Electrical – I have a circuit with a 9V battery, followed by 3 10 ohm resistors, and an led all in series…

There are two steps to finding the Thevenin equivalent circuit: finding the Thevenin voltage and the Thevenin resistance.

Thevenin voltage is the voltage across the two points you interested in (Vin). In this case it is easy to calculate as there is no current flowing in the 43 and 60 \$\Omega\$ resistors thus no voltage drop. Thus the voltage at Vin is the same as the voltage form the source, 72 V.

Thevenin resistance is calculated by 'turning off' all independent current and independent voltage sources and calculating the resistance between the two points. Turning off a voltage source sets the voltage across it to 0, which results in a short (0 \$\Omega\$) in parallel with the 275 \$\Omega\$ resistor. Any resistor combined in parallel with a short results in a short, leaving you with the 43 and 60 \$\Omega\$ resistors now in series, giving a Thevenin resistance of 103 ohms.

Putting the two together gives you a voltage source of 72 V in series with a 103 \$\Omega\$ resistor for you Thevenin equivalent circuit.

The electrolytic cap in your circuit is reverse biased. The reverse biasing of the capacitor removes the isolating oxide layer, so it allows current to pass. If you connect the capacitor the right way after mistreating it this way, the electrochemical process that dissolved the oxide layer is reversed and the capacitor recovers. You have to be careful to limit the current through the capacitor, because high currents caused by leaks in the oxide layer can permanently damage the capacitor. The dropper resistor in your LED circuit is likely enough to limit the current to safe values, but do not connect an electrolytic capacitor that got reversed directly to a battery or lab supply without setting a current limit.

The isolating oxide layer can not only be damaged by reverse polarity, but also dissolve over time if no voltage is applied to the cap. As soon as forward voltage is applied, the leakage current cures the oxide layer by electrolysis, so the cap heals itself. This is why it is sometimes recommended to slowly ramp up the voltage on vintage tube radios that were unpowered for dozens of years, although in some configuration under-heating a tube is bad for the cathode. The process of creating or restoring the oxide layer is called forming.

So as you likely understood, you can abuse an electrolytic cap as rectifier, because it conducts current when reverse biased, but blocks current when polarized the correct way. People actually did use a similar configuration of electrodes and electrolyte as you find in an electrolytic cap as rectifiers before semiconductor rectifiers were invented. They were called electrolytic rectifiers.