Be careful when reading the textbook so you won't miss anything.
"Do you have any plans to go to graduate school?"
[(1) The average lifetime of radionuclides t=∫λNtdt/N0=… =1.1443T1/2
However, Professor Li's intuition was obviously not as sharp as that of Old Tang. He did not hear the uncertainty in Lu Zhou's tone at all. Instead, he nodded approvingly and continued.
The answer is M=29.634kg, which takes two digits and is approximately 30kg
Lu Zhou was dumbfounded.
"No, just stay here."
"Why don't you find a place to sit?"
However, after a summer of intense study, he stood here impeccably!
However, even though he guessed a little, Lu Zhou didn't say anything, picked up the A4 paper and started reading the questions.
Textbooks used by non-physics majors will not mention this more cutting-edge expression in the text, but they will still mention it in the appendix. Generally, students who are not interested in physics will not read there after graduation.
Lu Zhou:? ?
]
Li Rongen smiled and nodded: "Okay, take the leave note and go. There are more opportunities to participate in this kind of competition. If you are not good at politics, you can pay attention to the opportunity of postgraduate study. Or wait until you are a junior next year and ask me. , I can ask you, Teacher Tang."
He has successfully turned physics problems into high-mathematics problems.
"Lu Zhou."
But no matter what you say, this is the teacher's good intention.
"Okay, you've passed."
① Find the average life of the U238 core (ln2=0.693).
This is…
He immediately became unhappy.
The relationship between the decay constant and the half-life is: N0/2=N0e^(-λT1/2), from which we can get T1/2=ln2/λ
After reading the title, Lu Zhou sighed softly in his heart.
Even if Lu Zhou couldn't answer, he would only make a few jokes at most, but he would still approve in the end.
Substituting the data, we can get, t=6.49*10^9 years.
Lu Zhou couldn't laugh or cry in his heart. He was only in his sophomore year and was still far away from the postgraduate entrance examination.
The next step is to defeat them one by one!
"Professor, I'm not done yet..."
Are you suggesting that I enroll in his graduate program in the future
Right now-
Who can say what will happen in the future
Calculate the number of U238s in the submarine, and calculate the number of U238 cores from this.
The three days of the Mathematical Modeling Competition will coincide with two major physics classes.
The number of photons entering the detector…
Although he was asking for leave according to the order, considering that it would not be good to ask for leave in the first class, Lu Zhou decided to send the leave note in person so as not to bother the class monitor to do it for him.
"What? Are you scared?" Li Rongen asked with a smile.
"Our Physics Department at Peking University is considered to be at the top level among universities in the country. To be modest, Peking University and we are tied for first place. But if we look at specific research directions, superconducting physics and strongly correlated electronic systems, and In fields such as condensed matter physics and material design, they are not as good as us. If you are interested in physics, you can come to me when you take the postgraduate entrance examination. What our carbon nanotube laboratory needs are top talents in the field of mathematics."
The steps are correct, and the result is undoubtedly correct.
First question!
"Take the first question." Throwing a piece of A4 paper into Lu Zhou's hand, Li Rongen looked at Lu Zhou with a smile, and the old man said on the ground, "Write as much as you can, I want to see the process."
Suppose the efficiency of photons passing through the steel plate is n1. Calculate the number of photons passing through the steel plate in dt time as dN1=n0λNdt·n1
② Find the mass of U238 in the nuclear submarine (unit: kg, take 2 digits)
"Asking for leave?" After looking at the leave note, Professor Li took off his reading glasses, raised his head and smiled, "Your name is Lu Zhou, right?"
"No," Lu Zhou calmed down and shook his head, "Teacher, please come up with the question."
"What's wrong?"
Originally, he thought that Professor Li would hit him with a difficult problem that he couldn't write in the second question, and warned him not to be complacent or anything like that. But he didn't expect that Professor Li didn't follow the routine at all and just let him go. He did.
[It can be solved from the question stem information that the number of high-energy photons produced by decaying into U234 at time dt is -dN·n0=n0λNdt
In fact, it is a joke made on young people to say that only after answering the questions correctly will they be approved as false.
About five minutes passed, Lu Zhou's brows relaxed, and the tip of his pen moved in a swishing motion.
Hearing this question, Lu Zhou breathed a sigh of relief.
However, just when he was about to bring N into M=N*238u and complete the final blow to the enemy's spring, Professor Li Rongen suddenly spoke.
