of thermal energy. The unit in which it is measured is the ohm (Ω). A resistor is said to be linear if the current through the resistor is proportional to the pd across its terminals. Downloaded from hlayiso.com

Power (page 6) P= VI Where; P is power in watts I is current in amperes V is potential difference in volts Downloaded from hlayiso.com

Relationship between heat energy and power (page 7) Heat = I2 RT where I is the current R is the resistance of the wire T is the time during which the current flows Downloaded from hlayiso.com

RESISTIVITY R= pl/a Where: p type of a material l is type of a material A is cross-sectional area Downloaded from hlayiso.com

TEMPERATURE RISE IN ELECTRICAL MACHINES (page 8) The temperature of the windings of an electrical machine may be determined by: • Calculating the increase of resistance of the windings, and • Thermo-couples embedded in one or more of the windings Downloaded from hlayiso.com

TEMPERATURE COEFFICIENT OF RESISTANCE (page 9-11) The temperature of the windings of an electrical machine may be determined by: • Calculating the increase of resistance of the windings, and • Thermo-couples embedded in one or more of the windings Downloaded from hlayiso.com

KIRCHHOFF’S LAWS (page 11-17) • Kirchhoff’s Current Law states that the algebraic sum of currents entering a point will be equal to the algebraic sum of the currents leaving that point. • Kirchhoff’s Voltage Law states that the algebraic sum of the individual voltage drops in a closed network is equal to the algebraic sum of the applied voltage. • If an answer for a current results in a negative quantity, is simply means that the direction of the current was assumed incorrectly and is, therefore in the opposite direction to the Downloaded fromassumption made. hlayiso.com

Additional notes: This section will familiarise you with Kirchhoff’s first law (current law) and his second law (voltage law). When doing the calculations here, you must make sure of Kirchhoff’s laws. Solutions making use of other methods will not be accepted during tests and examinations. NB: Here are the steps to apply when solving circuit problems: Step 1: Consider the circuit diagram as two separate diagrams using loops Step 2: Set up the equations Step 3: Solve the equation Downloaded from hlayiso.com

Work through examples 1.5 to 1.7 on pages 13 to 17 of your textbook to understand how Kirchhoff’s laws can be applied to solve the equations. Complete Exercise 1.5 on page 22 Additional activities Previous Question Paper: August 2014 Question 2; 2.2 November 2014 Question 2; 2.3 Downloaded from hlayiso.com