Circuit diagram is an important part of electricity. Many electrical problems often begin with the phrase "in the circuit as shown in the figure". If the recognition of the circuit diagram is wrong, the calculation of physical quantities such as current intensity, voltage and resistance in the circuit will also be wrong, resulting in "total annihilation". Therefore, analyzing the circuit is the first step to learn electricity well.

A simple circuit diagram can be judged by analyzing the path through which the current passes in the circuit. The current in the circuit is always the same, which shows that this is a series circuit. If the current is divided into two or more paths in the circuit, it means that it is a parallel circuit. For complex circuit diagram, it is necessary to analyze the circuit by drawing equivalent circuit diagram. This method can be divided into four steps:

1, junior high school generally thinks that the resistance of ammeter is zero and that of voltmeter is infinite. Use ammeter as conductor and voltmeter as open circuit.

2. Mark the intersection of three wires in the circuit diagram with letters.

3. Starting from the positive pole of the power supply, draw a simple circuit diagram according to the current path and the position of each node.

4. Reset the ammeter and voltmeter.

Example 1: Please draw the equivalent circuit diagram of the circuit shown in Figure 1 and explain the functions of ammeter and voltmeter.

Solution: (1), use ammeter as conductor and voltmeter as open circuit.

(2) Mark the nodes A, B, C and D of the circuit diagram.

(3) According to the current path, the current starts from the positive pole of the power supply, passes through point D and R 1 to point A, and the current is divided into two paths at point A, one path passes through R2 to point B, the other path passes through R3 to point B, and the two paths meet at point B to form a loop. Draw an easy-to-understand circuit diagram (as shown in figure 1, 1). Therefore, R2 and R3 are connected in parallel and then connected in series with R 1.

(4) The ammeter and voltmeter are reset. As can be seen from the first circuit diagram, two points C and A are connected by wires and can be regarded as the same point. The two ends of the voltmeter are connected to point D and point A respectively, so the voltmeter measures the voltage across R 1. The current is divided into two paths through point A, one path passes through the ammeter and then reaches point B through R3, so the ammeter measures the current intensity through R3 (as shown in figure 1.2).

From the above example, we can see the importance of the analysis circuit. In the review, we should pay special attention to the necessity of analyzing circuits and drawing equivalent circuit diagrams, so that students can master this basic skill. And can form the habit of drawing a circuit diagram first when encountering electrical problems. Good habit of analyzing the circuit before encountering the circuit diagram.

Second, the application of ohm's law.

Ohm's law is the core of electrical calculation in junior high school. It reveals the relationship between the three most important physical quantities of electricity: current, voltage and resistance. When applying ohm's law, special attention should be paid to:

(1), it is necessary to clarify the U, I and R involved in the law. Are three physical quantities of the same part of the circuit. Never substitute U, I and R that are not in the same circuit into the formula for calculation.

(2) Formulas,, and are not fundamentally different in mathematical sense. But the first two formulas are mathematical expressions of ohm's law, and the latter is the definition of resistance. We cannot mistakenly think that "resistance is directly proportional to voltage and inversely proportional to current", nor can we think that "there is no resistance without voltage at both ends of a conductor".

Example 1: As shown in Figure 2, there are two resistors connected in series, the power supply voltage is 6V, the resistance value of the resistor is 10Ohm, and the voltage across the resistor is 2V. Find the current through the resistor and the resistance value of the resistor.

Solution: From the characteristics of series circuit, we can know that:

This example emphasizes the corresponding relationship between u, i and r when ohm's law is applied. Never divide the voltage at both ends or the power supply voltage by the resistance value to find the passing current, and never divide the power supply voltage by the current to find the resistance value.

Example 2: In the circuit shown in Figure 3, the power supply voltage is 6V, the resistance of resistor R 1 is 15 ohm, the resistance of resistor R3 is 10 ohm, and the reading of ammeter is 0.3 ampere. What is the resistance of resistor R2?

Analysis: This is a series-parallel circuit. The resistor R 1 is connected in parallel with R2 and then in series with R3. It is required that the resistance of resistor R2 must know the voltage across R2 and the current through R2. According to the characteristics of series and parallel circuits, the voltage across the resistor R 1 and R2 is equal, and the sum of this voltage and the voltage across the resistor R3 is equal to the power supply voltage. The sum of the currents passing through R 1 and R2 is equal to the main current, that is, the reading of the ammeter is 0.3a. ..

Solution: Voltage across R3: = 10ω×0.3A = 3V.

Voltage between R 1 and R2: = 6V-3V = 3V.

Current through R 1:

Current through R2: 0.3-0.2a = 0.1a.

Answer: The resistance of resistor R2 is 30 ohms.

