Why do you say that? Because most famous schools will put the activity of grabbing students before the second model exam, if you do well in the first model exam, you can recommend yourself to the famous school and get an invitation to take the independent enrollment exam, so that you have the opportunity to enter your favorite famous school through the independent enrollment exam. Students who are qualified for early admission don't have to meet the admission score of the school, which can greatly reduce the pressure of the senior high school entrance examination.
To sum up, we must attach importance to the model test.
As far as physics is concerned, the test sites of a model test include pressure, buoyancy and electricity.
Among them, the key and difficult points are the change of solid-liquid pressure, the comprehensive calculation of liquid pressure and buoyancy, dynamic circuit, circuit fault, electrical calculation and two important experiments of electricity: measuring resistance by voltammetry and measuring electric power of small bulb.
This paper focuses on the common skills of dynamic circuit analysis.
First, the opening and closing of the switch causes the dynamic change of the circuit.
Think about solving problems:
(1) Analyze the circuit connection types and measuring objects of each ammeter and voltmeter when the switch is opened and closed; The voltage representation of the power supply voltage will not change.
(2) If two resistors are connected in series to form one resistor in the circuit, the total resistance becomes smaller and the current becomes larger. The voltage and current of the short-circuit resistor are both zero, and the voltage and current of the other resistor become larger, which is the power supply voltage.
(3) When the double resistors in the circuit are connected in parallel to form a single resistor, the total resistance becomes larger and the total current becomes smaller. The voltage and current of the disconnection resistor are both zero, and the voltage and current of the other resistor remain unchanged, so the total current becomes smaller.
Example: (20 17 Songjiang District Physical Model No.8)
In the circuit shown in the figure, the power supply voltage is constant. When the S key is turned from closed to off, the voltmeter ().
A.v index drops, v index drops.
B.B.V index increases, and V index increases.
C.v index drops, and v index rises.
The D.v index rises and the v index falls.
Analysis: ① When the S key is closed, the circuit is a simple circuit, and the resistor R2 and the resistor R 1 are short-circuited; An ammeter measures the current in the circuit; The voltmeter V 1 is connected in parallel at both ends of R 1 to measure the voltage at both ends of R 1, so the indication is 0; Voltmeter V2 measures the voltage of R2, that is, the power supply voltage;
② When the S key is closed, the circuit is a series circuit of resistors R 1 and R2; The flowmeter measures the current in the circuit, the voltmeter V 1 measures the voltage of R 1, and the voltmeter V2 measures the voltage of R2, that is, the partial voltage of the series circuit.
(3) When a single resistor is changed to a double resistor in series in the circuit, the total resistance becomes larger and the current becomes smaller. The voltage and current of the original short-circuited resistor become larger, and the voltage and current of the other resistor become smaller, so the V index becomes larger and smaller.
Answer: d
Secondly, the movement of the sliding blade of the sliding rheostat causes the dynamic change of the circuit.
Think about solving problems:
(1) Judge the circuit connection type and the objects to be measured by each ammeter and voltmeter;
(2) Determine the position where the sliding rheostat is connected to the circuit, and judge the change of resistance when the slider moves;
(3) Series circuit, constant resistance current I and voltage U are similar, sliding rheostat current I and voltage U are larger and smaller, and voltage changes? U=？ U, constant resistance R=? U/？ Me. The small bulb is a constant resistance;
(4) In the parallel circuit, the current, voltage and resistance of the branch where the fixed resistor is located are unchanged; The voltage of the branch where the sliding rheostat is located is constant, and the current is inversely proportional to the resistance; The main current resistance is the same as the branch where the sliding rheostat is located, and the current variation is the same.
Note: The ratio of voltage to current can be converted into resistance.
Example: (20 17 Xuhui District Physical ModelNo. 16)
In the circuit shown in the figure, the power supply voltage remains unchanged, the key S is closed, and the circuit works normally. In the following data: ① Indication V of voltmeter; (2) the ratio of voltmeter v to ammeter a; (3) the product of the voltage indication number v and the ammeter indication number a; (4) the ratio of ammeter A to ammeter A; (5) the ratio of voltage indicator v to ammeter a indicator.
When the slider P of the sliding rheostat R moves to the right, it becomes larger; What gets smaller is. (Please fill in the serial number of relevant data)
Analysis: ① The sliding rheostat R is connected in parallel with the fixed value resistor R. The ammeter A 1 measures the current flowing through R 1, the ammeter A2 measures the current flowing through R2, the ammeter A measures the main line current, and the voltmeter V measures the voltage at both ends of each appliance, that is, the power supply voltage, indicating that it is unchanged.
(2) The current and voltage resistances of the branch where the fixed-value resistor R is located remain unchanged, that is, the index A remains unchanged; In the branch where the sliding rheostat is located, when the slider P moves to the right, the resistance increases, the current decreases, and the total current also decreases, that is, the number A and the number A decrease by the same amount.
③ The ratio of voltmeter V to ammeter A is the resistance of sliding rheostat, which becomes larger; The ratio of voltage indicator v to ammeter a is the total resistance of the circuit, which becomes larger; Both ammeters A and A are reduced by the same amount, so the ratio becomes larger.
So the answer is: 245; ③。
Exercise:
Reference answer: 1, unchanged; Larger 2d
Dynamic circuit analysis is inseparable from everything, and seemingly difficult ratios or products can be obtained through the most basic physical quantities. Therefore, mastering and analyzing the most basic changes of resistance, voltage and current is the key for students to solve the problem of dynamic circuit analysis. I hope that some of the above problem-solving skills can help everyone.