關於電的問題

Ok. I am a graduate from electrical engineering. Your questions are basic and fundamental. It's good you can ask such questions, reflecting you have a habit to THINK. pls dont mind if i use English. I will try to make my words simple

*** First question: 點解electrons本身有電荷。但如果冇electric supply，就唔能用free electrons的電荷產生電能？***

In conductors (導電體), there are free electrons. they move in random speed and directions. some electrons move forward but some move backwards. So, the macroscopic effect results in zero NET movement.

Electrical supply is like a pump. It pumps the electrons to move in one specific direction. Remember, current (電流) is the flow of electrons. So, you can do nothing with the electrons alone. the electrons have to flow in a specific direction to constitute a current.


***Second question: 點解electrons一定要向正極先有電流？如果open curcuit，free electrons move randomly咁就冇電流？***

The current direction is conventionally defined. In old days, people thought the current flow is due to the positve charge flow. But now we know the current flow is due to the flow of electrons. However, we just stick to the old convention.

Hence, we say the current flows from the positive terminal of the battery.

In case of open-circuit, the current will have nowhere to flow. you can think this way. In case of open-circuit, the current faces a huge resistance brought by air. In fact, the electrons have no way to flow, resulting zero current.

Of course, if the voltage across the open-circuit is high enough, air will be ionized and complete the path of circuit. Current flows in this case.

***third question: 點解電池入面D chemical會用完？用完係因為chemical唔再o係電池入面，抑或D chemical變左另一種matter？***

Basically, the battery we used is chemical battery. Chemical energy is stored in the chemical inside. Through the electrochemical reactions, the chemical energy is converted into electrical energy. However, the chemicals will react each other and another product is formed. Until all reactants are used(直至所有反應物用完), all chemicals inside will become a product (生成物).

In simple words, when you buy a new battery, there is reactant A inside. When the battery is running low, chemical inside the battery will become product B. In the process of FROM A TO B, electrical energy is released.

In your words, that is **D chemical變左另一種matter**

***Forth question: 點解ammeter在light bulb前或後，測到的electric current係一樣？***

the ammeter is connected in series with the light bulb. the current in every point of the series circuit is the same. Hence, the measured electric current is the same.

You can think this way. You need to measure how many cars pass through the tunnel. So you can stand at the entrance of the tunnel, or in the middle of the tunnel or even at the exit of the tunnel. The measured results should be the same, unless some cars are crashed inside the tunnel..

**Final question: 點解個curcuit入面冇左light bulb，ammeter測到的electric current大左好多？**

there is a tungsten wire inside every light bulbs (traditional one). That wire has huge resistance(電阻). If you take away a bulb, you also take away that amount of resistance. The current now faces less resistance and is happy. So the current should increase. It is simple to understand. If the resistance seen by the current is smaller, the electrons tend to move more easily.

Thank you for reading. Please kindly choose my answer to be the best one if you are happy with my explanation.

2007-03-17 11:56:54 補充：
Remember the electrons always move, even without energy supply. The point is the movement is random. the electrons have to unit together and move together in order to constitute a current. Also, I am sorry to say that I found many mistakes and wrong concepts in the first answer

「點解electrons本身有電荷。但如果冇electric supply，就唔能用free electrons的電荷產生電能？」

electrons 不能說動就動，要有能量驅動

「點解electrons一定要向正極先有電流？如果open curcuit，free electrons move randomly咁就冇電流？」

唔係好清楚你問乜。

「點解電池入面D chemical會用完？用完係因為chemical唔再o係電池入面，抑或D chemical變左另一種matter？」

因為D chemical 以經變成 electricity，用左就冇


「點解ammeter在light bulb前或後，測到的electric current係一樣？」
因為成個 series circuit current 點都係同一個大細，不同的是 voltage


「點解個curcuit入面冇左light bulb，ammeter測到的electric current大左好多？」
因為 resistance 細左