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Hey there guys, Paul here from theengineeringmindset .com.
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In this video we're going to be looking at Ohm's Law
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to understand how it works as well as how to use it.
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There are also two problems at the end of this video for you to test your knowledge
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and see if you can solve.
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So what is Ohm's Law?
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Ohm's Law is the relationship between voltage, current and resistance and how they relate to each other.
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Ohm's law was developed by a German physicist named George Ohm
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who undertook many experiments to develop his theory including measuring current by touching live electrical circuits to see how much it hurt.
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As you might imagine the higher the current the more it hurt.
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Now there are three formulas we need to use for Ohm's law
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but we don't actually need to remember these and I'll show you a super easy tip in just a moment.
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So the three formulas we use for Ohm's law are voltage equals current multiplied by resistance, current equals voltage divided by resistance,
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and resistance equals voltage divided by current.
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If that seems like a lot to remember then don't worry because we don't need to remember them.
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All we need to remember is Ohm's triangle which looks like this.
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So you just need to remember these three letters in order V, I, R.
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Then we just write those letters in a triangle with V at the top
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and we draw a line to separate the letters.
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In fact you don't even need to remember those
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because I've made a free PDF guide for you with some worked examples
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which you can keep on your PC or your mobile and access wherever you need.
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Links for that can be found in the video description down below.
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Now all we do when we need to use a formula is cover up the letter we need.
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So if we want to find the voltage then we write V equals
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and then we cover up the V in the triangle.
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That leaves us with I and R so we write I multiplied by which means voltage equals current multiplied by resistance.
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You can write a little multiplication symbol in the triangle between the two letters if it helps you.
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Now I know what you're thinking, why is current represented with the letter I and not a C for current,
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or even a letter A for the unit of ampere?
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Well the unit of current is the ampere or the amp, and this is named after André Ampere, a French physicist.
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A couple of hundred years ago he undertook lots of experiments many involved varying the amount of electrical current.
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So he called this intensity du current or the intensity of current.
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So when he published his work they took the letter I and it became standard until this day.
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Now you might also come across formulas where the letter E is used instead of V.
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The letter E stands for EMF or electromotive force
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but don't worry about that just stick to using V
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and substitute V for E if you see it used in Ohm's law questions.
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Anyway so by covering V we get voltage equals current multiplied by resistance.
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If we want to find current then we write down I equals
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and then we cover up the letter I in the triangle.
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That gives us V and R
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so as V is above the R like a fraction
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we can write V divided by R therefore current is equal to voltage divided by resistance.
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If we want to find resistance then we write down R equals and we cover up R in the triangle.
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That leaves us with V and I so we write V divided by I
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which gives us resistance equals voltage divided by current.
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Let's look at some examples for how to use these formulas.
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First let's see how we find voltage and how it relates to the other parts.
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Let's say we have a simple electrical circuit with a battery and a resistor.
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We don't know what the voltage of the battery is though.
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The resistor is three ohms and when we connect a multimeter into the circuit, we see that we get a reading of two amps of current.
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We want to find the voltage, so using ohms triangle, we can cover up the V and that gives us V equals I multiplied by R.
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We know the current is two amps, so we can write that in, and we know the resistance is three ohms,
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so we can write that in also.
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Therefore, two amps multiplied by three ohms gives us six volts.
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The battery is therefore 6 volts.
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Now if you want to check your answers for ohms questions then I've built a free calculator on the website.
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You can just drop your numbers in and it will do the calculation for you.
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Links for that again in the video description down below.
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Coming back to the circuit, if we now double the voltage by connecting two 6 volt batteries in series we get 12 volts.
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If we now connect this to the same circuit the current also doubles from 2 amps to 4 amps.
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If we double the voltage again to 24 volts, the current will also double to 8 amps.
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So what's the relationship here?
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We can see that current is therefore directly proportional to voltage.
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If we double the voltage, we double the current.
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Remember voltage is like pressure, is the pushing force in the circuit.
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It pushes the electrons around the wires
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and we place things like lamps in the way of these electrons
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so that they have to flow through these and that causes the lamp to light up.
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By doubling the voltage we see that the current also doubles,
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meaning that more electrons are flowing and this occurs as we apply more pressure or more voltage.
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This is just like if we were to use a bigger water pump then more water will flow.
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Okay, so what about finding current?
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Let's say we now have a 3 ohm lamp connected to a 6 volt power supply.
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To find the current we write down I equals and then we cover up I in the triangle.
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That gives us V divided by R, so current equals voltage divided by resistance.
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We know the voltage is 6 volts and the resistance is 3 ohms, so the current is therefore 2 amps.
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And that's what we see on the multimeter.
