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 (Bac Ninh).
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
in phase: cùng pha.
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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
Activity 1: Review of knowledge.
- Teacher (T):
Question 1: What is expression of
instant current and instant voltage in an
AC circuit?
Note: In where I 0=I √ 2 ; U 0=U √ 2
correspond to the maximum of i and u.
Question 2: The phase difference
between voltage and current in the AC
circuit with only a resistor?
Question 3: The relation of Ohm's law
for the simple AC circuits only a
resistor?
Question 4: The phase difference
between voltage and current in the AC
circuit with only a capacitor?
Question 5: The formula for
calculating capacitive reactance ZC.
Explain each quantity in the formula
and its unit?
Question 6: The relation of Ohm's
Law for the simple AC circuits only a
capacitor?
Question 7: The phase difference
between voltage and current in the AC
circuit with only a pure inductance
coil?
Question 8: The formula for
calculating Inductive reactance ZL.

Contents
I.THE KNOWLEDGE NEEDED TO USE:
1. Some formula provide to problem:
Expression of instant intensity and instant voltage:
i=I √ 2 cos (ωt +φi ) (A)
(1)
u=U √2 cos (ωt +φu ) (V) (2)
In where I 0=I √ 2 ; U 0=U √ 2
a.The AC circuit with only a resistor.
+ The phase difference between u and I: φu=φi.

+ The Ohm's law:
b. The AC circuit with only a capacitor.
+ The phase difference between u and I: φu=φi-π/2

+

capacitive reactance:

+ The Ohm's law:

c.The AC circuit with only a pure inductance coil.
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Explain each quantity in the formula + The phase difference between u and I: φu=φi+π/2
and its unit?
Question 9: The relation of Ohm's
Law for the simple AC circuits only a
nductive reactance:
pure inductance coil.?
+I
Question 10: Who does give unit of
R; Z L; ZC ; Z; I; U ?

Question 11: Can you indicate method
+ The Ohm's law:
to write the expression of instant
voltage?
Ω Z ( Ω) Z (Ω); Z(Ω); I (A); U(V)
- Student (S): Answer the teacher's + Unit: R( ); L ; C
question.
2. The problem about writing the expression of
the instant intensity or the instant voltage
Method to solve:
+ Step 1: Determine maximum value:

Activity 2: Problems in simple AC
circuits.
1. Problem 1:
T: Gives students problem 1 and gives
a suggestion.
What quantities we need to write the
expression of the amperage?
S: Listen to teacher's lectures, discuss
and answer the teacher's question.

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
phase difference between voltage and current to find
initial phase of u when know initial phase of i and
vice versa.
II. PROBLEMS TO EXERCISE
1.Problem 1:
An AC circuit consists of a capacitor with the pure

(maximum I0 = I
, the angular
frequency of current, and initial phase).
.Given that the
Note: the angular frequency of current inductance coil with L =
equals the angular frequency of instant voltage at two ends of the circuit:
u=220 √ 2cos(100 πt )(V )
voltage)
a) Calculate ZL ?
T: Comments on student's answer.
b) Calculate effective intensity I?
c)Write the expression of i.
S: Summary and give the solutions.

Summary: u = 220

cos(100πt) (V)

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L=
T: calls student to the board to solution a)ZL =?
of problem.
b)I = ?
T: Comments on student's solution.

c) The expression of i = ?
Solution:
a)Impedance:
=100Ω
b)Apply the relation of Ohm's Law for the circuit
only consisting of a pure inductance coil, we have:

2.Problem 2:
T: Gives students problem 2 and gives
=2.2(A).
a suggestion.
What formulas we need to determine c)Since the instant amperage in the circuit is phase
lag of π/2 compared with the instant voltage at two
capacity C?
ends of the circuit, therefore: φi = φu-π/2 = -π/2 (rad).
S: Listen to teacher's lectures, discuss The expression of amperage flowing in the circuit is:
and answer the teacher's question.
ZC = U/I and C =1/(ω.ZC)

i =2. 2 cos(100πt-π/2) (A).
2.Problem 2:
The current flowing in the circuit consisting of a

T: Comments on student's answer.
S: Listen to teacher's lectures, discuss
and answer the teacher's question.
S: Summary and give the solutions.

capacitor: i = 5 cos(100πt+π/3) (A).
The effective voltage in the circuit is U = 100 V.
a)Determine C.
b)Write the expression of u.
Summary:

T: Comments on student's solution.
3. Problem 3:
(multiple choice of problem)
T: Gives students problem 3 and gives
a suggestion.
Choose the correct answer? Explain?
S: Listen to teacher's lectures, discuss
and answer the teacher's question.
(Answer: C)
(Apply the relation of Ohm's Law for
the circuit only consisting a resistor.
the phase difference between the

i = 5 cos(100πt+π/3) (A).
U = 100 V.
a)C=?.
b) The expression of u?
Solution:
a)From the relation:

,hence: ZC = U/I = 20 (Ω).

Since
, we have:C = 1/(ω.ZC) =10-3/(2π) (F).
b)Since the instant voltage at two ends of the circuit is
phase lag of π/2 compared with the instant amperage
in the circuit, therefore:
φu = φi-π/2 = -π/6 (rad).
The expression of u is:u = 100
3.Problem 3:

cos(100πt - π/6) (V)

Put the voltage u = 220 cos(100πt) (V) between to
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voltage and the current).
T: Comments on student's answer.
S: Summary and give the solutions.

ends of a circuit only consisting a resistor: R = 50Ω.
The expression of i ?
A. i=4.4 √ 2 cos(200πt) (A).
B. i=4.4 cos(100πt) (A).
C. i=4.4

T: Comments on student's solution.

4. Problem 4:
(multiple choice of problem)
T: Gives students problem 3 and gives
a suggestion.
Choose the correct answer? Explain?
(What quantities you use to determine
this element?)
S: Listen to teacher's lectures, discuss
and answer the teacher's question.
(Answer: B)
(The phase difference between the
voltage and the current is φu–φi = π/2).
T: Comments on student's answer.
S: Summary and give the solutions.
T: Comments on student's solution.
T: Determine L?
S: Determine L:
From the relation:
,
hence: ZL = U/I=50 (Ω).
Since ZL=ω.L, we have:
L = ZL/ω=1/(2π) (H).
T: Comments on student's solution.

√ 2 cos(100πt) (A).
√ 2 cos(100πt +π/2) (A).

D. i=4.4
Summary:

u = 220 cos(100πt) (V)
R =50Ω, i =?
Solution:
Apply the relation of Ohm's Law for the circuit only
consisting a resistor, we have:
= 1(A)
Since the instant amperage in the circuit is in phase
with the instant voltage at two ends of the circuit,
therefore:
φi = φu = 0 (rad).
The expression of amperage flowing in the circuit is:
i =4.4 cos(100πt)(A).
4. Problem 4:
The simple AC circuits only consist of a kind of
element with u=100

cos(100πt+π/6)(V), then i=2

cos(100πt-π/3).
This element is:
A. Capacitor.
B. Pure inductance coil.
C. Resistor.
D. Inductance coil with resistance r.
Answer: B
Summary:
u = 100

cos(100πt+π/6).

i = 2 cos(100πt-π/3)
Determine this element?
Solution:
Since: φu – φi = π/2 (rad).
We have the instant voltage at two ends of the
circuit is phase leading of π/2 compared with the
instant amperage in the circuit, therefore this element
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is a pure inductance coil.

III. CONSOLIDATION.
-The formula for calculating capacitive reactance Z C, 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.
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?

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