Great Country Academician

After the routine inspection, Xu Chuan entered the examination room and got the test paper.

The instruments placed on the table let him guess the general content of the exam, and when he picked up the white test paper, it was indeed similar to what he expected.

"National Middle School Students Physics Contest Experimental Exam Questions."

Question 1: Test the equivalent circuit parameters of the coaxial cable.

Introduction: In ordinary circuits, when the frequency of the current or voltage signal is low, the electrical signal is basically transmitted along the wire loop.

If the signal frequency is as high as hundreds of kHz, which is the so-called radio frequency signal, the ordinary wire will emit electromagnetic waves into the space like an antenna, so it cannot be an effective channel for electrical signal transmission.

In order to transmit radio frequency signals, a type of transmission line with a special structure is usually used, such as a coaxial cable. When used, one end is connected to the signal source and the other end is connected to the load.

In this experiment, different methods are used to measure multiple characteristic parameters of the same uniform coaxial cable. Neglect the loss of the coaxial cable. All results are without an estimate of uncertainty.

The experimental device is a coaxial cable to be tested with a length of 5.00 meters, a signal source (internal resistance is R0, which can generate signals of different frequencies and different waveforms), two AC voltmeters (which can measure the effective value of AC voltage), two-channel digital Oscilloscopes, resistors, wires, etc.

Experiment content:

A. Measure the equivalent capacitance and equivalent inductance of the coaxial cable with a low-frequency sinusoidal signal (8 points)

When the frequency of the sinusoidal signal is very low and the wavelength is much longer than the length of the coaxial cable to be tested, the coaxial cable to be tested can be approximately equivalent to the circuit shown in Figure 2, where C and L are the entire coaxial cable to be tested under low frequency conditions. The equivalent capacitance and equivalent inductance of the shaft cable, and satisfy the following relationship:

1/2πC\u003e\u003e2πL

B. Measure the characteristic impedance, wave velocity, equivalent capacitance and equivalent inductance per unit length of coaxial cable with pulsed square wave signal (32 points)

(1) When the signal frequency is high and the wavelength is close to or much smaller than the length of the coaxial cable, the current flowing through the center conductor and the voltage between the inner and outer conductors fluctuate along the cable. If the coaxial cable is divided into several small sections, so that the length of each small section Ax

(2) The characteristic impedance is a basic parameter of the coaxial cable. If the two characteristic impedances are respectively z. and Z's coaxial cable are connected together, as shown in Figure 4(a), transmission and reflection will occur at the interface connecting the two. Let the input signal pass through the characteristic impedance as Z. The voltage at the coaxial cable reaching the interface is V

One topic, two sets of experiments, three contents.

It took Xu Chuan a few minutes to browse through the title in its entirety.

Compared with the written test, the experimental questions are not very innovative, they are nothing more than high school physics content such as current, optics, sound waves, and magnetic fields.

Compared with top physics, there are not many experiments left for students to do in high school physics.

Even if the knowledge of the students who can participate in the competition is far beyond that of ordinary high school students, they have not yet entered the university and have not entered the society. Those mid-to-high-end instruments and dangerous experiments, such as high-voltage current experiments, are not very important. may be open to them.

This time, the National Judgment is about equivalent circuit, equivalent inductance and pulse signal.

Putting it into physics experiments, it is relatively common, and the difficulty of the questions is not very high, but the experimental steps are quite cumbersome, and the students who need to take the exam are very careful.

After all, there are many steps, which means that there are quite a lot of places where points can be deducted.

Even Xu Chuan was conscientious in conducting experiments with standard procedures.

"Boom~"

Suddenly, a sound like something falling on the ground appeared in the examination room, which shocked everyone who was doing the experiment to raise their heads and interrupted the operation.

Xu Chuan was no exception. He followed the voice and turned his head to look. It turned out that a classmate had swept the electric meter on the table to the ground.

The black and white electric meter rolled twice on the ground, smashed a corner, and splashed two small pieces of glass.

As for the candidate on the experimental table where the electric meter belonged, it was probably the first time to participate in the national competition. After shaking the instrument and dropping it, he stood there trembling, with a disheveled expression on his face, not knowing what to do.

The sound caught the attention of the invigilator, and he walked straight over, picked up the broken electric meter from the ground, and checked it.

On the side, the trembling examinee saw the teacher who was checking the equipment, and felt that he had gotten into trouble. Under the huge pressure of the National Judgment, his eyes darkened, his body softened, and he passed out directly.

This is a good time, it was the invigilator's turn to tremble.

Ignoring the broken electric meter, he hurried over and helped the fainted examinee to his feet.

"Hello!"

"Student, classmate."

"Wake up, wake up."

Someone fainted, and the invigilator became anxious immediately. Two or three invigilators immediately came over to give first aid to the student, while the other invigilators continued to maintain the examination room.

"Where's the medical team, a student fainted in examination room No. 3, come quickly."

There was chaos in the examination room, and the invigilator pinched the person and pressed his temple to give first aid, and at the same time called the medical staff outside the examination room.

Soon, the medical staff who received the call for help rushed over and carried the fainted student out.

The fainted student was sent away, and the invigilator returned to the examination room, continuing to support the invigilation work, but interrupted by this, the atmosphere in the entire examination room changed.

Some people are getting more and more nervous, and some people's hands holding the power supply are shaking

Xu Chuan shook his head and continued his experiment.

In fact, no matter whether it is mathematics competition or physical competition, there are people who faint in the examination room every year.

Especially for those students who entered the national finals for the first time and took part in the national examination, under the tremendous pressure, any slight disturbance may cause the tense nerves in their minds to break.

However, compared to the chemistry competition, mathematical physics is still much better, at least there is no external danger.

There are no experimental questions in Shujing, and the experimental questions in Wujing are basically not dangerous.

But the chemistry competition is different. Various dangerous substances sometimes appear in the experimental questions of the chemistry competition.

Such as paraformaldehyde, glycine, phenylenediamine, phenol. And other chemical substances.

These things are common and commonly used materials in chemical competitions, but they are all toxic, some are even highly toxic, and can easily enter the human body through the skin and air, causing harm.

Anyone who often does experiments in chemistry laboratories should know that there are not a few students who are poisoned every year because of chemistry competitions.

And what the teachers of the chemical competition said the most during the competition was to remind students to pay attention to safety.

So participating in the competition is actually dangerous.