In PN junction diode how does the majority electrons that flow from n--p leave behind an immobile positive charge as they are free electrons already ? since on doping we already got free electrons in n type

Which best represents the law of conservation of mass ?

Awater-filled u-tube manometer is used to measure the pressure inside a tank that contains air. the water level in the u-tube on the side that connects to the tank is 5 ft above the base of the tank. the water level in the other side of the u-tube (which is open to the atmosphere) is 2 ft above the base. determine the pressure within the tank.

What are the north and south poles of a solenoid change with?