Electric Current
Introduced| English | Meaning |
|---|---|
| current/ˈkʌrənt/ | The rate of flow of electric charge. |
| ampere/ˈæmpeə/ | The SI unit of electric current, equivalent to one coulomb of charge passing per second. |
| conventional current/kənˈvenʃənl ˈkʌrənt/ | Electric current defined in the direction that positive charge would move. |
| drift velocity/drɪft vəˈlɒsɪti/ | The average velocity of charge carriers due to an applied electric field. |
What is actually "flowing" in a wire carrying electricity?
- Flip a switch and something moves through the wire at once.
- That something is charge — mostly free electrons drifting along.
- Current 电流 measures how fast that charge flows past a point.
- Everything about circuits starts from this one flow.
Current is the rate of charge flow
- Current is charge per unit time: $I = \dfrac{dQ}{dt}$.
- Its unit is the ampere 安培 (1 A $= 1$ C/s).
- Over a steady flow, the average is just $I = \dfrac{Q}{t}$.
- The derivative form catches a current that changes moment to moment.
Drive a current
Build a simple loop and watch current flow around it from the battery.
Electric current is:
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$I = dQ/dt$ — charge passing a point per second.
If $10\ \text{C}$ passes a point in $5\ \text{s}$, find the current (in A).
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$I = Q/t = 10/5 = 2\ \text{A}$.
One ampere equals one coulomb per ____.
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1 A $= 1$ C/s.
Conventional current runs the "wrong" way
- Conventional current 常规电流 points the way a $+$ charge would move.
- The actual electrons drift the opposite way (they are negative).
- We keep the convention because the physics works out the same.
- Arrows on a circuit always mean conventional (positive) current.
Conventional current points in the direction that:
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Conventional current follows a $+$ charge — opposite to electron drift.
Electrons drift slowly, the signal is fast
- Individual electrons crawl along at a drift velocity 漂移速度 of about a millimetre per second.
- Yet the light comes on instantly — the field pushes the whole line at once.
- Think of a full pipe of water: push one end, the far end moves right away.
- Slow drifters, fast message.
Select all true statements about current.
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Amps, opposite-to-drift convention, slow drift. A bulb uses energy, not current.
Current is conserved in a series wire
- In a single unbranched wire, the current is the same everywhere.
- Charge doesn't pile up or vanish — what flows in flows out.
- A bulb does not "use up" current; it uses up energy.
- The same amps that enter a series bulb also leave it.
In a single series loop, the current is largest at the battery and smaller after each resistor.
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False. Charge is conserved, so the same current flows through every element of a series loop. What drops across each resistor is the potential, not the current.
A wire carries $6\ \text{C}$ of charge past a point in $2\ \text{s}$.
- $I = \dfrac{Q}{t} = \dfrac{6}{2} = 3\ \text{A}$.
- Three coulombs stream past every second.
A component uses energy, not current. The current leaving a bulb equals the current entering it — don't say the bulb "uses up" the amps. What drops across it is voltage (energy per charge), not current.
Current is the rate of charge flow, $I = dQ/dt$, in amperes (C/s). Conventional current runs the way a $+$ charge would go (opposite to electron drift). In a series wire current is conserved — a component uses energy, not current.
Write from a current of 5 amperes
- 20 coulomb pass a point in 4 seconds.
- Find the current. Say whether a series resistor uses up current.
A wire carries 20 coulomb past a point in 4 seconds. What is the current, and does a series resistor use that current up?
20 divided by 4 is 5 amperes. Charge is conserved along a single series path.
What is the current?
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20 coulomb divided by 4 seconds is 5 amperes, and series current is not consumed.