Since the "double reduction", in order to respond to the call of the country, the difficulty of the physics test questions in various places has decreased. Even so, students still have a headache with some of the question types, and teachers are even more confused and don't know why. Let's take a look at the following question and find the reason through the analysis of the test questions.
2023 BJ) As shown in the figure, it is a circuit designed by Xiaojing that triggers an alarm when the fuel in the car tank is insufficient, and the voltage at both ends of the power supply remains the same, and one of the resistors is a fixed-value resistor and the other is a varistor. The varistor is installed at the bottom of the tank, and its resistance increases with the decrease of the amount of oil in the tank, and when the voltage representation is greater than a certain value, an alarm will be triggered. When the voltage is indicated, the amount of oil in the tank is the warning oil level. The following statement is true.
a is a varistor
b As the amount of oil in the tank decreases, the current through the fixed resistance increases
c If you change to a fixed value resistor with a higher resistance, the warning oil quantity will be reduced
d If you switch to a higher voltage power supply, the amount of warning oil will increase.
Analysis of the original question] According to the circuit diagram, the resistor R1 and R2 are connected to the circuit in series, and the voltmeter measures the voltage at both ends of the resistor R1
a, b: the resistance of the varistor increases with the decrease of the oil in the oil tank, when the oil in the oil tank decreases, the resistance of the varistor becomes larger, according to the voltage division law of the series circuit, the varistor shares the voltage to become larger;According to the voltage law of the series circuit, the voltage at both ends of the fixed value resistor becomes smaller, because the voltage representation number is greater than a certain value, the alarm will be triggered, so R1 is a varistor;If the total resistance of the circuit becomes larger, according to Ohm's law, the current in the circuit becomes smaller, so option A is correct and option B is wrong.
C: The alarm voltage of the voltmeter remains unchanged, the voltage at both ends of the fixed value resistor remains unchanged, if the fixed value resistance is replaced with a larger resistance value, according to Ohm's law, the current in the circuit becomes smaller, according to R=U i, the resistance value of the varistor becomes larger, and the warning oil amount will become smaller, so the C option is correct.
D: The alarm voltage of the voltmeter remains unchanged, if the power supply with a larger voltage is used, the voltage at both ends of the fixed value resistance becomes larger, according to Ohm's law, the current in the circuit becomes larger, according to R=U I, the resistance of the varistor becomes smaller, and the warning oil volume will become larger.
So the answer is: ACD.
Comments] Through analysis, it is not difficult for us to find that understanding and mastering the voltage division law of series circuit is the basis for solving this problem. Aspiring students can also use letters to express how much U1 and U2 are equal to, and use the total voltage U, resistance values R1, and R2 to express it.
I wish the teachers and students progress!