http://hyperphysics.phy-astr.gsu.edu/hbase/electric/watcir.html +-----------------------------------------------------------------------------------------------------+ | DC Circuit Water Analogy | | | | | | This is an active graphic. Click any part of it for further details. | | | [watdc] | | | | | | 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 | Index | | basic electric ciruits. In the water circuit, the pressure P drives | | | the water around the closed loop of pipe at a certain volume flowrate | DC | | F. If the resistance to flow R is increased, then the volume flowrate | Circuits | | decreases proportionately. You may click any component or any | | | relationship to explore the the details of the analogy with a DC | | | electric circuit. | | | | | | +----------------------------------+ | | | | What is wrong with this analogy? | | | | +----------------------------------+ | | | +------------------------------------------------------------+ | | | | Ohm's law | Voltage law | Current law | Power relationship | | | | +------------------------------------------------------------+ | | | +--------------------------------+ | | | | Basic DC circuit relationships | | | | +--------------------------------+ | | |-----------------------------------------------------------------------------------------+-----------| | | | | | Go Back | | HyperPhysics***** Electricity and Magnetism R | | | Nave | | +-----------------------------------------------------------------------------------------------------+ +-----------------------------------------------------------------------------------------------------+ | 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 | Index | | and ejects it at high voltage. | | | | DC | | [volpre] | Circuits | | | | | +------------------------------+ | | | | Water analogy to DC circuits | | | | +------------------------------+ | | |-----------------------------------------------------------------------------------------+-----------| | | | | | Go Back | | HyperPhysics***** Electricity and Magnetism R | | | Nave | | +-----------------------------------------------------------------------------------------------------+ +-----------------------------------------------------------------------------------------------------+ | Current-Flowrate Analogy | | | | | | Volume Electric | | | flowrate in current flow | | | liters/min, cm [curflo2] [curflo3] in coulombs/ | | | ^3/sec, m^3/ sec = | | | sec, etc. amperes. | | | A large pipe offers A wire offers very | | | very little little resistance | | | resistance to flow, to charge flow | Index | | as shown by according to Ohm's | | | Poiseuille's law. law. | DC | | | Circuits | | 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. | | | | | | +------------------------------+ | | | | Water analogy to DC circuits | | | | +------------------------------+ | | |-----------------------------------------------------------------------------------------+-----------| | | | | | Go Back | | HyperPhysics***** Electricity and Magnetism R | | | Nave | | +-----------------------------------------------------------------------------------------------------+ +-----------------------------------------------------------------------------------------------------+ | 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". | | | | | | [resflo] | | | | | | The severe constriction will have more resistance than the remainder | | | of the pipe system. Likewise a resistor in an electric circuit will | Index | | generally have much more resistance than the wire of the circuit. If | | | the single elements represented are the only resistances in the | DC | | circuit, then essentially all the pressure or voltage will drop | Circuits | | 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. | | | | | | +------------------------------+ | | | | Water analogy to DC circuits | | | | +------------------------------+ | | |-----------------------------------------------------------------------------------------+-----------| | | | | | Go Back | | HyperPhysics***** Electricity and Magnetism R | | | Nave | | +-----------------------------------------------------------------------------------------------------+ +-----------------------------------------------------------------------------------------------------+ | 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. | | | | | | [grnd] | | | | | | 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. | Index | | | | | Nevertheless, the image of the earth as a charge reservoir is helpful | DC | | in understanding the energetics of the entire electrical supply | Circuits | | 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). | | | +------------------------------+ | | | | Water analogy to DC circuits | | | | +------------------------------+ | | |-----------------------------------------------------------------------------------------+-----------| | | | | | Go Back | | HyperPhysics***** Electricity and Magnetism R | | | Nave | | +-----------------------------------------------------------------------------------------------------+