There is a close analogy between charge flowing around an electrical circuit and water flowing through a set of pipes. Consider first a fluid system - this is a closed system, so no fluid is added to or removed from the system.

The fluid system:

• water flows because a pump maintains a pressure difference
• the current (how quickly the fluid flows) depends on both the pressure difference and on the overall resistance to flow in the set of pipes
• energy can be extracted from the fluid to do work (e.g., turn a water wheel)

The electrical system:

• charge flows because a battery maintains a potential difference
• the current (how quickly the charge flows) depends on both the potential difference and on the overall resistance to flow in the circuit
• energy can be extracted from the charges to do work (e.g., light a bulb)