After the routine inspection, Xu Chuan entered the examination room and got the test paper.
The instruments placed on the table allowed him to guess the general content of the exam. When he picked up the white test paper, he saw that it was pretty much what he had expected.
"Experimental Examination Questions for the National Physics Competition for Middle School Students."
Question 1: Test the equivalent circuit parameters of 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 or more, which is a so-called radio frequency signal, ordinary wires will emit electromagnetic waves into space like an antenna, and therefore cannot become 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.....
This experiment uses different methods to measure multiple characteristic parameters of the same uniform coaxial cable. Ignore the losses in the coaxial cable. All results are not evaluated for uncertainty.
The experimental device consists of a coaxial cable to be tested with a length of 5.00 meters, a signal source (the internal resistance is R0, which can generate signals of different frequencies and different waveforms), two AC voltmeters (can measure the effective value of the AC voltage), and two dual-channel digital Oscilloscopes, resistors, wires, etc.
Experiment content:
A. Use low-frequency sinusoidal signals to measure the equivalent capacitance and equivalent inductance of coaxial cables (8 points)
When the frequency ƒ₀ of the sinusoidal signal is very low and the wavelength is much larger 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 C₁ and L₁ are respectively the entire length to be measured under low frequency conditions. The equivalent capacitance and equivalent inductance of the coaxial cable satisfy the following relationships:
1/2πƒ₀C₁>>2πƒ₀L₁
B. Use pulse square wave signal to measure the characteristic impedance, wave speed, equivalent capacitance and equivalent inductance per unit length of the coaxial cable (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 central conductor and the voltage between the inner and outer conductors will fluctuate along the cable. If the coaxial cable is divided into several small segments such that the length of each segment is Ax...
(2) Characteristic impedance is a basic parameter of coaxial cable. If the characteristic impedance of the two roots is z. When the coaxial cable with Z₃ is 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 Z. The voltage at the coaxial cable reaching the interface is V...
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One question, two sets of experiments, three contents.
Xu Chuan spent a few minutes browsing through the questions completely.
Compared with the written test, the experimental questions don't actually have much new ideas. They are just high school physics content such as electric current, optics, sound waves, and magnetic fields.
Compared with top physics, high school physics does not leave many experiments for students to do.
Even though the knowledge of students who can participate in the competition is far superior to that of ordinary high school students, after all, they have not yet entered university and have not entered society. Those mid-to-high-end instruments and dangerous experiments, such as high-voltage current experiments, are not important. May be open to them.
This time, the national finals tested equivalent circuit, equivalent inductance and pulse signal.
When it comes to physics experiments, they are relatively ordinary. The difficulty of the questions is not very high, but the experimental steps are quite complicated and students who need to take the exam are very careful.
After all, there are many steps, which means there are many places where points can be deducted.
Even Xu Chuan worked diligently to conduct experiments according to 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. He turned around following the sound and saw that a classmate had swept the electric meter on the table to the ground.
The black and white electric meter rolled around on the ground twice, smashed into a corner, and splashed two small glass fragments.
The candidate at the experimental table where the electric meter belonged was probably participating in the national finals for the first time. His hands shook and he dropped the instrument. Then he stood there shivering, with a disheveled look on his face, not knowing what to do.
The sound attracted the attention of the invigilator, who walked over, picked up the broken meter from the ground, and checked it.
Beside, 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 finals, his vision went dark, his body went limp and he fainted.
Good thing this time, it was the invigilator's turn to tremble.
Ignoring the broken electricity meter, he quickly walked over and helped the fainted candidate up.
"Hello!"
"Classmate, classmate."
"Wake up, wake up."
Someone fainted, and the invigilator immediately became anxious. Two or three invigilators immediately came over to give first aid to the student, while the other invigilators continued to keep the examination room going.
"Where are the people from the medical team? A student has fainted in Examination Room 3. Come here quickly."
There was chaos in the examination room. The invigilator pinched the person and pressed his temples to give first aid while calling 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 students were sent away, and the invigilator returned to the examination room and continued to support the invigilation work. However, with this interruption, 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, whether it is a math competition or a physical competition, people faint in the examination room every year.
Especially for those students who are entering the national finals and taking the national examination for the first time, under huge pressure, any slight disturbance may cause the tight nerves in their minds to break.
However, compared to the chemistry competition, mathematical physics is still much better, at least there are no external dangers.
There are no experimental questions in the mathematics competition, and there are basically no dangers in the experimental questions in the physics competition.
But chemistry competitions are different. Various dangerous substances sometimes appear in the experimental questions of chemistry competitions.
Such as paraformaldehyde, aminoacetic acid, phenylenediamine, phenol... and other chemical substances.
These are common and commonly used materials in chemistry 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.
The most common thing Huajing teachers say when leading competitions is to remind students to pay attention to safety.
Therefore, participating in competitions is actually dangerous.
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