01
The basic principle of circuit simplification
Complex circuits in junior high school physical electricity can be simplified by the following principles:
☀ First: do not count the wire resistance, determine the R line ≈ 0. The voltage at both ends of the wire where the current flows is zero, and the voltage can be measured across the switch when disconnected (there are no other breakpoints in the circuit).
☀ Second: the opening and closing time is equivalent to a wire; the switch is equivalent to a broken circuit when the switch is open, which can be removed from the two nodes of the circuit. When the current is flowing through the switch, the voltage across the switch is zero, and the voltage can be measured at both ends of the switch when the switch is opened (there are no other breakpoints in the circuit).
☀ Third: the internal resistance of the ammeter is very small, and when analyzing the connection mode of the circuit, the place where there is a ammeter can be regarded as a wire.
☀ Fourth: the internal resistance of the voltmeter is very large, when analyzing the connection mode of the circuit, the place where there is a voltmeter can be regarded as a circuit break, removed from the two nodes of the circuit.
☀ Fifth: short circuit of the appliance (resistance): When the appliance (resistor) and the wire (switch, ammeter) are connected in parallel, there is no current passing through the appliance (as shown below), and the appliance can be removed (removed) from the two nodes of the circuit.
☀ Sixth: The Pa segment of the sliding varistor is not working by the wire (metal rod), after removing the Pa segment, the following figure a becomes Figure B.
☀ Seventh: According to the current and voltage law of the series and parallel circuits, the relationship between the total current, the total voltage and the current division and the voltage division is analyzed.
☀ Eighth: The ammeter and which appliance are connected in series to measure the current of which appliance, and the voltmeter and which appliance are connected in parallel to measure the voltage of which appliance. The voltage measured by the voltmeter can be judged by the slip method and the de-source method.
☀ Ninth: In principle, the voltmeter requires parallel connection in the circuit, and when the power supply voltage is measured separately, it can be directly at both ends of the power supply.
In general, if the voltmeter is connected in series in the circuit, the measured voltage is the voltage across the power supply (see note for details). The ammeter is directly connected to both ends of the power supply and will be burned out, and the power supply will be short-circuited and the power supply will be burned out.
☀ Tenth: If there is no electrical appliance (except for switches and ammeters) on the wire (between nodes), then the points on the wire can be regarded as a point, which can be merged, separated, increased or decreased at will. (This method is also known as the node method) For example:
02
Circuit simplification steps
☀ Step 1: Remove the disconnected switch and turn the closed switch into a wire according to the requirements of the question.
☀ Step 2: Turn the ammeter into a wire (as appropriate).
☀ Step 3: Remove the voltmeter.
☀ Step 4: Merge (or transpose) the nodes on the wire. (This step is used when there are more electrical appliances in the circuit and they are entangled with each other)
☀ Step 5: Draw an equivalent circuit diagram to determine whether each appliance is connected in series or parallel.
☀ Step 6: Use principles 7 and 8 in the original circuit to determine which appliance the ammeter and voltmeter each measure the current and voltage.
☀ Step 7: Add the voltmeter and ammeter to the equivalent circuit diagram and analyze the relationship between each ammeter and the voltage representation. (Principle VII of use)
03
Selected classic examples
☀ Example 1: In the circuit diagram below, after the switch S is closed, the indication of voltmeter V1 is 2.5V, the indication of V2 is 1V, if the indication of A2 is 0.2A, then what is the indication of A1? Try the voltage across the two bulbs.
Analysis: Step 1: Turn the switch into a wire;
Step 2: Turn the ammeter into a wire;
The third step removes the voltmeter.
Step 4 (Omitted)
Step 5: Determine the connection method of the electrical appliance
Obviously, the two bulbs in the circuit are connected in series.
Step 6: According to principle 4, know that the voltmeter V2 is disconnected, so there is no current flowing from A1, the number is zero; according to the slip method, know that V1 and the power supply are connected in parallel, and measure the voltage at both ends of the power supply. V2 and L2 are connected in parallel to measure the voltage across L2.
Step 7: The equivalent circuit diagram is as follows:
According to the equivalent circuit diagram, it can be seen that the resulting I1 = I2 = 0.2A
U1=2.5V-1V=1.5V U2=1V
☀ Example 2: As shown, after the switch s is closed, the relationship between the following voltage representations is ___, and the relationship between the current representations is ___.
Analysis: Step 1: Turn the switch into a wire;
Step 2: Turn the ammeter into a wire;
Step 3: Remove the voltmeter; step 4 (omitted);
Step 5: Determine the connection method of each electrical appliance:
Step 6: According to principles 7 and 8 and slip method or de-source method, the ammeter and voltmeter are determined by each object.
(1) In the original circuit, remove the voltmeter, it can be found that the ammeter A1 and A2 are connected in series with the bulb, so the current representation number is equal;
(2) The voltage across the L1 can be determined by using the slip method, the voltage at both ends of the power supply measured by V2, and the voltage at both ends of the L2 measured by V3.
☀ Example 3: After the circuit is opened and closed, the indications of v1, v2, and v3 are 8V, 9V, and 4V, respectively, so what are the voltages at L1, L2, and L3
The voltage relationship equation is: U2=U1+U3 I1=I2
Solution: According to the "ten principles, seven steps" can obtain the equivalent circuit diagram of the circuit as follows:
From the equivalent plot, the following system of equations can be obtained:
UL1+UL2=8V
UL2+UL3=9V
UL3=4V
解得:UL1=3v UL2=5v UL3=4V
☀ Example 4: As shown in the figure, it is known that the two bulbs L1 and L2 are connected in series, then the correct judgment in the three meters (1), (2) and (3) (ammeter or voltmeter) is ( )
(1) ammeter, (2) and (3) voltmeter;
B. (1) and (3) are voltmeters, (2) ammeters;
C. (1) and (2) ammeters, (3) voltmeters;
D. (1)(2) and (3) are both ammeters
Analysis: This question can be solved by elimination. Fill in the forms for answer A
Of the three positions, according to the "Ten Principles, Seven Steps" circuit diagram simplified to:
Obviously, it does not fit the point.
By filling in the B answer table in the diagram, the circuit can be simplified to:
This filling is in line with the meaning of the topic.
Fill in the C answer table into the circuit, circuit L1 is disconnected, and the simplified circuit diagram is the same as the A answer, which is not satisfactory. Fill in the D answer table into the figure, the power supply is short-circuited, and there is no current flowing through the two bulbs, which is not satisfactory. So choose Answer B.
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