Electric current
| English | Chinese | Pinyin |
|---|---|---|
| current | 电流 | diànliú |
| charge carrier | 载流子 | zài liú zi |
| conventional current | 常规电流 | cháng guī diàn liú |
| quantised | 量子化 | liàng zǐ huà |
| elementary charge | 基本电荷 | jī běn diàn hè |
| coulomb | 库仑 | kù lún |
| drift velocity | 漂移速度 | piāo yí sù dù |
| semiconductor | 半导体 | bàn dǎo tǐ |
Slower than a snail
- Flip a switch and the light comes on instantly.
- Yet the electrons themselves drift through the wire slower than a snail.
- The push (the electric field) travels fast; the charges crawl.
What current 电流 is
- An electric current is a flow of charge carriers 载流子 (electrons in a metal, ions in a liquid).
- Conventional current 常规电流 points the way positive charge would flow — opposite to the electrons.
High-voltage power lines carry electrical energy across the country
Current, voltage and resistance
Current is the rate of flow of charge. Raise the voltage and current rises; raise the resistance and it falls — I = V / R.
Conventional current points in the direction that:
Conventional current is the flow of positive charge — opposite to the electron drift in a metal wire.
Charge comes in lumps
- Charge is quantised 量子化: the smallest free unit is the elementary charge 基本电荷 $e = 1.60 \times 10^{-19}\ \text{C}$.
- Every free charge is a whole-number multiple of $e$. Unit of charge: the coulomb 库仑 (C).
I-V characteristic of an ohmic conductor (metal wire at constant temperature)
The smallest free unit of charge is the ____ charge, $e = 1.6 \times 10^{-19}\ \text{C}$.
All free charges are whole-number multiples of the elementary charge $e$.
Current = charge per second
- $I = \dfrac{Q}{t}$, so $Q = It$. Unit: the ampere ($1\ \text{A} = 1\ \dfrac{\text{C}}{\text{s}}$).
- For a changing current, the charge is the area under an $I$–$t$ graph.
A wider conductor has less resistance
A current of $2.0\ \text{A}$ flows for $5.0\ \text{s}$. How much charge passes?
$Q = It = 2.0 \times 5.0 = 10\ \text{C}$.
The charge that has flowed equals the area under an $I$–$t$ graph.
Yes — current is the rate of flow of charge, so the area (current × time) gives the total charge.
Drift velocity 漂移速度
- $I = Anvq$ — area $A$, carrier density $n$, drift speed $v$, charge $q$ each.
- Same current in a thinner wire → faster drift; a semiconductor 半导体 (small $n$) → much faster drift.
Which equation gives the current in terms of the drift velocity?
Current = (area)(number density)(drift speed)(charge per carrier) $= Anvq$.
In a thinner wire carrying the same current, the electrons drift faster.
From $I = Anvq$, a smaller area $A$ at the same $I$ needs a larger drift speed $v$.
You've got it
- current is a flow of charge; conventional current = direction of positive flow
- $I = \dfrac{Q}{t}$, and charge is the area under an $I$–$t$ graph
- drift: $I = Anvq$ (charge comes in units of $e = 1.6 \times 10^{-19}\ \text{C}$)