TheAlgorithms/Python · error · ValueError

Resistance must be positive.

Error message

Resistance must be positive.

What it means

Thrown by charging_capacitor() when resistance <= 0. The RC time constant tau = R*C appears in the exponent of the charging curve; zero or negative resistance makes the model undefined (instant/inverted charging), so the library rejects it.

Source

Thrown at electronics/charging_capacitor.py:63

    Traceback (most recent call last):
        ...
    ValueError: Source voltage must be positive.

    >>> charging_capacitor(source_voltage=20,resistance=-2000,capacitance=30,time_sec=4)
    Traceback (most recent call last):
        ...
    ValueError: Resistance must be positive.

    >>> charging_capacitor(source_voltage=30,resistance=1500,capacitance=0,time_sec=4)
    Traceback (most recent call last):
        ...
    ValueError: Capacitance must be positive.
    """

    if source_voltage <= 0:
        raise ValueError("Source voltage must be positive.")
    if resistance <= 0:
        raise ValueError("Resistance must be positive.")
    if capacitance <= 0:
        raise ValueError("Capacitance must be positive.")
    return round(source_voltage * (1 - exp(-time_sec / (resistance * capacitance))), 3)


if __name__ == "__main__":
    import doctest

    doctest.testmod()

View on GitHub (pinned to f5988cc097)

Solutions

1. Use a strictly positive resistance, e.g. the actual series resistor value in ohms.
2. If you assumed an ideal circuit with no resistor, this model cannot represent it — add a realistic ESR/series resistance.
3. Verify keyword arguments; positional calls easily swap resistance and capacitance.

Example fix

# before
charging_capacitor(source_voltage=20, resistance=-2000, capacitance=30, time_sec=4)

# after
charging_capacitor(source_voltage=20, resistance=2000, capacitance=30, time_sec=4)

if resistance <= 0:
    raise ValueError("resistance must be positive")
v = charging_capacitor(source_voltage, resistance, capacitance, time_sec)

When it happens

Trigger: charging_capacitor(source_voltage=20, resistance=-2000, capacitance=30, time_sec=4) or resistance=0. This check runs after the source-voltage check.

Common situations: Ideal-wire assumptions (R=0) carried over from schematic simplifications; unit confusion (kOhm vs Ohm) leading to negative/zero computed values; parameter-order mix-ups.

Related errors

• Electron concentration cannot be negative in a semiconductor
• Hole concentration cannot be negative in a semiconductor
• Intrinsic concentration cannot be negative in a semiconducto
• Source voltage must be positive.
• Capacitance must be positive.

AI-assisted analysis of TheAlgorithms/Python@f5988cc097 (2026-08-14). Data as JSON: /api/errors/e4c1e54d13666bdf. Report an issue: GitHub.