To determine the electromotive force (EMF) and internal resistance of an electrochemical primary cell by measuring terminal potential difference as a function of load current, evaluate the short-circuit current and maximum power transfer, and investigate instrument loading effects. Explore a virtual apparatus, collect readings and analyse a graph.
Ready for a virtual observation.
Adjust the apparatus, then take a reading.
Timing is generated from the model; a short on-screen trial represents a longer physical interval. Animation is illustrative.
Model calculations assume constant ambient temperature (20.0 °C), negligible battery heating, and ideal linear Butler-Volmer kinetics without irreversible chemical degradation or deep discharge hysteresis. Real secondary and primary cells exhibit transient polarization relaxation times (τ ~ 5 to 30 s), non-linear Butler-Volmer overpotentials at very high discharge rates, and finite internal thermal capacitance.
Provides a stable chemical electromotive force (nominal open-circuit EMF ℰ = 1.500 V) with an inherent internal electrolytic and polarization resistance (r = 2.500 Ω) arising from finite ionic mobility in the ammonium chloride / zinc electrolyte paste.
Provides calibrated non-inductive standard load resistances from 2.0 Ω to 50.0 Ω (tolerance ±0.1%), allowing smooth and repeatable adjustment of the external circuit current without introducing reactive impedance.
Measures the terminal potential difference V across the cell output posts with 0.001 V resolution and an input impedance exceeding 10 MΩ to ensure that voltmeter loading current is strictly negligible (< 0.15 μA).
Measures the closed-loop load current I from 0.0 mA to 350.0 mA with 0.1 mA resolution, engineered with a low-burden shunt resistance (R_A ≤ 0.05 Ω) to minimize insertion voltage drops.
Closes and opens the discharge circuit intermittently with negligible contact resistance (< 5 mΩ), preventing prolonged cell discharge and minimizing thermal heating or cumulative polarization during observation intervals.
Forms robust low-resistance electrical interconnections between circuit components with total lead resistance under 0.02 Ω, ensuring that all measured potential drops represent the cell and intended load.
To determine the electromotive force (EMF) and internal resistance of an electrochemical primary cell by measuring terminal potential difference as a function of load current, evaluate the short-circuit current and maximum power transfer, and investigate instrument loading effects. Suggested independent settings: 50.0, 100.0, 150.0, 200.0, 250.0, 300.0 mA.
Inspect the open-circuit cell using the high-impedance digital voltmeter with the main knife switch open; record the quiescent open-circuit terminal voltage to verify baseline electromotive force ℰ₀ = 1.500 V prior to drawing discharge current.
Establishing the open-circuit EMF before current flow verifies the chemical integrity of the cell and confirms that zero initial polarization exists.
Connect the circuit in series: primary cell positive terminal to ammeter input, ammeter output to the decade resistance box, and resistance box back to the cell negative terminal through the knife switch. Connect the high-impedance voltmeter directly across the cell battery posts (Kelvin voltage sensing configuration) to exclude lead resistance drops.
Kelvin 4-wire potential sensing ensures that the voltmeter measures the true terminal voltage across the cell electrodes, completely bypassing ohmic drops along the high-current connecting wires.
Set the decade resistance box to its highest load setting (producing minimum initial current I = 50.0 mA). Momentarily close the knife switch for 3 to 5 seconds, read both the ammeter current I and the digital voltmeter terminal potential difference V, and immediately open the switch to prevent chemical exhaustion or polarization buildup.
Starting at minimum current limits initial chemical consumption, while brief 3 to 5 second pulses prevent cumulative concentration polarization and internal Joule heating.
Allow the cell a 60-second recovery period for ionic diffusion to re-establish chemical equilibrium at the electrode interfaces. Step down the load resistance progressively to draw currents of 100.0 mA, 150.0 mA, 200.0 mA, 250.0 mA, and 300.0 mA, recording the stabilized terminal voltage V at each setting.
The 60-second recovery interval allows depolarizing agents to clear accumulated hydrogen films, ensuring each observation begins from identical equilibrium conditions.
Tabulate the paired values of load current I (in amperes) and terminal potential difference V (in volts). Plot terminal voltage V on the vertical axis against current I on the horizontal axis; fit an ordinary least-squares straight line to determine the slope m and the vertical intercept b.
Linear regression across multiple discharge levels averages out meter quantization noise and contact resistance fluctuations, yielding statistically robust fit parameters.
Extract the internal resistance as r = −m and the true electromotive force as ℰ = b. Calculate the extrapolated short-circuit current I_sc = ℰ / r, evaluate the maximum deliverable external power P_max = ℰ² / (4r), and compare the fitted parameters against manufacturer reference specifications.
Extrapolating from the linear regression determines the absolute physical operating bounds of the cell without requiring destructive short-circuit testing.
Points are your observations. The fit uses SI x and y values before display scaling.
| # | Load current I (mA) | Terminal voltage V (V) | Load current I (A) | Terminal voltage V (V) | Remove |
|---|
No readings yet. Take a virtual measurement and record it.