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By the way, if you don't have a multimeter then I highly recommend you get one.
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It's essential for troubleshooting but also building your essential electrical knowledge.
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I'll leave some links down below for which one to get and from where.
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So we saw what happens when we use a resistance of 3 ohms in the circuit.
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But if we double the resistance to 6 ohms by placing another 3 ohm lamp into the circuit, the current halves to just 1 amp.
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If we double the resistance again to 12 ohms, the current will half again to 0 .5 amps.
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We can visually see this because the lamps will become less bright as the current reduces from the increase in resistance.
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So what's the relationship here?
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We can see that the current is inversely proportional to the resistance.
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When we double the resistance the current will decrease by half.
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If we half the resistance the current will double.
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Current is the flow of electrons or the flow of free electrons.
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For us to make this lamp shine we need to push electrons through it.
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How do we do that?
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We apply a voltage across the two ends.
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The voltage will push the electrons.
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The atoms inside the copper wire have free electrons in their valence shell
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which means they can very easily move to other copper atoms.
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They will naturally move to other atoms by themselves
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but this will be in random directions which is of no use to us.
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For the lamp to turn on we need lots of electrons to flow in the same direction.
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When we connect to voltage source
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we use the pressure of the battery to push the electrons through the circuit all in the same direction.
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For example to power this 1 .5 ohm resistive lamp with a 1 .5 volt battery requires 1 amp of current.
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This is equal to 6 ,242 ,000 ,000 electrons passing from the battery
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and through the lamp every second and if we can achieve this then the lamp will stay at full brightness.
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If the voltage or current reduces or the resistance of the circuit increases then the lamp will become dimmer.
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Okay now let's look at finding resistance.
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Say we have a resistive lamp connected to a 12 volt power supply.
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We don't know how much resistance is adding to the circuit but we measure the current at 0 .5 amps.
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To find the resistance we write down R equals and and then we cover up the R in the triangle.
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We're left with V and I, so resistance equals voltage divided by current.
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We know the voltage is 12 volts and the current is 0 .5 amps, so 12 divided by 0 .5 gives us 24 ohms of resistance.
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Resistance is the opposition to the flow of electrons.
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It tries to prevent electrons from flowing.
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That's why we use resistors in circuits to reduce the current and protect components such as an LED.
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If we try to connect an LED directly to a 9V battery, it will blow out because the voltage and the current are too high.
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But when we add a resistor into the circuit, then these are reduced, so the LED is protected and will shine brightly.
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So given the circuit, we can increase the current by increasing the voltage, or we can also increase the current by reducing the resistance.
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We can also reduce the current by increasing the resistance.
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Okay, time for you to test your skills.
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Can you solve these problems?
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I'll leave a link for the answer and the solution in the video description down below.
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Problem one, let's say we have this lamp which has a resistance of 240 ohms.
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If we plug this into an outlet in the US which uses 120 volts, what will the current be?
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Problem two, if I plug the same 240 ohm resistive lamp into an outlet in the UK, we get a current of 0 .958 amps.
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So what is the voltage being applied here?
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Okay guys, that's it for this video, but to continue your learning,
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then check out one of the videos on screen now and I'll catch you there for the next lesson.
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Don't forget to follow us on Facebook, Twitter, Instagram, LinkedIn, as well as theengineeringmindset .com.
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Từ vựng và ghi chú luyện nói cho bài học này
Bài luyện nói này dựa trên video “Ohms Law Explained - The basics circuit theory”. Những từ được nhắc lại nhiều nhất trong bài: current, voltage, resistance, Amp, lamp. Video này có 108 câu và 1796 từ để luyện shadowing. Phần lời nói dài 9:49. Người nói nói nhanh, khoảng 183 từ mỗi phút, nên sẽ có nhiều chỗ nối âm và nuốt âm. Chỉ 80% số từ nằm trong 3.000 từ tiếng Anh thông dụng nhất, nên từ vựng khá khó.
