Hello everyone, I am Song Geon-woong from the Electric Power Department. Continuing from our last session, I would like to discuss the exam questions that have been asked five or more times over the past eight years for the Electric Power Technician certification. As I mentioned last time, the content I bring is based on actual questions that have appeared in the exams, checked against data on how many times they have been asked. Therefore, you can study these questions with confidence regarding their reliability. Please remember that you must be familiar with these questions.

Today, we will dive into Part 2. The first question I have brought is one that has appeared nine times. It started in 2017 and has been asked a total of nine times up to 2023. The question is about relative permittivity.

I would like to talk about the types of capacitors that use titanium oxide as a dielectric, which are non-polar and have excellent performance for their price, making them widely used. Here, we should focus on a couple of key points. First, titanium oxide has been mentioned. Second, it is non-polar. If we keep these two points in mind, we can immediately identify that we are referring to ceramic capacitors. 

On the other hand, electrolytic capacitors, unfortunately, are polar. Because they are polar, they do not apply to this topic. Next, there are mylar capacitors, which are somewhat similar to ceramic capacitors; however, they are also used in applications like bypass filters or other types of filters. The same goes for ceramic capacitors. But when we come across keywords like titanium oxide, non-polar, and good cost-performance ratio, we can confidently conclude that we are indeed talking about ceramic capacitors.

Next, we have the mica capacitor. This type has good high-frequency characteristics and is primarily used in filter circuits. Since the topics of tantalum or non-polarized capacitors and cost-effectiveness are not relevant here, you can opt for ceramic capacitors. Of course, non-polarized capacitors are uniform, but due to the other conditions, you need to choose ceramic capacitors. So, what should you summarize from this issue? What do you need to clarify? You can just take this with you. In your problems, if you come across tantalum and it is non-polarized and cost-effective, you should not hesitate to choose ceramic capacitors. Then, expanding a little further, if a film capacitor comes up, remember that it has the characteristic of being non-polarized. If you are asked what types of non-polarized capacitors there are, the answer would be film capacitors.

If you provide a capacitor, that would be great. So, this problem is that the ceramic capacitor is the correct answer, and since it has been asked a total of nine times, I hope you will definitely take this problem with you. Now, let's take a look at the second question of today. This question has been asked about six times, starting from the fourth round in 2017 up to the second round in 2023. Let me read the question for you. Such questions come up quite often and can be unexpectedly tricky. So, I will help you to clearly understand this question. 

When five resistors of equal value are connected in series, what is the equivalent resistance? The question is asking how many times the equivalent resistance in series is compared to the equivalent resistance when connected in parallel. In other words, it is asking whether you can distinguish between the situation when five resistors are connected in series and when they are connected in parallel.

So, if you do it once, you can try it. I will connect five resistors in series. So, R in series, we are trying five resistors. Then, this is how it looks: one, two, three, four, five resistors are connected. Now, it means we need to combine them into one, as it's a combined resistance. So, we can express it as one. But how many ohms is one resistor? It’s said to be 5 ohms. Let's write that down. 

Now, if you connect five 5-ohm resistors in series, you can simply add them up since they are in series. So, it becomes 25 ohms. Therefore, when resistors are connected in series, the total resistance is 25 ohms. Did you catch that? Next, let's look at parallel connections. So, it's connected like this: one, two, three, four, five, connected in parallel, and we need to combine them into one.

Let's combine this. So, this is also a series of five-ohm resistors connected together. Now, how should we calculate this? There are two methods: one is to use a formula, and the other is to use a standard approach. The standard approach is to add them up when they are connected in parallel: 1/5 + 1/5 + 1/5 + 1/5 + 1/5. This gives us the correct calculation. However, when the same resistance is connected in parallel multiple times, we have a formula for that, which is R/n. Here, n refers to how many are connected in parallel. So, what is the value of n? It's five, right? And the exact value of one is five ohms. Since we have five of them in parallel, we calculate it as 5 divided by 5, which equals 1. So, when we calculate this, we find that it results in 1 ohm.

According to a certain formula, the combined resistance formula is represented as r over n. However, this applies when multiple resistors of the same value are connected. If the resistor values are different—let's say one is 593 ohms and the other is 1 ohm—then we have to approach it using the standard method I showed earlier. 

Now, we have solved the problem. Let's compare the results. Here, we see that when connected in series, the resistance is 25 ohms, while in parallel, it's significantly different. How many times greater is it? When we multiply, we find it's 25 times greater, right? Therefore, the combined resistance in series is 25 times that of the combined resistance in parallel. So, the correct answer is option 1. 

Some of you might find this confusing when you first encounter the problem, but that's not unusual. It's perfectly normal to be confused because comparing two different setups in electricity can be quite challenging.

For those who are encountering this for the first time or are non-majors starting from scratch, it can feel quite awkward and heavy, which is completely understandable. However, since we've worked through it together, you can definitely make it your own. So, we can say it's 25 times, and while it has only been tested 6 times, this number could change, and it could have been tested more often. Therefore, this type has not just been tested 6 times but rather 10 or even more than 12 times. So, even if it's given as a 15-point question instead of a 5-point one, you should be able to solve it just fine. Now, we've looked at the second problem together, and we’re moving on to the last one. I've brought the third question, which has been tested 8 times, starting from the fourth test in 2018 up to the third test in 2023. Let me read the question: Which of the following is not a ferromagnetic material? Ah, I've heard this one quite a bit.

What are ferromagnetic materials? Ferromagnetic materials refer to substances that exhibit a significant degree of magnetization. Now, just take this: iron, nickel, cobalt, and manganese. Then there are paramagnetic materials. Paramagnetic materials are substances that are magnetized, although not to the same extent as ferromagnetic materials. You should know five examples: platinum, oxygen, nitrogen, aluminum, and tin. Now, for diamagnetic materials, you can think of them as everything else that is not included in the previous categories. So, you don't need to memorize anything special about diamagnetic materials. You can simply remember that if it's not one of the two mentioned, then it is a diamagnetic material. Therefore, we don't need to memorize all three categories; just remember the first two. So, ferromagnetic materials include iron, nickel, and manganese.

If we change the order of the mnemonic, we have Nico Cheolmang, which means Nico got caught in the iron mesh. There is a mnemonic technique for this. Anyway, you can remember it as either Cheol Nico or Nico Cheolmang. It might be a bit embarrassing, but if you can memorize this, you can do everything. So what do we say? Nico Cheolmang. Ah, Nico Cheolmang got caught. We're going with Nico Cheolmang. And this is just to remember Baeksanji. Let's go with 137R, 137 Alju, Baeksanji. What was Baek Jong-won's way of speaking? He speaks with that Alju accent, right? So, thinking of Baek Jong-won, we remember Baeksanji like this. That's how we do it. Baeksanji Alju. Next, for the paramagnetic substance, you just need to consider the remaining elements, not the first two. So, for the lecture, we are not talking about the paramagnetic substance. Since it's Nico Cheolmang, it’s not platinum. Platinum is a diamagnetic substance.

We are now facing a problem, but in reality, we have memorized everything, so whether it's a strong magnetic body, a box magnetic body, or a weak magnetic body, we can solve them all. This is because we know about nickel-copper alloys and the Baeksan system. So, I hope you can take just this much with you. Today, we discussed the written exam questions for the regular technician certification that have been asked more than five times in the past eight years. How did you find today's session? The first question and the third question were somewhat memorization-based, while the second question involved a rather tricky problem related to the resistance values in series or parallel connections and comparing them. I hope you will reflect on these problems once more and internalize them. I have prepared three questions for you today.

In the next video, I will bring you better and more useful content. Thank you for today as well.

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