Water Model (Pictures)

• Power supplies always should produce 5V (otherwise something is wromg).  When a power supply is used properly it must supply the current required by the circuit while maintaining a constant voltage. Battery or DC power supply is similar to a pump and a water column. The pump maintains the water in the column at some fixed height. You can imagine some detector that signals the pump to start/stop pumping whenever the water level drops below/above a certain level. If water flows out at a rapid rate due to high demand the pump must work faster. If water is not flowing out rapidly then the pump can run at a slow rate.

• If a pump cannot maintain the rate required by the system then the height of the column will decrease. A large hole at the bottom of the column would certainly overtax a pump.  If you build a circuit that has a high current demand then the power supply might sag. The voltage from the power supply will be less than the nominal 5V.  (Also notice that as the water runs out the hole at the bottom and the height of the water drops then the push that is forcing the water out the hole is decreasing. At some point the pump might be able to keep up with the demand but at a significantly smaller water column height.)

• The height of the column of water provides the push or pressure to move the water through the system. If you double the height of the column you double the pressure.  Heights can be measured and directly used to indicate how water will flow. If there is a height difference then water will flow. A height difference indicates that there is a pressure difference. If the pressure at one point is greater than at another then water will be pushed towards the lower pressure.

• On way to visualize the varying pressure in a water system is to place a column (or imagine a column) at any point in the circuit. The water will be forced up the column due to the pressure in the system. The height of the water in the column can be used to indicate the pressure at that point. Height is a valid way to think about the push in a water circuit. All the rules that you apply based on pressure are the same as those based on height.

• Pinching or blocking a tube is the same as cutting or removing a wire. A break in an electrical circuit is the same as a block in a water circuit. If you cut a wire electricity does not leak out into the room.  Pinching a piper or tube so that no water can pass is the same as cutting a wire.  This can be a point of confusion when trying to predict how a circuit will respond if you cut or remove a wire.  You need to remember that this breaks the path and so nothing flows and the current drops to zero. If I pinch the tube no water can flow through the tube.