On complex circuit diagrams using a dual supply the earth symbol is often used to indicate a connection to 0V, this helps to reduce the number of wires drawn on the diagram.

In this circuit the 4V across the resistor and the 2V across the LED add up to the battery voltage: 2V + 4V = 6V.

This flow of electricity is called conventional current and it is the direction of flow used throughout electronics.

1mA = 0.001A, or 1000mA = 1A