ground - Grounding in circuit diagrams

Often I see in circuit diagrams that the circuit is connected to the ground:

I understand that this creates a reference for zero potential, but why does all of the current not simply flow right into the ground? I must have a fundamental misunderstanding somewhere.

Answer

The current doesn't all just flow to ground because of Kirchoff's Current Law. Kirchoff's Current Law states that the sum of the currents entering and leaving a node must be equal to zero. If x amps flow from the power supply, x amps must return to the power supply.

In your schematic, before the earth connection was added, the voltages had no reference. Adding the earth connection puts the negative terminal of the supply at earth potential, making all the voltages earth referenced. If you prefer to think of it in terms of Spice, the negative terminal and earth nodes were merged. The current still has to flow back to the power supply, where it came from. Additionally, if any other point in the circuit were tied to earth, the current would still flow back to the negative terminal of the power supply through earth.