These two examples are relatively simple, and the key is to emphasize the corresponding relationship of three physical quantities: U, I and R, as well as the characteristics of series-parallel circuits and the flexible application of Ohm's law.

Third, the application of electric power, electric power and Joule's law formula.

After learning Ohm's Law, junior middle school students still feel that electricity is not very difficult. Because only one formula or its variant can solve the problem. However, when we study electric power (), electric power () and Joule's Law (), these three very close and closely related concepts are easily confused. Some students reported that there are more than a dozen formulas for the size of this part of the content, and they often use the wrong formula. The key to learn electricity well and break through the difficulties of electricity lies in the flexible use of these dozens of formulas. In fact, there is a deep correlation between these dozens of formulas, which can be remembered by the following figure.

Pure resistance circuit (definition of electric power)

Formula, is often used in parallel circuits (when the voltage is constant), and formula, is often used in series circuits (when the current is constant). Formula summation is only applicable to pure resistance circuits. For non-pure resistance circuits, Joule's law can only be used to calculate the heat generated by current.

Example 1: Two identical resistors are connected in series to the power supply, and the heat generated is Q. If they are connected in parallel to the same power supply, the heat generated in the same time is ().

a，Q； b，Q； c、4Q； 2Q Washington, DC.

Analysis: according to the meaning of the question, the same power supply is connected twice, and the voltage is equal. It is more convenient to calculate by formula. Let two identical resistors be R, the heat released after series connection is zero, and the heat released after parallel connection is zero. According to the ratio of the two formulas, the answer should be C.

Example 2: The lamps A and B are marked with the words "4V, 2W" and "6V, 3W" respectively. If two lights are connected in parallel in the circuit, which one is brighter? Two lamps are connected in series in the circuit, which lamp is brighter? (The applied voltage will not melt the filament)

Analysis: The brightness of a light bulb depends on its actual electric power consumption. The greater the electric power consumption, the brighter the light bulb. When the voltage across the bulb changes, it will cause the change of electric power. The resistance of the filament will not change with the change of voltage. By electric light bulb

The resistance of the filament can be obtained from the rated voltage and rated power marked on the table. When two lamps are connected in parallel, the voltage at both ends of the bulb is equal, so the formula is used. When two lamps are connected in series, the current passing through the filament is the same, so the formula is used.

Solution: First calculate the resistance of two filaments.

When two lamps are connected in parallel, the voltage is equal.

Because that is, a lamp is brighter.

When two lamps are connected in series, the currents are equal.

because

A: When two lamps are connected in parallel on the circuit, the first lamp is brighter. When two lamps are connected in series in the circuit, the second lamp is brighter.

Fourth, the circuit changes.

Many students reported that "the circuit of change is very difficult, not just where to start". This is because the analysis of the change circuit involves a wide range of contents and deep consideration. Analysis of the relationship among resistance, current intensity, voltage and electric power, a little carelessness will cause wrong connection reaction and draw wrong conclusions. The key of circuit change is to analyze the change of resistance. The method of analyzing circuit changes can usually be divided into four steps:

1, from the change of resistance.

2, according to the characteristics of series-parallel circuit, see the change of resistance.

3. The change of total current is obtained by constant total voltage (power supply voltage).

4. According to the characteristics and meaning of the circuit, judge the changes of current, voltage and electric power in some circuits.

Example 1: In the circuit shown in Figure 4, the slider P of the sliding rheostat R2 moves to the right. Please analyze the changes of ammeter and voltmeter.

Solution: This is a series circuit with resistors R 1 and R2 in series. The ammeter measures the main current, and the voltmeter measures the voltage across the sliding rheostat. The slider P moves to the right, and the resistance of the sliding rheostat connected to the circuit is R2. The analysis process is as follows:

(1), analyze the change of ammeter:

raise

Fixed r total growth

U always decreases, and the ammeter reading becomes smaller.

(2) Analyze the change of voltmeter: decrease, increase and unpredictable change. The change of resistance current in the circuit can not be directly analyzed, and the corresponding voltage change can not be obtained. At this time, it is necessary to analyze the changes of other parts of the circuit and the characteristics of the circuit to get the changes of the research circuit. This problem can be solved by analyzing the change of voltage across the resistor:

decrease

Invariant reduction

Invariant reduction

Sustained growth

The reading of voltmeter increases.

Example 2: In the circuit shown in Figure 5, what will happen to the brightness of the light bulb after the switch S is closed?

Solution: The brightness of the light bulb is determined by the actual electric power of the light bulb. But for the same (or the same type) bulb, the resistance is the same. The light bulb is a pure resistance electrical appliance, and we can know the change of the brightness of the light bulb by analyzing the change of the voltage at both ends of the light bulb.

