diff --git a/electronics/charging_capacitor.py b/electronics/charging_capacitor.py
new file mode 100644
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+++ b/electronics/charging_capacitor.py
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+# source - The ARRL Handbook for Radio Communications
+# https://en.wikipedia.org/wiki/RC_time_constant
+
+"""
+Description
+-----------
+When a capacitor is connected with a potential source (AC or DC). It starts to charge
+at a general speed but when a resistor is connected in the  circuit with in series to
+a capacitor then the capacitor charges slowly means it will take more time than usual.
+while the capacitor is being charged, the voltage is in exponential function with time.
+
+'resistance(ohms) * capacitance(farads)' is called RC-timeconstant which may also be
+represented as τ (tau).  By using this RC-timeconstant we can find the voltage at any
+time 't' from the initiation of charging a capacitor with the help of the exponential
+function containing RC.  Both at charging and discharging of a capacitor.
+"""
+from math import exp  # value of exp = 2.718281828459…
+
+
+def charging_capacitor(
+    source_voltage: float,  # voltage in volts.
+    resistance: float,  # resistance in ohms.
+    capacitance: float,  # capacitance in farads.
+    time_sec: float,  # time in seconds after charging initiation of capacitor.
+) -> float:
+    """
+    Find capacitor voltage at any nth second after initiating its charging.
+
+    Examples
+    --------
+    >>> charging_capacitor(source_voltage=.2,resistance=.9,capacitance=8.4,time_sec=.5)
+    0.013
+
+    >>> charging_capacitor(source_voltage=2.2,resistance=3.5,capacitance=2.4,time_sec=9)
+    1.446
+
+    >>> charging_capacitor(source_voltage=15,resistance=200,capacitance=20,time_sec=2)
+    0.007
+
+    >>> charging_capacitor(20, 2000, 30*pow(10,-5), 4)
+    19.975
+
+    >>> charging_capacitor(source_voltage=0,resistance=10.0,capacitance=.30,time_sec=3)
+    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()