Labo Élec

electrical lab simulator
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Reminder sheet

Voltage & current

Bastaya

Voltage pushes, current flows. Keep that picture: voltage is measured between two points, current is counted in a branch.

The voltage U between two points is the difference of their potentials (V_A − V_B), in volts: it sets the charges in motion. The current I is the flow of charge, in amperes: 1 A = 1 C/s. By convention it flows from + to − outside the generator (electrons go the other way).

The quantity of electricity Q = I·t is counted in coulombs or ampere-hours. The current density J = I/S (A/mm²) limits the heating of a conductor. The European mains delivers 230 V phase to neutral, 400 V between phases.

Formulas

Quantity of electricityQ = I · t
QuantitySymbolUnit
Quantity of electricityQC
CurrentIA
Timets
Current densityJ = IS
QuantitySymbolUnit
Current densityJA/mm²
CurrentIA
Cross-sectionSmm²
Cross-section of a round wireS = π · d²4
QuantitySymbolUnit
Cross-sectionSmm²
Diameterdmm

Worked example

A 40 Ah battery delivers a steady current of 2 A. How long can it last? What quantity of electricity, in coulombs, does that represent? A 1.5 mm² wire carrying 10 A: what current density?

Q = I · t so t = Q / I = 40 / 2 = 20 h.

In coulombs: 40 Ah × 3 600 = 144 000 C (1 Ah = 3 600 C).

J = I / S = 10 / 1.5 = 6.7 A/mm²: too much for a domestic installation (5 to 6 A/mm² is accepted), the wire would heat up.

Common mistakes

  • Saying that "the voltage flows through" a component: voltage is measured between two points, it is the current that flows through.
  • Mixing the conventional direction (+ to −) with the electron flow (the opposite): the course and diagrams use the conventional direction.
  • Q = I · t with t in hours gives ampere-hours, not coulombs: for coulombs, t must be in seconds.

IEC symbols

GVoltage source
VVoltmeter
AAmmeter

Mini circuit

Open in the sandbox

Frequently asked questions

What is the difference between voltage and current?
Voltage is the "pressure" that pushes the charges, in volts, between two points; current is the resulting flow of charges, in amperes, through a conductor. Without a closed circuit there is a voltage but no current.
How many coulombs is 1 Ah?
3 600 C: one ampere for one hour, i.e. 3 600 seconds. A 40 Ah battery thus stores 144 000 C.

22 exercises in this topic

This chapter is covered by the paths: Discovery, General electricity, ISL-oriented exercises

All sheets · All formulas · Glossary of electrical quantities · Standard electrical symbols (IEC 60617)

Practise this topic
Simulated time00:00:00
Q = I·t-
W = P·t-

Workbench

Circuit diagram CEI 60617

How does it work?

The perfboard: each big hole is a circuit node. You push a single leg of a component (resistor, lamp...) into it. The 4 small holes around it are connected to it: that is where chips and leads plug in.

  1. Drag a component onto the board: it snaps onto the grid. R rotates it.
  2. To join neighbouring big holes, click Jumper chip in the palette (or C) then drag on the board: a line of chips is laid at once. Bridged holes become a single node.
  3. For long links and for the power supply (placed next to the board), drag a lead from one big hole to another (or to a terminal).
  4. As soon as a loop is closed, the circuit is solved in real time (Ohm + Kirchhoff) and the diagram follows.
  5. Zoom with the wheel (or + / −, two-finger pinch); drag the background to pan, 0 fits the board. The + buttons around the board add a row or a column.

Del delete · R rotate · C chip brush · Esc cancel · wheel zoom · drag the background to pan

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