Giáo án Vật lí 12 - Period 24: Problems in simple AC circuits - Nguyễn Thị Quyên

Nội dung tài liệu: Giáo án Vật lí 12 - Period 24: Problems in simple AC circuits - Nguyễn Thị Quyên

1. BAC NINH DEPARTMENT OF EDUCATION & TRAINING TEACHING PHYSICS IN ENGLISH LESSON PLAN Period 24: PROBLEMS IN SIMPLE AC CIRCUITS Time: 45 minutes Teacher’s name: Nguyen Thi Quyen. Writing date: 12/11/2019 Teaching date: 15/11/2019 I. OBJECTIVES: At the end of this lesson, the students will be able to: -Calculate capacitive reactance ZC, and inductive reactance ZL - Apply Ohm’s Law to solve problems involving simple AC circuits. - Write expression of amperage when know expression of voltage and vice versa. II. TEACHING STEPS: 1. Subject Matter - Reference books: Physics for high- school (Vietnamese) textbook grade 12. - Materials: sheets of paper, projector. 2. Starting the lesson: Introduce the lesson: Good morning every one. Today, the teacher in our school attend our lesson. Thus, I offer all student clap your hand to welcome teachers. We have studied the theory of simple AC circuits, the simple AC circuits only consisting a kind of element (resistor, capacitor or pure inductance coil). Today, I’m going to give a lesson on problems in simple AC circuits. The focus of this lesson is problem in writing the expression of i and u. The lesson has 4 activities: + Firstly: Vocabulary of the lesson + Secondly: Knowledge needed to use + Thirdly: Practice + Finally: Consolidation 3. Introducing terms used in the lesson: Firstly, I give you the vocabulary of this lesson: resistor: điện trở. capacitor: tụ điện. pure inductance coil: cuộn dây thuần cảm. impedance: trở kháng. capacitive reactance: dung kháng. inductive reactance: cảm kháng. phase difference: độ lệch pha 1
2. in phase: cùng pha. phase leading: sớm pha. phase lag: trễ pha. effective intensity: cường độ hiệu dụng effective voltage: điện áp hiệu dụng. instant intensity: cường độ tức thời instant voltage: điện áp tức thời angular frequency: Tần số góc. initial phase: Pha ban đầu. Secondly: Knowledge needed to use Teacher and Students’ activities Contents Activity 1: Review of knowledge. I.THE KNOWLEDGE NEEDED TO USE: - Teacher (T): 1. Some formula provide to problem: Question 1: What is expression of Expression of instant intensity and instant voltage: instant current and instant voltage in an 푖 = 2 표푠(휔푡 + 휑푖) (A) (1) AC circuit? = 푈 2 표푠(휔푡 + 휑 ) (V) (2) Note: In where I0 = I 2; U0 = U 2 In where I0 = I 2; U0 = U 2 correspond to the maximum of i and u. Question 2: The phase difference a.The AC circuit with only a resistor. between voltage and current in the AC + The phase difference between u and I: φu=φi. circuit with only a resistor? Question 3: The relation of Ohm’s law U + The Ohm’s law: I for the simple AC circuits only a R resistor? Question 4: The phase difference b. The AC circuit with only a capacitor. between voltage and current in the AC + The phase difference between u and I: φu=φi-π/2 circuit with only a capacitor? Question 5: The formula for calculating 1 + capacitive reactance: ZC capacitive reactance ZC. Explain each C quantity in the formula and its unit? Question 6: The relation of Ohm’s Law for the simple AC circuits only a U + The Ohm’s law: I capacitor? ZC Question 7: The phase difference between voltage and current in the AC c.The AC circuit with only a pure inductance coil. circuit with only a pure inductance coil? + The phase difference between u and I: φu=φi+π/2 Question 8: The formula for calculating Inductive reactance ZL. Explain each quantity in the formula and its unit? 2
3. Question 9: The relation of Ohm’s Law + Inductive reactance: ZL L for the simple AC circuits only a pure inductance coil.? Question 10: Who does give unit of R; U + The Ohm’s law: I 푍퐿; 푍 ; Z; I; U ? ZL + Unit: R(Ω); 푍퐿(Ω); 푍 (Ω); Z(Ω); I (A); U(V) Question 11: Can you indicate method to write the expression of instant 2. The problem about writing the expression of voltage? the instant intensity or the instant voltage - Student (S): Answer the teacher’s Method to solve: question. + Step 1: Determine maximum value: Use: I0=I 2 ; U0=U 2 ; Apply Ohm’s Law. + Step 2:Determine the angular frequency: The angular frequency of current equals the angular frequency of voltage. + Step 3: Find the initial phase. Determine the kind of element in circuit, based on the Activity 2: Problems in simple AC phase difference between voltage and current to find circuits. initial phase of u when know initial phase of i and vice 1. Problem 1: versa. T: Gives students problem 1 and gives a II. PROBLEMS TO EXERCISE suggestion. 1.Problem 1: What quantities we need to write the An AC circuit consists of a capacitor with the pure expression of the amperage? 1 S: Listen to teacher’s lectures, discuss inductance coil with L = (H ) .Given that the instant and answer the teacher’s question. voltage at two ends of the circuit: (maximum I = I 2 , the angular 0 u 220 2 cos(100 t)(V ) frequency of current, and initial phase). a) Calculate Z ? Note: the angular frequency of current L b) Calculate effective intensity I? equals the angular frequency of voltage) c)Write the expression of i. T: Comments on student’s answer. Summary: u = 220 2 cos(100πt) (V) 1 L= (H ) S: Summary and give the solutions. a)ZL =? b)I = ? c) The expression of i = ? Solution: T: calls student to the board to solution a)Impedance: Z L =100Ω of problem. L 3
