Electrical Energy Practice Questions PDF

FOR ONLINE USE ONLY DO NOT DUPLICATE Chapter Seven Electrical energy Introduction In Standard Four, you learnt about different sources of...

FOR ONLINE USE ONLY DO NOT DUPLICATE Chapter Seven Electrical energy Introduction In Standard Four, you learnt about different sources of electrical LY energy. Also, you learnt about different uses of electrical energy. In this chapter, you will learn about series circuits and parallel circuits. Similarly, you will learn about uses of series and parallel circuits. N Also, you will learn how to use electric devices to measure electric currents and voltages in circuits. O Simple electric circuits SE Examine Figure 1 and then answer the questions that follow. U Switch Dry cell Bulb E N Wire LI N O Figure 1: Simple electric circuit Questions R 1. What will happen to the bulb when you turn ON the switch? FO 2. Explain what will happen to the bulb if you cut off the wire while the switch is ON. 3. Explain what will happen if the dry cell is reversed and the switch is turned ON. 112 SCIENCE AND TECHNOLOGY STD V.indd 112 7/30/21 11:03 AM  FOR ONLINE USE ONLY DO NOT DUPLICATE When the bulb in Figure 1 lights up, it means that the circuit is complete. A circuit is a closed path or loop around which an electric current flows. It is complete if it has a source of electricity, wire, a switch and a resistor. The source of electricity can be a dry cell, a battery, a dynamo, a solar cell or a generator. A resistor refers to a device that opposes the flow of electric currents. Examples of resistors are bulbs and an electric cookers. An electric current is a flow of electric charges. In order for an electric current to flow, it requires two things: a battery and a path. The dry cell has two terminals, a negative terminal (–) and a positive terminal (+). Electricity LY flows from the positive terminal to the negative terminal. Symbols that are used in electric circuits N Table 1: Symbols and names used in electric circuits O No. Circuit symbol Name 1. SE Dry cell U 2. Battery E 3. Switch N LI 4. OR Bulb N O 5. Wire OR R 6. OR Resistor FO 113 SCIENCE AND TECHNOLOGY STD V.indd 113 7/30/21 11:03 AM  FOR ONLINE USE ONLY DO NOT DUPLICATE Types of electric circuits There are two types of electric circuits: series circuits and parallel circuits. Series circuits are closed circuits that allow electric currents to flow through a single path. Parallel circuits provide several paths for electric currents to flow. Series circuits LY Experiment 1: Observing how a series circuit works Aim: To examine if the circuit will work when one bulb is disconnected from the circuit N Materials: Two dry cells, connecting wires, a switch, two bulbs, two O bulb holders and a dry cell holder Procedure 1. Set up the circuit as shown in Figure 2. SE 2. Make sure the dry cells are arranged in such a way that the positive U terminal of one cell touches the negative terminal of the other. 3. Turn ON the switch. What do you see? E 4. Turn OFF the switch and then remove one bulb from the circuit. N 5. Turn ON the switch again. What have you seen? Switch LI Dry cell holder Dry cell N Bulb holder O Bulb R Wire FO Figure 2: Series circuit 114 SCIENCE AND TECHNOLOGY STD V.indd 114 7/30/21 11:03 AM  FOR ONLINE USE ONLY DO NOT DUPLICATE Results When one bulb is disconnected from the circuit, the other bulb will not light up because the two bulbs are in the same path that completes the electric circuit. Conclusion In this experiment, you have seen that, if the circuit is not complete, the bulb cannot light on. Parallel circuits LY Experiment 2: Observing how a parallel circuit works Aim: To examine whether the circuit will work if one bulb is removed N from the circuit Materials: Two dry cells, a piece of wire, a switch, two bulbs, two O bulb holders and a dry cell holder Procedure 1. SE Connect the circuit as shown in Figure 3. 2. Turn ON the switch. What do you see? U 3. Turn OFF the switch and then remove one bulb from the circuit. 