Why voltage have negative sign?

Answer 1

The strength of a electrical flow is measured in Voltage and Electrons are negative.

Alternating current (AC) oscillates between positive and negative as the direction of flow periodically reverses direction. An alternating current of 60Hz therefore changes direction 60 times a second, and can be visualised as a sine wave that oscillates above and below the zero baseline, with a period (a complete cycle) of 1/60th of a second.

Direct current (DC) infers constant polarity, meaning the direction of flow is unidirectional (one direction only). While DC can oscillate as a full-wave rectification or a half-wave rectification, it is always indicated as a positive voltage (the line or wave is always above the zero baseline).

A voltage or current indicated with a negative symbol therefore implies alternating current although AC is more correctly indicated by the ± symbol. Direct current is generally indicated by a long horizontal bar above three short horizontal bars.