I can make an LED glow very dimly in all 4 following circuits:

^{simulate this circuit – Schematic created using CircuitLab}

What is the physics behind this? I understand that there occurs a potential difference between me and positive AND me and negative terminals of my power supply but I don't know how and why this happens. Also I understand that depending of the circuit, my electrical potential becomes:

1. Higher than the negative terminal
2. Lower than the negative terminal
3. Higher than the positive terminal
4. Lower than the positive terminal

Since the LED glows in all 4 configurations. How is this happening?

I can make an LED glow very dimly in all 4 following circuits. The voltage source in the circuits is the output of an AC to DC converter fed into a breadboard. Also, one end of the voltage source is always floating, as shown on the schematics.

^{simulate this circuit – Schematic created using CircuitLab}

What is the physics behind this? I understand that there occurs a potential difference between me and positive AND me and negative terminals of my power supply but I don't know how and why this happens. Also I understand that depending of the circuit, my electrical potential becomes:

1. Higher than the negative terminal
2. Lower than the negative terminal
3. Higher than the positive terminal
4. Lower than the positive terminal

Since the LED glows in all 4 configurations. How is this happening?