4. Turn ON the switch again. What have you seen? E Switch N Dry cell LI N O Bulb R Wire FO Figure 3: Parallel circuit 115 SCIENCE AND TECHNOLOGY STD V.indd 115 7/30/21 11:03 AM  FOR ONLINE USE ONLY DO NOT DUPLICATE Results When one bulb is removed from the circuit, the second bulb will light on. Conclusion In this experiment, you have seen that, when the electric current circuit is complete, the bulbs light on. When one bulb is removed from the circuit, it does not affect the other bulb. Therefore, the two bulbs are not in the same path that completes the electric current circuit; they are parallel instead. Uses of series circuits and parallel circuits LY Series circuits are used in various appliances like torches as well as the decorative and blinking string lights, as shown in Figure 4. They are used in situations where there are few devices using electricity. In this type N of circuit, the connected appliances depend on each other. Therefore, a fault in one electric appliance affects the whole circuit. Thus, it is difficult O to identify a faulty in series circuits. Also, too many electric appliances in a series circuit reduce the amount of electric currents. SE U E Decorative light N Torch LI Figure 4: Devices that use series circuits N O R FO Traffic lights Figure 5: Device that uses a parallel circuit Parallel circuits are used in places where many electric appliances are 116 SCIENCE AND TECHNOLOGY STD V.indd 116 7/30/21 11:03 AM  FOR ONLINE USE ONLY DO NOT DUPLICATE using one electric path, for example in homes, on roads and in factories. When one device is damaged, the other electric devices will continue working. Thus, the traffic lights shown in Figure 5 use a parallel circuit. The voltage across each device in a parallel circuit is the same. Therefore, when more electric devices are added to parallel circuits, there will be no drop in voltage across the devices. Figure 5: Device that use a parallel circuit. Exercise 1 LY Answer the following questions. 1. Draw a picture of a series circuit showing a dry cell, a switch, wire and a resistor. N 2. Explain the advantages and disadvantages of series and parallel circuits. O 3. Mention the type of circuit used in a torch. Draw a picture of a torch circuit. SE 4. Name three devices or systems that use parallel circuits. Measurement of electricity U Electromotive force (voltage) makes electricity flow in a circuit. The flowing of electricity in a circuit is called an electric current and is denoted by the letter I. The international system unit (SI) of electric currents is ampere E (A) and it is measured using a device called ammeter. N The material that allows currents to pass through it is called a conductor. Examples of conductors are metals like copper and silver. The non- LI metal material that does not allow currents to pass through it is called an insulator. Examples of insulators are wood, rubber, plastic and glass. N When an electric current is flowing in a circuit, there is resistance offered by the component that resists the flow of electricity through it. The SI unit O of resistance is ohm, which is denoted by the greek letter Omega (Ω). The device used to measure this resistance is called ohmmeter. Voltage R is measured using a voltmeter. The SI unit of voltage is volt, which is denoted by the letter V. The names and symbols that are used to denote FO electric currents, resistance and voltage measurement devices are given in Table 2. 117 SCIENCE AND TECHNOLOGY STD V.indd 117 7/30/21 11:03 AM  FOR ONLINE USE ONLY DO NOT DUPLICATE Table 2: Symbols of devices used in electric circuits Symbol Device Voltmeter Ammeter Ohmmeter LY Measurement of electromotive force N Activity 1: Measuring an electromotive force in a circuit O Materials: A voltmeter, two dry cells, a bulb, a piece of wire and a switch Procedure 1. Set up the circuit as shown in Figure 6. SE 2. Turn on the switch. What is the reading of the voltmeter? 3. Add the second dry cell and then read the voltmeter. What is the U reading of the voltmeter? Dry cell Switch E Wire N LI Bulb X N Voltmeter V O Figure 6: Measurement of voltage in a circuit R The voltmeter shows that the voltage of one dry cell is about 1.5 V. FO When two dry cells are combined in a series, the voltage doubles to about 3 V. The voltage in a circuit increases when the number of the dry cells connected in a series increases. 