A zinc half-cell and a copper half-cell are connected by a wire with a load and by a salt bridge. Electrons flow through the wire from zinc to copper, while ions move through the bridge to balance charge. The lab computes the emf from the Nernst equation and the electrode mass rates from Faraday's law.
• A zinc anode, a copper cathode, an external circuit and load, a salt bridge and half-cell ion markers. • Sliders for log₁₀ zinc-ion activity and log₁₀ copper-ion activity (−3 to 0) and for external load resistance (10 to 1000 Ω), with toggles for closing the circuit, the salt bridge and explanatory particles. • Readouts: open-circuit emf, model current, loaded terminal voltage, load power, zinc loss rate and copper gain rate in mg/s. • Experiments: an open circuit keeps the emf measurable but current and deposition at zero; removing the salt bridge stops sustained current despite a connected wire.
Zn + Cu²⁺ → Zn²⁺ + Cu with E = 1.10 − (RT/2F) ln(a_Zn²⁺/a_Cu²⁺) at T = 298.15 K. The current is I = E/(R_load + 5 Ω) and the mass rate is IM/(2F). Raising copper activity or lowering zinc activity increases the emf.
The model assumes ideal activities, a fixed 25°C, an assumed 5 Ω internal resistance, and no polarization or changing reservoirs. Removing either connection prevents sustained modeled current. Ion and mass animations are explanatory; the emf and Faraday rates use the entered parameters.
It lets ions migrate between the half-cells so charge does not build up. Without it, sustained current stops even with the wire connected, as the Remove salt bridge experiment shows.
The potential difference between the half-cells exists whether or not current flows. With the circuit open the emf is still measurable but the current and deposition rates are zero.
Through the Nernst term (RT/2F) ln(a_Zn/a_Cu). Higher copper activity or lower zinc activity raises the emf above the standard 1.10 V.
No. The reservoirs are assumed constant, so activities do not change as zinc dissolves and copper deposits. The mass rates are reported as steady rates for the entered parameters.