Current, e.m.f. and potential difference · 电流、电动势与电势差
| English | 中文 | Pinyin · 拼音 |
|---|---|---|
| electric current/ɪˈlektrɪk ˈkʌrənt/ | 电流 | diàn liú |
| potential difference/pəˈtenʃl ˈdɪfrəns/ | 电势差 | diàn shì chà |
| amperes/ˈæmpeəz/ | 安培 | ān péi |
| ammeter/ˈæmiːtə/ | 电流表 | diàn liú biǎo |
| conventional current/kənˈvenʃənl ˈkʌrənt/ | 常规电流 | cháng guī diàn liú |
| direct current/daɪˈrekt ˈkʌrənt/ | 直流电 | zhí liú diàn |
| alternating current/ˈɔːltəneɪtɪŋ ˈkʌrənt/ | 交流电 | jiāo liú diàn |
| volts/vəʊlts/ | 伏特 | fú tè |
| voltmeter/ˈvəʊltmiːtə/ | 电压表 | diàn yā biǎo |
What is flowing?
- Switch on a lamp and something flows through the wires — an electric current 电流.
- A cell gives that flow the push it needs.
- Three quantities describe it all: current, e.m.f. and potential difference 电势差.
在流动的是什么?
- 打开一盏灯,某物流过导线——一个电流(electric current)。
- 一个电池给那个流动它需要的推力。
- 三个量描述这一切:电流、电动势和电势差。
Electric current
- An electric current is a flow of electric charge, measured in amperes 安培 (A) with an ammeter 电流表 joined in series.
- Current is the charge passing a point each second:
- $I$ in amperes, $Q$ in coulombs (C), $t$ in seconds.
Conventional current 常规电流 flows from the positive terminal to the negative terminal, through the external circuit. The ammeter (A) is connected in series to measure the current.
Worked example. A current of $2\ \text{A}$ flows for $30\ \text{s}$. Charge $= It = 2 \times 30 = 60\ \text{C}$.
Conventional current vs electron flow. Conventional current is drawn from $+$ to $-$, but the actual electrons in a metal wire drift the opposite way ($-$ to $+$). The convention was set before electrons were discovered, and we still use it today.
电流
- 一个电流是一个电荷的流动,以安培(amperes,A)测量,用一个串联连接的电流表(ammeter)。
- 电流是每秒经过一个点的电荷:
- $I$ 以安培计,$Q$ 以库仑(C)计,$t$ 以秒计。
常规电流从正端流向负端,通过外部电路。电流表(A)被串联连接以测量电流。
例题。 一个 $2\ \text{A}$ 的电流流动 $30\ \text{s}$。电荷 $= It = 2 \times 30 = 60\ \text{C}$。
常规电流对电子流动。 常规电流被画成从 $+$ 到 $-$,但一根金属导线中实际的电子漂移相反的方向($-$ 到 $+$)。这个惯例在电子被发现之前就被确立了,而我们今天仍使用它。
Ohm's law · 欧姆定律
V = IR
For an ohmic conductor the current is proportional to the voltage — the gradient is 1/resistance. · 对一个欧姆导体,电流与电压成正比——斜率是 1/电阻。
A charge of $30\ \text{C}$ flows past a point in $10\ \text{s}$. What is the current, in A? · 一个 $30\ \text{C}$ 的电荷在 $10\ \text{s}$ 内流过一点。电流是多少,以 A 计?
$I = \dfrac{Q}{t} = \dfrac{30}{10} = 3.0\ \text{A}$. · $I = \dfrac{Q}{t} = \dfrac{30}{10} = 3.0\ \text{A}$。
A current of $2.0\ \text{A}$ flows for $5.0\ \text{s}$. How much charge passes, in C? · 一个 $2.0\ \text{A}$ 的电流流动 $5.0\ \text{s}$。通过多少电荷,以 C 计?
$Q = It = 2.0 \times 5.0 = 10\ \text{C}$. · $Q = It = 2.0 \times 5.0 = 10\ \text{C}$。
How is an ammeter connected to measure the current through a lamp? · 一个电流表如何被连接以测量通过一盏灯的电流?
An ammeter goes in series so the same current flows through it and the lamp. (A voltmeter goes in parallel.) · 一个电流表串联,所以相同的电流流过它和灯。(一个电压表并联。)
Direction and type of current
- Conventional current flows from + to − round the outside of the cell.
- The free electrons in a metal really drift the other way (− to +).
- Direct current 直流电 (d.c.) always flows one way (a battery). Alternating current 交流电 (a.c.) keeps swapping direction (the mains).
Direct current is steady in one direction; alternating current keeps swapping direction, crossing zero many times each second
Resistors, components that control the current in a circuit
电流的方向和类型
- 常规电流(conventional current)绕电池外面从 + 到 − 流动。
- 一根金属中的自由电子真的漂移另一个方向(− 到 +)。
- 直流(direct current,d.c.)总是单向流动(一个电池)。交流(alternating current,a.c.)不断交换方向(市电)。
直流在一个方向上平稳;交流不断交换方向,每秒多次穿过零
电阻器,在一个电路中控制电流的元件
Conventional current flows around the outside of a cell from: · 常规电流绕一个电池外面从以下流动:
Conventional current is taken from + to −. The electrons themselves drift the opposite way (− to +). · 常规电流被取为从 + 到 −。电子本身漂移相反的方向(− 到 +)。
E.m.f. and potential difference
- Both are measured in volts 伏特 (V) and both are work done per unit charge:
- e.m.f. is the energy a source gives each coulomb to drive it round the whole circuit.
- potential difference (p.d.) is the energy each coulomb gives up in one component.
- A voltmeter 电压表 measures them, joined in parallel (across the component).
电动势和电势差
- 两者都以伏特(volts,V)测量且都是每单位电荷做的功:
- 电动势(e.m.f.)是一个源给每库仑驱动它绕整个电路的能量。
- 电势差(potential difference,p.d.)是每库仑在一个元件中放出的能量。
- 一个电压表(voltmeter)测量它们,并联连接(跨过元件)。
A cell does $120\ \text{J}$ of work driving $60\ \text{C}$ of charge round a circuit. What is its e.m.f., in V? · 一个电池做 $120\ \text{J}$ 的功驱动 $60\ \text{C}$ 的电荷绕一个电路。它的电动势是多少,以 V 计?
$E = \dfrac{W}{Q} = \dfrac{120}{60} = 2.0\ \text{V}$. · $E = \dfrac{W}{Q} = \dfrac{120}{60} = 2.0\ \text{V}$。
A voltmeter is connected: · 一个电压表被连接:
A voltmeter measures the p.d. across a component, so it is connected in parallel with it. · 一个电压表测量跨过一个元件的 p.d.,所以它与它并联连接。
Match each electrical quantity to its meaning. · 把每个电学量与它的含义配对。
Each item links the term to its correct meaning. · 每一项把术语与它正确的含义联系。
You've got it
- current = flow of charge: $I = \dfrac{Q}{t}$; ammeter in series, in amperes
- conventional current + → −; electrons drift − → +; d.c. one way, a.c. swaps
- e.m.f. (whole circuit) and p.d. (one component) are both $\dfrac{W}{Q}$ in volts
- voltmeter in parallel
你掌握了
- 电流 = 电荷的流动:$I = \dfrac{Q}{t}$;电流表串联,以安培计
- 常规电流 + → −;电子漂移 − → +;d.c. 单向,a.c. 交换
- 电动势(整个电路)和 p.d.(一个元件)都是以伏特计的 $\dfrac{W}{Q}$
- 电压表并联