Inhaltsverzeichnis

Block 01 — Physical Quantities, Units, Charge & Current

Learning objectives

90-minute plan

  1. Warm-up (10 min): SI prefixes speed-drill; unit sanity checks (▶ quick quiz).
  2. Core concepts & derivations (60 min): SI system & prefixes → charge and the elementary charge → current as charge per time; conventional vs electron flow; reference arrows in circuits.
  3. Practice (15 min): ✎ Conversions & short calculations (prefixes; Q–I–t triangle); direction questions with mixed charge carriers.
  4. Wrap-up (5 min): Recap key formulas and common mistakes; preview: voltage & potential (next block).

Conceptual overview

What’s the game? In circuits we count how much charge moves (Q, coulombs) and how fast it moves (I, amperes). SI units and prefixes let us express tiny sensor signals and huge lightning currents on one common scale. Current direction is a convention (positive-charge movement) and must not be confused with the motion of electrons, which are negatively charged and usually move the other way.

Core definitions & formulas

SI base & derived (used today)

Prefixes (selected)

Charge (discrete and continuous)

Current (definition)

Conventional vs electron flow

Symbol Meaning SI unit Typical values
$Q$ Electric charge C $\mathrm{pC}$ (sensors) … $\mathrm{mC}$
$e$ Elementary charge C $1.602\times 10^{-19}~\mathrm{C}$
$n$ Number of charges/particles $10^3 \ldots 10^{20}$ (context dependent)
$t$ Time s $\mathrm{ms}$ … $\mathrm{s}$
$I$ Electric current ($\mathrm{d}Q/\mathrm{d}t$) A $\mathrm{\mu A}$ … $\mathrm{A}$
Tab. 1: Symbols, units, typical values

Worked example(s)

Example 1 — Prefix fluency & charge moved

A sensor draws $3.6~\mathrm{mA}$ continuously. a) Express this in $\mathrm{A}$ and in $\mathrm{\mu A}$. b) How much charge passes in $250~\mathrm{ms}$?

Solution. a) $3.6~\mathrm{mA}=3.6\times 10^{-3}~\mathrm{A}=3600~\mathrm{\mu A}$. b) $Q = I \cdot t = 3.6\times 10^{-3}~\mathrm{A}\cdot 0.250~\mathrm{s}=9.0\times 10^{-4}~\mathrm{C}=0.90~\mathrm{mC}$.

Example 2 — From particles to current

A current in a thin gold wire is due to electrons. In $20~\mathrm{ms}$, $n=7.5\times 10^{15}$ electrons pass a cross-section. What average current flows?

Solution. Total charge $Q = n e = 7.5\times 10^{15}\cdot 1.602\times 10^{-19}~\mathrm{C}\approx 1.20\times 10^{-3}~\mathrm{C}$. $I \approx Q/t = (1.20\times 10^{-3})/0.020 \approx 0.060~\mathrm{A}=60~\mathrm{mA}$. Direction: electron motion right→left implies conventional current left→right.

Example 3 — Mixed carriers & current direction

In an electrolyte between faces $A_1$ and $A_2$, during $\Delta t=1~\mathrm{s}$, $\Delta Q_p=+40~\mathrm{\mu C}$ moves from $A_1$ to $A_2$ and $\Delta Q_n=-25~\mathrm{\mu C}$ (negative) moves from $A_2$ to $A_1$. What is the algebraic current from $A_1$ to $A_2$?

Solution. Total charge transfer $\Delta Q=\Delta Q_p-\Delta Q_n = 40~\mathrm{\mu C}-(-25~\mathrm{\mu C})=65~\mathrm{\mu C}$. $I=\Delta Q/\Delta t=65~\mathrm{\mu A}$ from $A_1$ to $A_2$ (positive).

Quick checks

Embedded resources

Common pitfalls & misconceptions

Mini-assignment / homework (optional)

⚠ Safety: When measuring current, never put a multimeter in voltage mode across a source; use the current input and series connection to avoid a short circuit.