Current, e.m.f. and potential difference
Introduced| English | Meaning |
|---|---|
| electric current/ɪˈlektrɪk ˈkʌrənt/ | The rate of flow of electric charge. |
| potential difference/pəˈtenʃl ˈdɪfrəns/ | The energy transferred per unit charge between two points. |
| amperes/ˈæmpeəz/ | The SI unit of electric current, equivalent to one coulomb of charge passing per second. |
| ammeter/ˈæmiːtə/ | An instrument connected in series to measure electric current. |
| conventional current/kənˈvenʃənl ˈkʌrənt/ | Electric current defined in the direction that positive charge would move. |
| direct current/daɪˈrekt ˈkʌrənt/ | Current flowing in one direction only, as supplied by a battery. |
| alternating current/ˈɔːltəneɪtɪŋ ˈkʌrənt/ | Current that repeatedly reverses direction, as supplied by the mains. |
| volts/vəʊlts/ | The SI unit of potential difference, equal to one joule of energy transferred per coulomb of charge. |
| voltmeter/ˈvəʊltmiːtə/ | An instrument connected across two points to measure their potential difference. |
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.
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.
Ohm's law
V = IR
For an ohmic conductor the current is proportional to the voltage — the gradient is 1/resistance.
A charge of $30\ \text{C}$ flows past a point in $10\ \text{s}$. What is the current, in A?
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$I = \dfrac{Q}{t} = \dfrac{30}{10} = 3.0\ \text{A}$.
A current of $2.0\ \text{A}$ flows for $5.0\ \text{s}$. How much charge passes, in C?
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$Q = It = 2.0 \times 5.0 = 10\ \text{C}$.
How is an ammeter connected to measure the current through a lamp?
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An ammeter goes in series so the same current flows through it and the lamp. (A voltmeter goes in parallel.)
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
Conventional current flows around the outside of a cell from:
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Conventional current is taken from + to −. The electrons themselves drift the opposite way (− to +).
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).
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?
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$E = \dfrac{W}{Q} = \dfrac{120}{60} = 2.0\ \text{V}$.
A voltmeter is connected:
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A voltmeter measures the p.d. across a component, so it is connected in parallel with it.
Match each electrical quantity to its meaning.
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Each item links the term to its correct meaning.
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