SET Chapter 2.pdf

Learning Outcomes Explain how an electric circuit works State the three basic electrical quantities: voltage, current and resistance State the units of measurement for voltage, current and resistance Describe the use of a voltmeter for measuring voltage D...

Learning Outcomes Explain how an electric circuit works State the three basic electrical quantities: voltage, current and resistance State the units of measurement for voltage, current and resistance Describe the use of a voltmeter for measuring voltage Describe the use of an ammeter for measuring current Describe the use of an ohmmeter for measuring resistance State the different uses of a multimeter Use the multimeter to measure voltage, current and resistance, and to check for continuity of an electrical installation Exercise safety precautions when handling and using measuring instruments 1.2.1 Electric Circuits An electric circuit is the physical pathway for current to flow. A simple electric circuit consists of: a source of electromotive force (emf), such as a battery or generator; a load (which has resistance), such as a lamp; conducting/connecting wires to connect the various parts of the circuit; and additional components such as a switch, fuse or circuit breaker, and a measuring instrument. Switch Connecting A Wire t Battery - Lamp - - open : off circuit Fig. 1.2-1: Simple electric circuit - closed : On circuit Unit 1.2 | Electric Circuits 6  1.2.2 Voltage, Current and Resistance The three basic electrical quantities in a basic electric circuit are: (a) supply voltage or electromotive force; (b) current; and compare to height of waterfall. enta ↳ (c) resistance. Voltage is the driving force to cause current (a) Supply Voltage / Electromotive Force ↑ to flow i n a circuit. It is the electrical quantity that causes current in a closed circuit. It is represented by V (supply voltage) or emf (electromotive force) and is measured in volts (V). > - volts is the unit (b) Current electrial load= electrical equipment for ↳ (lightbulb fan charger air con). voltage/ef moving charges , , ,. It is the electrical quantity that is needed for the electrical load to function, i.e., current allows the lamp in Fig. 1.2-1 to light up. It is represented by I and is measured in amperes (A). ↳ unit for current (c) Resistance is amperes. Resistance helps limit the size of current in the circuit. It is represented by R and is measured in ohms (Ω). When there is no or very little resistance in the circuit, such as when a short circuit occurs, the current will be very high and will damage the equipment if the circuit is not protected from this high current. current flowing short circuit will have a high. This causes a heat and lot of eventually fire 1.2.3 Voltmeter cause a. may voltage = electromotive force f A voltmeter is used for measuring the emf of the cell, battery, supply voltage and voltage across theO load in the circuit. It is always connected in parallel to the load or supply source, -S and at its two ends, where the voltage is to be measured. Eg Lamp Resistance Voltmeter get.. > - The symbol for a voltmeter is - V. must be connected in PARALLEL. Fig. 1.2-2 shows how a voltmeter can be connected to measure the voltage across the lamp in an electric circuit. across the lamp parallel. V Lamp Fig. 1.2-2: Voltmeter connected parallel to the lamp and at its two ends Unit 1.2 | Electric Circuits 7  Parallel circuit : the current will split path. Series circuit : the current only have ONE = - 1.2.4 Ammeter path. Does not split up An ammeter is used for to measure current flowing in a circuit. It is always connected in series to the load in the circuit. *Ammeter connected The symbol for an ammeter is A. in SERIES. - ! Fig. 1.2-3 shows how an ammeter should be connected to measure the current in an electric circuit. A Lamp Fig. 1.2-3: Ammeter connected in series to lamp 1.2.5 Ohmmeter An ohmmeter is used for measuring the resistance of an electrical load in a circuit. It is always connected in parallel to the load and at these two ends, where the resistance is to be measured. Ohmmeter connected The symbol for an ohmmeter is - Ω. in PRALLEL Fig. 1.2-4 shows how an ohmmeter should be connected to measure the resistance of an electrical load. NOTE: When using an ohmmeter to measure resistance, make sure its power is switched off. Otherwise, the user may receive an electric shock and the ohmmeter may be damaged. Fig. 1.2-4: Ohmmeter connection Unit 1.2 | Electric Circuits 8  1.2.6 Multimeter ① ③ A multimeter is a device that can measure voltage, current and resistance. It can also be used to diagnose electrical problems. -- check if 2 ↳ continuity tester There are two kinds of multimeter: points are connected. the analogue multimeter (Fig. 1.2-5), which uses an indicator needle with a measurement scale; and the digital multimeter (Fig. 1.2-6), which uses a numeric LCD display. The multimeter uses a rotating switch to select the quantity to be measured. It has two metal- tipped wires called probes, one red (+) and one black (-). probes. ↓ t > - Fig. 1.2-5: Analogue multimeter Fig. 1.2-6: Digital multimeter When using a multimeter to measure resistance, make sure its power is switched off. However, the measurement of voltage can only be carried out with the power on. Due to the risk of electric shock, only trained individuals should conduct voltage tests. Unit 1.2 | Electric Circuits 9