跟讀練習: Ohms Law Explained - The basics circuit theory - 通過影片學習英語口語
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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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本課的詞彙與口語要點
這節口語課以影片“Ohms Law Explained - The basics circuit theory”為素材。 影片中反覆出現的詞有：current, voltage, resistance, Amp, lamp。 這段影片共有 108 個句子、1796 個單詞可供跟讀。 講話部分時長為 9:49。 說話人語速較快，每分鐘約 183 個詞，會出現較多連讀和弱讀。 只有 80% 的單詞屬於英語最常用的 3,000 詞，詞彙難度較高。
影片中的重點詞彙
影片中 15 個值得學習的單詞，附發音和釋義：
| 單詞 | 發音 | 釋義 |
|---|---|---|
| voltage 名詞 | /ˈvəʊltɪd͡ʒ/ | 電壓 /電壓 |
| lamp 名詞 | /ˈlæ̝mp/ | 燈 /燈, 電燈 /電燈 |
| divide 動詞 | /dɪˈvaɪd/ | 分, 分裂 |
| triangle 名詞 | /ˈtɹaɪˌæŋɡəl/ | 三角, 三角形 |
| connect 動詞 | /kəˈnɛkt/ | 連線 /連線 |
| litter 名詞 | /ˈlɪt.ɚ/ | 墊料 |
| formula 名詞 | /ˈfɔː.mjʊ.lə/ | 公式 |
| metre 名詞 | /ˈmiːtəɹ/ | 米, 公尺 |
| atom 名詞 | /ˈætəm/ | 原子 |
| wire 名詞 | /ˈwaɪɚ/ | 線 /線, 鐵絲 /鐵絲 |
| solve 動詞 | /sɒlv/ | 解決 /解決 |
| experiment 名詞 | /ɪkˈspɛɹ.ə.mənt/ | 實驗 /實驗 |
| copper 名詞 | /ˈkɒpə/ | 銅 /銅 |
| leaf 名詞 | /lif/ | еезы, еер |
| shine 動詞 | /ʃaɪn/ | 發亮 /發亮, 照 |
影片中出現的短語動詞
| 單詞 | 釋義 |
|---|---|
| cover up 動詞 | 掩蓋 /掩蓋, 掩飾 /掩飾 |
| write down 動詞 | 寫下來 /寫下來, 寫下 /寫下 |
| check out 動詞 | 退房 |
影片中的語法
說話人最常用的結構，並附上影片中的原話：
| 結構 | 影片中的用法 |
|---|---|
| 被動語態 be + 過去分詞 — 強調發生了什麼，而不是誰做的 | was developed · can be found · is named |
| 現在完成時 have/has + 過去分詞 — 過去發生但與現在仍有關聯的事 | I've made · I've built |
| 定語從句 who / which + 從句 — 補充說明人或事物 | directions which is · lamp which has |
需要注意的發音
說話人用了 16 次縮略和弱讀形式，例如 don't, I'll, I've。請按聽到的簡短形式來說。
- “sh” 和 “zh” 音: shine /ʃaɪn/, shell /ʃɛl/, fraction /ˈfɹæk.ʃən/, calculation /ˌkælkjuˈleɪʃn̩/, multiplication /ˌmʌltɪplɪˈkeɪʃən/
- 長單詞——注意重音位置: electrical /ɪˈlɛktɹɪkəl/, experiment /ɪkˈspɛɹ.ə.mənt/, naturally /ˈnæt͡ʃ.(ə.)ɹə.li/, intensity /ɪnˈtɛnsɪti/, visually /ˈvɪzju.əli/
中文母語者容易讀錯的音：
- /v/ — 不要讀成 /w/，上齒輕觸下唇: voltage /ˈvəʊltɪd͡ʒ/, divide /dɪˈvaɪd/, solve /sɒlv/, wherever /(h)wɛɹˈɛvɚ/, vary /ˈvɛəɹi/
- 詞尾子音 — 要讀清楚，後面不要加母音: voltage /ˈvəʊltɪd͡ʒ/, circuit /ˈsɝ.kɪt/, lamp /ˈlæ̝mp/, divide /dɪˈvaɪd/, connect /kəˈnɛkt/
- /r/ 和 /l/ 的區別: triangle /ˈtɹaɪˌæŋɡəl/, electrical /ɪˈlɛktɹɪkəl/, relate /ɹɪˈleɪt/, naturally /ˈnæt͡ʃ.(ə.)ɹə.li/, brightly /ˈbɹaɪtli/
如何用這段影片練習
- 先完整聽一遍影片，不要開口，記下不認識的單詞。
- 先用 0.75 倍速逐句跟讀，熟練之後再回到正常速度。
- 錄下自己的聲音並與原聲對比，特別注意 voltage, lamp, divide 這類單詞。
什麼是跟讀法？
跟讀法 (Shadowing) 是一種有科學依據的語言學習技巧，最初開發用於專業口譯員的培訓，並由多語言者Alexander Arguelles博士普及。這個方法簡單而強大：您在聽英語母語原聲的同時立即大聲重複——就像是一個延遲1-2秒緊跟說話者的影子。與被動聽力或語法練習不同，跟讀法強迫您的大腦和口腔肌肉同時處理並模模擬實的講話模式。研究表明它能顯著提高發音準確性，語調，節奏，連讀，聽力理解和口語流利度——使其成為雅思口語備考和真實英語交流最有效的方法之一。