In 2023, this middle question in LN City uses the plumbing electric blanket as the background to test students' ability to apply the characteristics of series circuits and electrical power. The question stem does not take into account the heat loss, in fact, even if the heat loss is considered, the student should be able to do it. Let's take a look at this question.
2023 ln) is a simplified circuit diagram of a certain plumbing electric blanket, where r, r1, and r2 are heating resistors. The electric blanket has two gears: "temperature regulation" and "constant temperature", switch 1 is "temperature regulation" gear, and the power change range is 50W-100W, and switch 2 is "constant temperature" gear, and the power is 33W. The electric blanket is fitted with 1k**. c water = 42×103j/(kg·0c)
Find: 1) the current of the electric blanket in constant temperature mode;
2) the maximum resistance of the variable resistance r;
3) If the maximum power is used for heating, the time taken to increase the temperature of the water in the electric blanket from 200C to 500C regardless of heat loss.
Original problem analysis] Solution: (1) It can be obtained from p=ui, the current of the electric blanket in the constant temperature mode:
2) As can be seen from the figure, when the switch is connected to 1, the sliding blade is at the rightmost end, and only R2 is connected to the circuit in the circuit, which is the maximum power P height = 100W for the temperature control block.
When the switch is connected to 1, the sliding blade is at the leftmost end, the variable resistance r reaches the maximum value, r is connected in series with r2, the resistance in the circuit is the largest, and the minimum electrical power is p low = 50w,
So the maximum resistance of the variable resistance r:
3) Heat absorbed by water:
Because the heat loss is not counted, the electrical energy consumed:
From p=w t, the time taken to increase the temperature of the water in the electric blanket from 200C to 500C:
Comments] This question is a comprehensive question of electric heating, which mainly tests students' understanding and application of the characteristics of series circuits, electric power formulas, and endothermic formulas, and can correctly analyze the circuit connections in different states is the key to solving the problem. Although this question does not take into account the heat loss of the electric blanket, after completing the question, students should add a question: If the thermal efficiency of the electric blanket is , what is the result? Students will do the same.
I wish the teachers and students progress!