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

The instruments on the table allowed him to guess the general content of this exam, and when he picked up the white test paper, it was about the same as he expected.

"National Middle School Students' Physics Competition Experimental Examination 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 several hundred kHz or more, it is the so-called radio frequency signal, and ordinary wires will emit electromagnetic waves into space like antennas, so they cannot become an effective channel for electrical signal transmission.

In order to transmit RF signals, a special type of transmission line is usually used, such as axial cables, ......., when used, one end is connected to the signal source, and the other end is connected to the load..........

In this experiment, different methods were used to measure multiple characteristic parameters of the same homogeneous coaxial cable. Ignore the loss of coaxial cables. Uncertainty was not assessed for all outcomes.

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

Experiment:

A. Measurement of equivalent capacitance and equivalent inductance of a coaxial cable with a low-frequency sinusoidal signal (8 minutes)

When the frequency of the sinusoidal signal ƒ₀ is very low and the wavelength is greater than the length of the coaxial cable to be measured, the coaxial cable to be tested can be approximated with the circuit shown in Figure 2, where C₁ and L₁ are the equivalent capacitance and equivalent inductance of the entire coaxial cable to be measured under low frequency conditions, respectively, and meet the following relationships:

1/2πƒ₀C₁>>2πƒ₀L₁

B. Measurement of the characteristic impedance, wave velocity, equivalent capacitance per unit length and equivalent inductance of a coaxial cable using a pulsed square wave signal (32 minutes)

(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 central conductor and the voltage between the inner and outer conductors show fluctuating changes along the cable. If the coaxial cable is divided into several small segments, the length of each segment is Ax......

(2) Characteristic impedance is a basic parameter of coaxial cable. If the two characteristic impedances are z, respectively. When connected to a coaxial cable with Z₃, as shown in Figure 4(a), transmission and reflection occur at the interface connecting the two. Let the input signal pass through the characteristic impedance is Z. The voltage at which the coaxial cable reaches the interface is V......

.......

One question, two experiments, three contents.

Xu Chuan spent a few minutes going through the questions in their entirety.

Compared with the written test, the experimental questions are actually not too new, they are nothing more than the content of high school physics such as electricity, optics, sound waves, and magnetic fields.

Compared to top physics, high school physics does not have many experiments left for students to do.

Even if the students who can participate in the competition are far more knowledgeable than ordinary high school students, after all, they have not yet entered the university and society, and those mid-to-high-end instruments, as well as dangerous experiments, such as high-voltage current experiments, are unlikely to be open to them.

This time, the national decision is based on equivalent circuits, equivalent inductances and pulse signals.

In the physics experiment, it is relatively ordinary, 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 quite careful.

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

Even Xu Chuan is conscientious and does experiments with standard procedures.

.......

"Bang~"

Suddenly, a sound like something falling to the ground appeared in the examination room, startling everyone who was doing the experiment to look up and interrupt the operation.

Xu Chuan was no exception, turning his head to look at the voice, it turned out that a classmate swept the meter on the table to the ground.

The black and white meter rolled twice on the ground, smashing a corner and splashing two tiny shards of glass.

The candidate at the experimental table to which the meter belongs, probably participating in the national final for the first time, stood there trembling after dropping the instrument with a trembling hand, with a dishful face, not knowing what to do.

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

On the side, the trembling candidates saw the teacher who was checking the instrument, and felt that they were in trouble, and under the huge pressure of the national decision, their eyes were dark, and their bodies were soft and they fainted directly.

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

Ignoring the tattered meter, he hurriedly walked over and helped the fainted candidate up.

"Hey!"

"Classmates, classmates."

"Wake up, wake up."

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

"What about the people in the medical team, there are students fainting in the No. 3 examination room, come here."

There was chaos in the examination room, and the invigilator pinched people and pressed their temples for first aid, while calling the medical staff outside the examination room.

Soon, paramedics who received a message for help rushed over and carried the fainted student out.

The students who fainted were sent away, and the invigilator returned to the examination room to continue to support the invigilation work, but this interruption changed the atmosphere in the entire examination room.

Some people are getting more and more nervous, and some people's hands are shaking when they hold the power supply.....

Xu Chuan shook his head and continued his experiment.

In fact, whether it is a number competition or a physical competition, there are people who faint in the examination room every year.

Especially for those students who enter the national finals for the first time and take the national examination, under tremendous pressure, any slightest disturbance may lead to the broken string of the tense nerve in their minds.

However, compared to the chemistry competition, mathematics and physics are still much better, at least there are no external dangers.

There are no experimental problems in the number competition, and the experimental problems in the physical competition are basically not dangerous.

However, chemistry competitions are different, and various dangerous substances are sometimes present in the experimental questions of chemistry competitions.

For example, paraformaldehyde, aminoacetic acid, phenylenediamine, phenol, ..... and other chemicals.

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

If there are chemical competitors who often do experiments in the chemistry laboratory, they should know that there are not a few students who are poisoned by chemistry competitions every year.

And the most common thing that the teacher of Huajing said with the competition was to remind students to pay attention to safety.

Therefore, participating in the competition is actually dangerous.

.......