Electric Current
Introduced| English |
|---|
| current/ˈkʌrənt/ |
| ampere/ˈæmpeə/ |
A river of charge, counted by the second
- Switch on a lamp and charge starts flowing through the wire like water in a pipe.
- Current 电流 measures how much charge flows past a point each second.
- The more charge per second, the bigger the current and the brighter the lamp.
- It is the first quantity you need to understand any circuit.
Current is charge per second
- Current is the rate of flow of charge: $I = \dfrac{Q}{t}$.
- Its unit is the ampere 安培 ($1\ \text{A} = 1\ \tfrac{\text{C}}{\text{s}}$).
- A $2\ \text{A}$ current means $2$ coulombs of charge pass each point every second.
- Current needs a complete path to flow — a break stops it instantly.
Build a current loop
Close a simple circuit and see current flow around the complete loop.
$6\ \text{C}$ of charge flows past a point in $3\ \text{s}$. What is the current, in amperes?
$I = Q/t = 6/3 = 2\ \text{A}$.
Electric current is defined as:
$I = Q/t$ — charge flowing per second.
Current is measured in ____.
$1\ \text{A} = 1\ \tfrac{\text{C}}{\text{s}}$.
Conventional current vs electron flow
- By convention, current flows from + to − (the direction a positive charge would move).
- But in a metal wire it is really electrons that drift, from − to +.
- Both descriptions give the same circuit behaviour; we draw the conventional direction.
- So "current out of the + terminal" is the standard, even though electrons go the other way.
In which direction does conventional current flow?
Conventional current flows + → −, the way a positive charge would move.
In a metal wire, the electrons actually drift opposite to the conventional current.
Electrons drift − → +, opposite to the conventional (+ → −) current direction.
What drives it
- A battery provides the push (a voltage) that keeps charge moving around the loop.
- Without a voltage difference, charges have no reason to flow — no current.
- The current is the same all the way around a simple (single-loop) circuit.
- Charge isn't used up as it goes; it just carries energy from the battery to the components.
Select all true statements about electric current.
Current needs a loop, equals $Q/t$, and is driven by voltage. Charge carries energy but is not used up.
Conventional current (+ → −) is the opposite of the actual electron flow (− → +) in a metal. Diagrams and formulas use the conventional direction — don't let the electron drift confuse you. And current only flows in a complete circuit.
$6\ \text{C}$ of charge flows past a point in $3\ \text{s}$. What is the current?
- $I = \dfrac{Q}{t} = \dfrac{6}{3} = 2\ \text{A}$.
Double the charge in the same time and the current doubles to $4\ \text{A}$.
Current is the rate of charge flow, $I = \dfrac{Q}{t}$ (in amperes), and needs a complete circuit. Conventional current flows + → −; in a metal the electrons actually drift − → +. Diagrams use the conventional direction.