DC Circuit Water Analogy

In a direct current (DC) electrical circuit, the voltage (V in volts) is an expression of the available energy per unit charge which drives the electric current (I in amperes) around a closed circuit. Increasing the resistance (R in ohms) will proportionately decrease the current which may be driven through the circuit by the voltage.

Each quantity and each operational relationship in a battery-operated DC circuit has a direct analog in the water circuit. The nature of the analogies can help develop an understanding of the quantities in basic electric ciruits. In the water circuit, the pressure P drives the water around the closed loop of pipe at a certain volume flowrate F. If the resistance to flow R is increased, then the volume flowrate decreases proportionately. You may click any component or any relationship to explore the the details of the analogy with a DC electric circuit.

Voltage-Pressure Analogy

A battery is analogous to a pump in a water circuit. A pump takes in water at low pressure and does work on it, ejecting it at high pressure. A battery takes in charge at low voltage, does work on it and ejects it at high voltage.

Current-Flowrate Analogy

Volume flowrate in liters/min, cm3/sec, m3/sec, etc.			Electric current flow in coulombs/sec = amperes.
	A large pipe offers very little resistance to flow, as shown by Poiseuille's law.	A wire offers very little resistance to charge flow according to Ohm's law.

Connecting a battery to an appliance through a wire is like using a large pipe for water flow. Very little voltage drop occurs along the wire because of its small resistance. You can operate most appliances at the end of an extension cord without noticeable effects on performance.

Resistance to Flow

The resistance to flow represented by a severe constriction in a water pipe is analogous to the resistance to electric current represented by a common electric "resistor".

The severe constriction will have more resistance than the remainder of the pipe system. Likewise a resistor in an electric circuit will generally have much more resistance than the wire of the circuit. If the single elements represented are the only resistances in the circuit, then essentially all the pressure or voltage will drop across these single elements. The fact that essentially all the voltage drop appears across a resistor or an ordinary electrical appliance makes possible the operation of such appliances from an extension cord, or the operation of several appliances in parallel on a single circuit in your home.

Ground-Reservoir Analogy

The function of a ground wire in an electric circuit is in many ways analogous to the reservoir attached to the water circuit. Once the pipe is filled with water, the pump can circulate the water without further use of the reservoir, and if it were removed it would have no apparent effect on the water flow in the circuit.

The reservoir provides a pressure reference, but is not part of the functional circuit. Likewise, the battery can circulate electric current without the ground wire. The ground provides a reference voltage for the circuit, but if it were broken, there would be no obvious change in the functioning of the circuit. The ground wire protects against electric shock and in many cases provides shielding from outside electrical interference.

This view of a ground is not adequate to explain the function of an appliance ground wire because just a connection to the earth is not sufficient to trip a circuit breaker in case of an electrical fault. To be effective in preventing shock hazards, an appliance ground must connect back to the supply through the neutral wire.

Nevertheless, the image of the earth as a charge reservoir is helpful in understanding the energetics of the entire electrical supply system. At a power plant, charge can be drawn from the earth and the generation process does work on the charge to give it energy. This energy is described by stating its voltage (1 volt = 1 joule/coulomb = energy/charge). The energy can be transported cross-country at high voltages and then supplied to end users at lower voltages with the use of step-down transformers. The energy can then be used and the charge discharged to the earth. The charge upon which the work is done at the power plant does not have to be transported cross-country, and the "spent" charges do not have to be transported back to the power plant, but just dumped into the "reservoir".

All such analogies have their drawbacks, and you can generate spirited discussions at all levels of expertise about analogies for grounding. Some object to the reservoir approach because is creates the image of some sort of limitless supply of charge, and that there is something "special" about it. It also creates the mistaken impression that you can pull some charge out of it without putting some in. The Earth is just a good conductor of charges, but like all electrical circuits, must ultimately make a closed circulation path in order to conserve charge ( a hard and fast conservation law).