After the switch is closed, the circuit becomes a resistor R in parallel with the bulb L 1 and then in series with the bulb L2. The total resistance of parallel connection between R and L 1 is R, and the voltage across L 1 before and after turning off is sum, and the voltage across L2 before and after turning off is sum. According to the characteristics of parallel circuit:

The brightness of R<& lt bulb L 1 will be dimmed.

Principle of constant voltage and voltage division in parallel circuit

& lt

& gt the brightness of the light bulb L2 will be brighter.

After the first four topics, students should be instructed to sum up some rules: (1), analyze the circuit before calculation, and make clear the connection modes of several electrical appliances in the circuit. (2) The power supply voltage in the circuit is usually constant. (3) Mastering the calculation of current intensity and finding current is the key to finding other physical quantities. (4) The reason for the current change in the circuit is that the sliding rheostat changes the resistance, and the resistance is removed or connected in the circuit, which changes the resistance of the circuit. (5) No matter whether several electrical appliances are connected in series or in parallel, their total power is equal to the sum of the electrical power of each electrical appliance (…). (6),,,, are usually used in parallel circuits. (7),,,, are usually used in series circuits.

Fifth, electrical experiment.

Junior high school electricity requires students to master the circuit diagram according to the physical diagram, or connect the physical diagram according to the circuit diagram to design a simple circuit. Students are required to use ammeter and voltmeter to measure current and voltage, resistance and electric power. There are * * series circuits and parallel circuits in junior high school physics textbooks. Current is measured by ammeter, voltage is measured by voltmeter, resistance is measured by voltammetry, and light bulb is measured.

Five students, including electric power, conducted experiments in groups. The first three experiments are the preparation of knowledge and skills for the latter two experiments, so the review should focus on the two experiments of measuring resistance by voltammetry and measuring electric power of small bulbs. According to the connection of various knowledge points, the review of electrical experiments is divided into three steps.

1, connect the physical diagram according to the circuit diagram of the physical diagram or according to the circuit diagram. It is required that the two diagrams are connected in the same way, including the positive and negative poles of power supply, ammeter and voltmeter, as well as the positions of all electrical appliances and the functions of switches. The way to draw a circuit diagram is to understand the physical diagram first, especially the connection relationship of various electrical appliances and the function of switches. Then it is processed in the mind to form a complete circuit diagram. Finally, according to the requirements of "horizontal and vertical" of the circuit diagram, draw the corresponding circuit diagram. The connection method of physical diagram is to understand the distribution of circuit diagram and physical diagram first, and then connect the physical diagram section by section from one pole of power supply according to the current flow direction in the circuit diagram. When connecting, try to avoid wires crossing each other.

2. Simple circuit design. The circuit design should analyze the connection mode and switch of each electrical appliance according to the requirements of the topic.

The control object of. For example, according to the requirement that "one switch controls two light bulbs on or off at the same time", we can know that the circuit connection mode is as follows.

There are two possibilities: one is that the switch is connected in series with two light bulbs; Second, the two parallel switches of the light bulb are connected to the main road. According to the requirement that "one switch controls one light bulb, the other switch controls two light bulbs at the same time, and the failure of one light bulb will not affect the other light bulb", it can be known that two light bulbs are connected in parallel, one switch is connected to the branch of one light bulb, and the other switch is connected to the main road to control the main switch. Then draw the circuit diagram according to the connection mode of each electrical appliance.

3. Students review the experiment in groups. Voltammetric measurement of resistance and electric power are two important experiments in junior high school. There are many similarities between the two experiments, which can be reviewed by comparison.

Experimental device

Experimental circuit diagram

computing formula

Resistance measurement experiment

There are 1 ammeter, 1 voltmeter, 1 sliding rheostat, 1 battery pack, 1 switch, 1 resistor and several wires to be tested.

Electric power measurement experiment

There are 1 ammeter, 1 voltmeter, 1 sliding rheostat, 1 battery pack, 1 switch, 1 bulb to be tested and several wires.

During the experiment, * * * also pay attention to: (1), and use ammeter and voltmeter correctly. (2) The switch must be turned on when connecting the lines. (3) Connection of sliding rheostat. (4) Adjust the sliding rheostat to the maximum resistance of the access circuit before electrifying, and then gradually increase the resistance of the access circuit during the experiment.

The difference between the two experiments is that when measuring the resistance, the voltage across the resistance can be changed within a certain range by sliding the rheostat. When measuring rated electric power, use sliding rheostat to adjust the voltage at both ends of the bulb to make it equal to the rated voltage of the bulb.

During the implementation of the electrical review of the above five topics, some exercises and homework should be added to each topic. Through the review of these five topics, the related knowledge of electricity is well linked. Let students look at this part of knowledge from a higher angle, have new understanding and new improvement, and receive the review effect of highlighting key points, analyzing difficulties and breaking through difficulties.