Labo Élec

electrical lab simulator
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Reminder sheet

Electrostatics

Bastaya

Everything starts with charges attracting or repelling each other. Understand Coulomb's force and the field, and current will hold no mystery.

Matter is made of atoms: a nucleus (protons +, neutrons) surrounded by electrons (−). A body losing electrons becomes positive, one gaining electrons negative; the elementary charge is e = 1.6 × 10⁻¹⁹ C.

In a conductor free electrons move; in an insulator they do not. Two charges exert the Coulomb force on each other: attraction for opposite signs, repulsion otherwise. An electric field E exists between two plates at different potentials.

Formulas

Coulomb's lawF = k · q · q'd²
QuantitySymbolUnit
ForceFN
ChargeqC
Distancedm
Constant (9 × 10⁹)kN·m²/C²
Field between two platesE = Ue
QuantitySymbolUnit
Electric fieldEV/m
VoltageUV
Plate spacingem
Force on a chargeF = q · E
QuantitySymbolUnit
ForceFN
ChargeqC
Electric fieldEV/m

Worked example

Two charges q = 2 µC and q' = −3 µC are 10 cm apart. What force do they exert on each other? Attraction or repulsion?

F = k · q · q' / d² with d = 0.10 m:

F = 9 × 10⁹ × 2 × 10⁻⁶ × 3 × 10⁻⁶ / 0.10² = 5.4 × 10⁻² / 0.01 = 5.4 N.

Opposite signs: attraction.

Common mistakes

  • Leaving the distance in centimetres, or forgetting to square it.
  • Getting the direction wrong: opposite signs = attraction, like signs = repulsion.
  • Confusing the elementary charge e (1.6 × 10⁻¹⁹ C) with the coulomb: one coulomb is 6.25 × 10¹⁸ electrons.

Illustration

+−F = k · q · q' / d²

Opposite charges: attraction; the force decreases as 1/d².

Frequently asked questions

Has a negatively charged body gained or lost electrons?
It has gained some: the electron carries the negative charge. A body that loses electrons becomes positive.
Why does an insulator not conduct current?
Its electrons stay bound to their atoms: no free charge can move. In a metal, each atom releases an electron that travels through the whole conductor.

7 exercises in this topic

This chapter is covered by the paths: General electricity

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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