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The Volt is a measurement of Electron Moving Force or Electromotive Force (EMF).
Potential - is the measurement relative to ground which is assumed to be at zero Volts.
Potential difference - is the measurement between two points in a circuit, often unrelated to ground or earth.
Current - will flow if there's a potential difference and a complete circuit in which the electrons are free to move.
Vdd Vcc V+ The positive power supply to a circuit. The dd and cc come from the Drain and Collector pins of transistors and MOSFETS.
Gnd The ground, earth or zero volt connection to a circuit.
Vss Vee V- The negative power supply to a circuit. The ss and ee come from the Source and Emitter pins of transistors and MOSFETS.
In many circuits, this is also the ground connection.
Other circuits use two power rails with a separate ground connection.
Static Electricity - Lots of volts but usually a negligible current so it's not dangerous. Stick balloons to cats. Make your hair stand on end. Deflect a water jet. But lightning is an example of "static" electricity with a large current.
Breakdown - The voltage becomes so large that current flows where it normally would not, often destroying the device. Lightning and sparks are examples.
The voltages round a circuit add up to be equal to the battery or power supply voltage.
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