4. b)Apply the relation of Ohm’s Law for the circuit only U consisting of a pure inductance coil, we have: I T: Comments on student’s solution. ZL =2.2(A). c)Since the instant amperage in the circuit is phase lag 2.Problem 2: of π/2 compared with the instant voltage at two ends of T: Gives students problem 2 and gives a the circuit, therefore: φi = φu-π/2 = -π/2 (rad). suggestion. The expression of amperage flowing in the circuit is: What formulas we need to determine i =2. 2 2 cos(100πt-π/2) (A). capacity C? 2.Problem 2: The current flowing in the circuit consisting of a S: Listen to teacher’s lectures, discuss capacitor: i = 5 2 cos(100πt+π/3) (A). and answer the teacher’s question. The effective voltage in the circuit is U = 100 V. ZC = U/I and C =1/(ω.ZC) a)Determine C. b)Write the expression of u. T: Comments on student’s answer. Summary: S: Listen to teacher’s lectures, discuss i = 5 2 cos(100πt+π/3) (A). and answer the teacher’s question. U = 100 V. S: Summary and give the solutions. a)C=?. b) The expression of u? Solution: U a)From the relation: I ,hence: ZC = U/I = 20 (Ω). ZC 1 -3 Since Z , we have:C = 1/(ω.ZC) =10 /(2π) (F). T: Comments on student’s solution. C C b)Since the instant voltage at two ends of the circuit is 3. Problem 3: phase lag of π/2 compared with the instant amperage (multiple choice of problem) in the circuit, therefore: T: Gives students problem 3 and gives a φu = φi-π/2 = -π/6 (rad). suggestion. The expression of u is:u = 100 2 cos(100πt - π/6) (V) Choose the correct answer? Explain? 3.Problem 3: S: Listen to teacher’s lectures, discuss Put the voltage u = 220 2 cos(100πt) (V) between to and answer the teacher’s question. ends of a circuit only consisting a resistor: R = 50Ω. (Answer: C) The expression of i ? (Apply the relation of Ohm’s Law for A. i=4.4 2 cos(200πt) (A). the circuit only consisting a resistor. B. i=4.4 cos(100πt) (A). the phase difference between the C. i=4.4 2 cos(100πt) (A). voltage and the current). D. i=4.4 2 cos(100πt +π/2) (A). T: Comments on student’s answer. Summary: u = 220 2 cos(100πt) (V) 4
5. S: Summary and give the solutions. R =50Ω, i =? Solution: Apply the relation of Ohm’s Law for the circuit only consisting a resistor, we have: T: Comments on student’s solution. U I = 1(A) R Since the instant amperage in the circuit is in phase with the instant voltage at two ends of the circuit, 4. Problem 4: therefore: (multiple choice of problem) φi = φu = 0 (rad). T: Gives students problem 3 and gives a The expression of amperage flowing in the circuit is: suggestion. i =4.4 2 cos(100πt)(A). Choose the correct answer? Explain? 4. Problem 4: (What quantities you use to determine The simple AC circuits only consist of a kind of this element?) element with u=100 2 cos(100πt+π/6)(V), then i=2 S: Listen to teacher’s lectures, discuss 2 cos(100πt-π/3). and answer the teacher’s question. This element is: (Answer: B) A. Capacitor. (The phase difference between the B. Pure inductance coil. voltage and the current is φu–φi = π/2). C. Resistor. T: Comments on student’s answer. D. Inductance coil with resistance r. S: Summary and give the solutions. Answer: B T: Comments on student’s solution. Summary: T: Determine L? u = 100 2 cos(100πt+π/6). S: Determine L: i = 2 2 cos(100πt-π/3) U From the relation: I , Determine this element? Z L Solution: hence: Z = U/I=50 (Ω). L Since: φ – φ = π/2 (rad). Since Z =ω.L, we have: u i L We have the instant voltage at two ends of the circuit L = Z /ω=1/(2π) (H). L is phase leading of π/2 compared with the instant T: Comments on student’s solution. amperage in the circuit, therefore this element is a pure inductance coil. III. CONSOLIDATION. -The formula for calculating capacitive reactance ZC, inductive reactance ZL, the relation of Ohm’s Law, and the phase difference between voltage and current in simple AC circuits. Apply them to solve problems. - The method to write the expression of instant voltage, instant amperage is: determine maximum value, the angular frequency, and initial phase. 5
6. IV. THE QUESTIONS FOR PREPARING THE NEW LESSON. - Do the exercises 3, 4, 7, 8, 9 in the textbook, page 74. - Read unit 14: “A CIRCUIT WITH R, L, C CONNECTED IN SERIES” and answer the questions: Question 1: What is the impedance formula in an AC circuit with R, L, C connected in series? Question 2: State Ohm’s law for an AC circuit with R, L, C connected in series? 6