Classic Question 3: In an experiment, it is necessary to measure the voltage at both ends of a small light bulb and the current flowing through it. The smart but careless Xiao Gang connected the circuit as shown in the picture. Xiao Lin, who was in the same group, found something wrong and advocated dismantling and reconnecting, but there was no time. Xiao Gang rolled his eyes and added only one wire to the circuit shown in the picture, and the circuit worked normally.
Question (I) If the switch in the circuit shown in the diagram is closed, what is the approximate reading of the voltmeter ? What is the approximate reading of the ammeter ? How does the small light bulb glow?
⑵ Draw the wire that Xiaogang connected in the picture. How does the light bulb glow?
Analysis:
1 Read the question carefully, extract useful information, and analyze it carefully.
1.1 This is a physical wiring circuit experiment question. The purpose of the experiment is to measure the voltage at both ends of the small light bulb and the current flowing through it. Seeing this, can you recall the physical diagrams and circuit diagrams in the textbook?
1.2 The physical scenario is that the operator Xiaogang connected the physical object wrongly. Where is the mistake? This is the key to the problem.
1.2.1 We know that the voltmeter must be connected in parallel with the electrical appliance or power supply under test. If it is connected in series with the electrical appliance, it will measure the power supply voltage. It can be seen from the meaning of the question that to measure the voltage across the bulb, the voltmeter must be connected in parallel with the bulb. With this basic knowledge, we can see from Xiaogang's actual connection diagram that the voltmeter is not connected in parallel with the light bulb, which is a mistake.
1.2.2 We also know that the ammeter must be connected in series with the electrical appliance under test in the circuit. It can be seen from the picture that the ammeter, the light bulb and the fixed value resistor are connected in parallel, which is wrong.
1.3 If you have enough time, it is not a bad idea to dismantle and reconnect, but there is not much time now, so you must make mistakes and make local adjustments quickly based on the original connection line. Xiaogang still had a solid foundation, quick eyes and quick hands, and quickly added a wire, making the experiment a complete success.
1.4 Xiaogang solved the problem quickly, but it caused trouble for us. We must follow Xiaogang's ideas and explore Randomness! How to do
?
The most effective and direct way is to draw the corresponding circuit diagram based on the physical drawing that Xiaogang made the mistake of connecting, and then check and correct the error.
Let’s draw the circuit diagram based on the physical diagram:
1.4.1 Starting from the positive pole of the power supply, the first electrical appliance is the switch, the second electrical appliance is the sliding rheostat , and branching occurs when the third electrical appliance has a fixed value resistance. The other branch goes to the ammeter. It can be seen from this that this is a parallel circuit, as shown in the following figure:
1.4.2 Starting from the negative pole of the power supply, the first electrical appliance is a voltmeter, and there is a branch after the ammeter of the second electrical appliance. , extending to the light bulb, as shown in the picture below:
1.4.3 Put the two pictures into one picture, as shown in the picture below:
1.5 The physical picture of the wrong connection of the small steel is drawn into a circuit diagram as shown in the picture above. It is easy to know from the picture :
① The light bulb and the fixed value resistor are connected in series,
② The ammeter is connected in parallel with the light bulb and the fixed value resistor,
③ The voltmeter is connected in series on the main circuit.
It is very obvious that ② and ③ are wrong, so how to correct the error quickly? One idea is to connect the voltmeter and the light bulb in parallel. It is easy to know from the circuit diagram. Just add a wire as shown below.
 deformed into the following picture:
2 See the question
2.1 The first question is about the physical diagram that Xiaogang connected incorrectly. If the switch is closed, what will happen to the readings of the voltmeter and ammeter? The small light bulb Is it glowing or not?
① First look at the voltmeter. From the circuit diagram of the wrong connected object, we can see that the voltmeter is connected in series on the main road.We know that the resistance of the voltmeter is very large, so if the switch is closed at this time, the current through the circuit will be very small (close to zero), and all the voltage of the power supply is added to the voltmeter. It can be seen that the voltmeter is measuring The voltage across the power supply must have a reading, and the reading is the power supply voltage 6 V.
②Look at the ammeter. As can be seen from the picture, the ammeter is connected in parallel with the light bulb and the fixed-value resistor. The resistance of the ammeter is very small and can be regarded as a wire, so the reading will be very, very small, about zero.
③ Finally, look at the small light bulb. Because it is short-circuited by the ammeter, it will not emit light.
2.2 The second question is to draw the wire that Xiaogang connected in the picture, and ask how the light bulb glows? From the above analysis, it can be seen that the added wires are as shown in the figure below. At this time, the switch is closed, the circuit is passed, and the light bulb works normally.
[Answer] (1) The reading of the voltmeter is about 6 V, the reading of the ammeter is about 0, and the small light bulb does not light up.
(2) The answer is as shown below.
Summary: Some connected circuit diagrams make it unclear to us how the circuit is connected.
We can draw the corresponding circuit diagram according to the direction of the current. From the circuit diagram, it is very clear that the circuit connection error is wrong.