118 SCIENCE AND TECHNOLOGY STD V.indd 118 7/30/21 11:03 AM  FOR ONLINE USE ONLY DO NOT DUPLICATE Measurement of electric currents Activity 2: Measuring electric currents in a circuit Activity: To measure electric currents in a circuit Procedure 1. Set up the circuit as shown in Figure 7. 2. Turn on the switch. What is the reading of the ammeter in amperes? LY 3. Add the second dry cell and turn on the switch. What is the reading of the ammeter in amperes? 4. Add the third dry cell and turn on the switch. What is the reading of N the ammeter in amperes? O Dry cell Switch SE A Wire Ameter Bulb U X Figure 7: Measurement of electric currents E When the voltage is increased in the circuit, the ammeter shows that the N electric current also increases. This supports Ohm’s law, which states that the flow of electric currents increases as voltage increases. LI N O R FO 119 SCIENCE AND TECHNOLOGY STD V.indd 119 7/30/21 11:03 AM  FOR ONLINE USE ONLY DO NOT DUPLICATE Exercise 2 Answer the following questions. Section A: Answer all questions. 1. Choose the right answer. (i) An electric current that flows in a circuit is measured by ____. (a) ohmmeter LY (b) galvanometer (c) ammeter N (d) voltmeter O (ii) The voltage in an electric circuit is measured by ____. (a) ohmmeter (b) dry cell SE (c) voltmeter U (d) ammeter (iii) The two main types of circuits are ____. E (a) series circuits and a resistor N (b) parallel circuits and a conductor LI (c) series circuits and a parallel circuits (d) parallel circuits and an insulator N O (iv) The flow of electricity in the circuit is called ____. (a) a parallel circuit R (b) an electric current (c) a series circuit FO (d) electromotive force 120 SCIENCE AND TECHNOLOGY STD V.indd 120 7/30/21 11:03 AM  FOR ONLINE USE ONLY DO NOT DUPLICATE 2. Match the circuit symbols in List A with the corresponding names in List B. No. List A List B i A a Cell ii b Bulb or LY iii or c Switch iv or d Voltmeter N v e Ammeter O vi V f Wire vii SEg Resistor U 3. Write TRUE for a true statement and FALSE for a false statement. (a) Ammeter is a device that measures electrical resistance in a E circuit. __________ N (b) Voltmeter is a device used to measure electromotive force in a LI circuit. __________ (c) If you disconnect one bulb in a series circuit, the remaining N bulbs will continue giving light. __________ O (d) If one device in a parallel circuit is damaged, the whole circuit shuts down. __________ R (e) If you increase electromotive force, an electric current increases FO as well. __________ 121 SCIENCE AND TECHNOLOGY STD V.indd 121 7/30/21 11:03 AM  FOR ONLINE USE ONLY DO NOT DUPLICATE Section B: Fill in the blanks. 4. The flow of electricity in a circuit is called ___________. In order for that flow to occur, the circuit must be ___________. 5. Copper is a good _______ of electricity and plastic is an _________. 6. The SI unit of an electric current is called ___________. Section C: Answer the following questions. 7. What instrument is used to measure electric currents? LY 8. Mention the instrument used to measure electromotive force. 9. Draw a circuit to show how an electric current is measured. N 10. What instrument is used to measure resistance? O 11. Explain what will happen to the ammeter and voltmeter when more dry cells are added, each with 1.5 V, to a series circuit. SE 12. Describe what will happen to an electric current in a series circuit when resistors are added. U Vocabulary Ammeter an instrument for measuring electric currents E Ampere the unit of an electric current N Circuit a path through which electric currents pass LI Conductor the material that allows electric currents to pass through it N Insulator the material that does not allow electric currents to pass O through it Resistor a device that opposes the flow of electric currents R Unit the standard measurement of a quantity which has been FO adopted internationally by convention or by law 122 SCIENCE AND TECHNOLOGY STD V.indd 122 7/30/21 11:03 AM