After knocking on the door, Lu Zhou walked in, put the leave note on the desk, and explained his purpose.
[The half-life of the U238 core in a nuclear submarine is 4.5×10^9 years. There is a 0.7% probability of becoming a U234 core during decay, and a high-energy photon is emitted at the same time. 93% of these photons are absorbed by the submarine steel plate. In 1981, the high-energy photons transmitted outside the nuclear submarine numbered U137 of the former Soviet Union were measured by a detector 1.5m away from the nuclear source (processed as a point). The normal incident area of the instrument is 22cm^2, the efficiency is 0.25% (every 400 incident photons can generate a pulse signal), and 125 signals are measured per hour.
Lu Zhou smiled and expressed his gratitude. Without saying another word, he took the hard-won leave and left quickly.
]
"For a system with given energy, material composition, and parameters, there is such a stable equilibrium state: other states can always be reached through reversible processes." Lu Zhou repeated this statement as if he was reciting a text.
Generally speaking, there are two main expressions of the second law of thermodynamics, one is the famous Kelvin expression, and the other is the expression mentioned by Clausius in his 1850 paper.
Unable to resist the obsession with obsessive-compulsive disorder, he still wrote down the results at the end.
The number of signals generated in the detector…
Is there such an operation? !
After taking the pen and scratch paper handed over by Professor Li, Lu Zhou stood next to the desk, frowning and thinking for a moment.
Lu Zhou said sincerely: "Thank you, teacher."
By the way, the first general physics class will be held next Monday. Is it really okay to test myself on this kind of superficial nuclear physics knowledge
Since yesterday, he has approved dozens of leave slips for the Mathematical Modeling Competition. How could he be short of such a slip
Lu Zhou always felt that this guy had a premeditated plan. Otherwise, how could he explain why this question had been printed on A4 paper
"You have already calculated it to this point. Even a junior high school student can calculate it," Li Rongen said with a smile, "You can calculate it as you wish. It's up to you."
After this brief and in-depth investigation, Li Rongen can now confirm that there is indeed nothing to teach him in his course for undergraduate non-physics majors.
The atmosphere of doing the question was completely ruined, and Lu Zhou was speechless for a while.
"Yes." Lu Zhou nodded.
After writing these two lines of calculations, Lu Zhou's eyes flickered slightly.
It seems that Old Tang is indeed not bragging, this guy is a talent!
Li Rongen smiled and continued: "Don't be nervous. The second question is very simple. Just tell me about the Kennan formulation of the second law of thermodynamics."
As expected of Professor Li, who was named one of the "Four Demons of the Institute of Physics" by his predecessors, did he start with this kind of topic
Although Lu Zhou wanted to say "I don't want to", but thinking that the professor had not approved his leave note, he couldn't help but said: "Just say it."
Physics is really not his area of expertise.
Have you even prepared the questions? !
I've already finished my calculations here, can't you wait until I finish my calculations before talking
Lu Zhou substituted all the data and calculated the number of U238 cores in the submarine N=7.456*10^25.
Looking at the class schedule information provided by the Academic Affairs Office, Teacher Dawu’s name is Li Rongen, and he is an old professor with quite high qualifications. After asking for the phone number in the school group, Lu Zhou made a call first to confirm that the teacher was in the office, and then went over with the leave note.
Glancing at the A4 paper, Professor Li Rongen raised his eyebrows, with an interesting look in his eyes.
After signing the note, Li Rongen took out the roll call book in the drawer, drew two ticks on it, and said in an elder's tone.
Lu Zhou thought for a while and said in an uncertain tone: "Yes."
]
Professor Li smiled and said, "Okay, I'll test you with two questions. If you solve one, I'll approve your leave."
The pen tip moved quickly across the A4 paper.
Lu Zhou brought this question to a successful conclusion.
Not noticing the change in Professor Li's expression, Lu Zhou, who was highly focused on the paper, began to answer the next question.
[The decay law of radionuclides: N=N0e^(-λt)
What do you mean, let me pass
As for the Kennan expression, it is a relatively new expression method that has recently emerged.
"Not bad." Li Rongen nodded approvingly.
"Do you want to listen to the second question?" Li Rongen said with a smile.
If Lu Zhou could even master this obscure knowledge point so clearly, it would be enough to prove that he had thoroughly understood the entire "College Physics" textbook.
]