Labo Élec
electrical lab simulator
Reminder sheet
Two laws suffice for any circuit: at a node, what comes in goes out; around a loop, the voltages balance. Write them, solve, check in the simulator.
Junction rule: at a junction, the sum of incoming currents equals the sum of outgoing currents (charge does not pile up).
Loop rule: around a closed loop, the algebraic sum of voltages is zero. Choose a direction; an EMF counts + if crossed from − to +, a drop R·I counts − if following the current. With several sources, write as many independent equations as unknowns and solve.
| Quantity | Symbol | Unit |
|---|---|---|
| Current | I | A |
| Quantity | Symbol | Unit |
|---|---|---|
| EMF | E | V |
| Voltage drop | R·I | V |
Currents I₁ = 2 A and I₂ = 0.5 A enter a node; a single conductor leaves it: what current? Then a loop: E = 12 V in series with R₁ = 20 Ω and R₂ = 40 Ω. Current and voltage of each resistor?
Junction rule: I₃ = I₁ + I₂ = 2.5 A.
Loop rule, following the current: E − R₁·I − R₂·I = 0, so 12 − 20·I − 40·I = 0 and I = 12 / 60 = 0.2 A.
U₁ = 20 × 0.2 = 4 V, U₂ = 40 × 0.2 = 8 V; 4 + 8 = 12 V, the loop closes.