Từ vựng quan trọng trong video
15 từ đáng học trong video, kèm phiên âm và nghĩa:
| Từ | Phiên âm | Nghĩa |
|---|---|---|
| voltage danh từ | /ˈvəʊltɪd͡ʒ/ | điện áp, hiệu điện thế |
| lamp danh từ | /ˈlæ̝mp/ | đèn, đèn điện |
| divide động từ | /dɪˈvaɪd/ | chia |
| triangle danh từ | /ˈtɹaɪˌæŋɡəl/ | tam giác, hình tam giác |
| connect động từ | /kəˈnɛkt/ | nối |
| formula danh từ | /ˈfɔː.mjʊ.lə/ | công thức |
| metre danh từ | /ˈmiːtəɹ/ | mét, thước |
| atom danh từ | /ˈætəm/ | nguyên tử |
| wire danh từ | /ˈwaɪɚ/ | dây, dây thép |
| solve động từ | /sɒlv/ | giải quyết |
| experiment danh từ | /ɪkˈspɛɹ.ə.mənt/ | thí nghiệm |
| copper danh từ | /ˈkɒpə/ | đồng |
| leaf danh từ | /lif/ | lá |
| shine động từ | /ʃaɪn/ | toả sáng, chiếu sáng |
| plug danh từ | /plʌɡ/ | phích, đầu cắm |
Cụm động từ bạn sẽ nghe
| Từ | Nghĩa |
|---|---|
| write down động từ | viết ra, ghi xuống |
| check out động từ | tính tiền, trả phòng |
Ngữ pháp trong video
Những cấu trúc người nói dùng nhiều nhất, kèm đúng cụm từ trong video:
| Cấu trúc | Trong video |
|---|---|
| Câu bị động be + quá khứ phân từ — nhấn vào việc xảy ra, không phải người làm | was developed · can be found · is named |
| Thì hiện tại hoàn thành have/has + quá khứ phân từ — việc đã xảy ra nhưng còn liên quan đến hiện tại | I've made · I've built |
| Mệnh đề quan hệ who / which + mệnh đề — thêm thông tin về người hoặc vật | directions which is · lamp which has |
Phát âm cần chú ý
Người nói dùng 16 dạng rút gọn, ví dụ don't, I'll, I've. Hãy nói theo dạng ngắn đúng như bạn nghe.
- Âm “sh” và “zh”: shine /ʃaɪn/, shell /ʃɛl/, fraction /ˈfɹæk.ʃən/, calculation /ˌkælkjuˈleɪʃn̩/, multiplication /ˌmʌltɪplɪˈkeɪʃən/
- Từ dài — đặt trọng âm cho đúng: electrical /ɪˈlɛktɹɪkəl/, experiment /ɪkˈspɛɹ.ə.mənt/, naturally /ˈnæt͡ʃ.(ə.)ɹə.li/, intensity /ɪnˈtɛnsɪti/, visually /ˈvɪzju.əli/
Những âm người Việt thường gặp khó:
- Cụm phụ âm cuối — đọc đủ từng âm, đừng nuốt âm cuối: lamp /ˈlæ̝mp/, connect /kəˈnɛkt/, solve /sɒlv/, experiment /ɪkˈspɛɹ.ə.mənt/, component /kəmˈpoʊ.nənt/
- Âm /tʃ/, /dʒ/ và /ʒ/ — tiếng Việt không có, đừng đọc thành “ch” hay “gi”: voltage /ˈvəʊltɪd͡ʒ/, naturally /ˈnæt͡ʃ.(ə.)ɹə.li/, substitute /ˈsʌb.stɪ.tʃuːt/
- Âm /æ/ — mở miệng rộng hơn âm “e”: lamp /ˈlæ̝mp/, triangle /ˈtɹaɪˌæŋɡəl/, atom /ˈætəm/, naturally /ˈnæt͡ʃ.(ə.)ɹə.li/, fraction /ˈfɹæk.ʃən/
Cách luyện với video này
- Nghe hết video một lần, chưa cần nói, và ghi lại những từ bạn chưa biết.
- Bắt đầu ở tốc độ 0,75×, nói đuổi từng câu, rồi quay lại tốc độ bình thường khi đã quen.
- Ghi âm giọng mình rồi so với bản gốc, chú ý các từ như voltage, lamp, divide.
Phương Pháp Shadowing Là Gì?
Shadowing là kỹ thuật học ngôn ngữ có cơ sở khoa học, ban đầu được phát triển cho chương trình đào tạo phiên dịch viên chuyên nghiệp và được phổ biến rộng rãi bởi nhà đa ngôn ngữ học Dr. Alexander Arguelles. Nguyên lý cốt lõi đơn giản nhưng cực kỳ hiệu quả: bạn nghe tiếng Anh của người bản xứ và lặp lại to ngay lập tức — như một "cái bóng" (shadow) đuổi theo người nói với độ trễ chỉ 1–2 giây. Khác với luyện ngữ pháp hay học từ vựng bị động, Shadowing buộc não bộ và cơ miệng phải đồng thời xử lý và tái tạo ngôn ngữ thực tế. Các nghiên cứu khoa học xác nhận phương pháp này cải thiện đáng kể phát âm, ngữ điệu, nhịp điệu, nối âm, kỹ năng nghe và độ lưu loát khi nói — đặc biệt hiệu quả cho người luyện IELTS Speaking và muốn giao tiếp tiếng Anh tự nhiên như người bản ngữ.