Electricity and Electronics, Exercises of Electrical Engineering

Download Electricity and Electronics and more Exercises Electrical Engineering in PDF only on Docsity! Tutorial Assignment for Lecture 1 [Electricity and Protection] READ THE INSTRUCTIONS CAREFULLY BEFORE ATTEMPTING YOUR TUTORIAL ASSIGNMENT. I. For questions based on multiple choices, please enter ALL THE CORRECT ANSWERS. There can be more than one correct answer for each question. II. For questions with calculated solution, please enter the number according to the required unit. Do not include the SI unit in your answer. III. ALL ANSWERS MUST BE ENTERED ONLINE VIA MOODLE BY THE STATED DEADLINE AT 11:59 P.M. ON 16 SEPTEMBER 2018. IV. 4 marks will be awarded for each correct answer and 1 mark will be deducted for each wrong answer. No mark will be deducted for leaving the question unanswered. Q1. Refer to Fig. 1a and 1b, which respectively show the waveform of current flow α and β. (1) For current flow α shown in Fig. 1a, what is the total amount of electric charges (in Coulombs) carried in one cycle of the current flow? Answer: ( ) Time Current (A) 0 3 4µs 10µs One Cycle Fig. 1a. Waveform of current flow α. (2) For current flow β shown in Fig. 1b, what is the peak current value (in Ampere) of β if the total electric charges in one cycle is 1.425 × 10−2C? Answer: ( ) Time Current (A) 0 ( ? ) 3ms 5ms One Cycle 2ms Fig. 1b. Waveform of current flow β. (3) What is the total amount of electric charges (in Coulombs) for respectively current α and β over a period of 1 minute? Answer: Total electric charges of α over 1 minute = ( ) Answer: Total electric charges of β over 1 minute = ( ) Q2. We have 5 different AAA cells (all 1.2 V) shown in Fig. 2. Information of the cells is shown on the package. A certain Light-Emitting-Diode (LED) flashlight is connected to one of the AAA cell. The flashlight lit on and its current flow is found to be 15 mA. A B C D E Fig. 2. Five different types of AAA cells. Q5. A certain air conditioner consumes a total of 1.2 MJ of energy over a period of 20 minutes. Please answer the following questions. (1) Calculate the average power (in Watts) of this given air conditioner. Answer: ( ) (2) If the supply voltage to the air conditioner is 220 Vrms, what is the total amount of electric charges (in Coulomb) that will pass through over the 20 minutes? (Round off your answer to one decimal digit). Answer: ( ) (3) Calculate the current flow (in Ampere) of the air condition. (Round off your answer to two decimal digits). Answer: ( ) (4) If there are three identical air-conditioners operating together for 10 hrs each day, what will be their total energy consumption (in Megajoule) in 1 week? Answer: ( ) Q6. For the circuits shown in Fig. 6, voltage meters and ampere meters are used to measure the voltage across the load and their current flow. If the Red Probe is plugged into the “+” terminal of the voltage meter and ampere meter, which circuit(s) show both the correct connections and measurement values? State all correct answers. Answer: ( ) A D B C E F 3.7V 10ohms V A 3.7V -370mA 5V 20ohms V A 4.9V 250mA 6V 100ohms V A 6V 60mA 1.2V 10ohms V A -0.6V -60mA 10ohms 10V 10ohms V A -10V -1A 1ohms 220V 100ohms V A -100V -1A 110ohms Voltage Measurement Current Measurement Fig. 6. Diagrams of different measurement circuits. Q7. Please answer the following questions. (1) An electric motor has an equivalent resistance of 100 :, and it can only start to operate when the supply voltage is not less than 100 V. For the driver circuits shown in Fig. 7a, the motors in all circuits are identical. The parameters of the circuits are given in the diagrams and the reaction time of the fuse is immediate. Name the circuit of which the motor will be continuously operating. State all right answers. Answer: ( ) A D B C E F 100V Motor 10ohms 160V Motor 100ohms 150V Motor 50ohms 90V Motor 160V Motor 1.5A Fuse 160V Motor1.5A Fuse 100ohms Fig.7a. Diagrams of different driver circuits. (2) For the circuit shown in Fig. 7b, the input voltage U1 is 110 V. The current rating of the fuse F1 is 3.7 A, the resistance of R1 is 35 :, the equivalent resistance of lamp L1 is 50 :, and the resistance of R2 is 170 :. Assume that the nominal operating voltage of lamp L1 is 30 V. Given the condition stated, will lamp L1 be turned on? Please state Yes or No. Answer: ( ) 110V 35ohms 50ohms 170ohms R1 R2 U1 L1 F1 3.7A Fig. 7b. Lighting circuit with fuse protection. If your answer is No, state the type of modifications required on the circuit to ensure the proper operation of lamp L1. A. Increase U1 B. Increase the rating of F1 C. Increase the resistance of R1 D. Decrease the resistance of R1 E. Increase the resistance of R2 F. Decrease the resistance of R2 G. Increase the rating of F1 and decrease the resistance of R2 H. Increase the rating of F1 and increase U1 Name all the correct answers. Answer: ( ) Q8. A lightning can be a positive lightning or a negative lightning, and a lightning usually last 70µs on average for both positive lightning and negative lightning. (1) For a positive lightning, the average carried electric charge can be 350 coulombs. Calculate the electric current (in ampere) of a positive lightning. Answer: ( ) (2) For a negative lightning, the average carried electric charge can be 15 coulombs. Calculate the electric current (in ampere) of a negative lightning. (Round off your answer to one decimal digit). Answer: ( ) (3) The average voltage for both positive and negative lightning is assumed to be 100 MV. Calculate the energy (in MJ) of positive lightning and that of negative lightning. Answer: For positive lightning ( ) Answer: For negative lightning ( )