Quote from Batch:
It was humorous seeing a disembodied head and hand doing the talking and gesturing. I think analogs are OK for illustrating and explaining a particular point, but are confusing when coupled to another physics discipline. Do hydraulic engineers learn their craft by studying electrical technology? If not, then why should electrical students study hydraulics?
The analogy is meant to foster understanding by drawing analogies with systems that are more commonly familiar to the student, i.e less abstract than the ideas of electric fields etc. If hydraulics confuse you then ignore it. Analogs are also useful tools for solving real world engineering problems. Sometimes mechanical problems, for example, can be more easily solved by setting up an analogous electrical circuit and solving the circuit equations or merely observing the circuits behaviour. In fact this was often done physically with analog computers back in the mid 20th century before digital computers were prevalent.
To each his own method. But, it is one thing to use certain methods to solve problems, and another thing to teach and understand the science involved.
The professor seemed to imply that positive charges were present in a electrical circuit. That, of course is not true. Only negative charges are present in wires.
Of course there are positive charges in electrical circuits or there would be a humongous net negative charge. The positive charge carriers are called protons.
Tilt! There are no positive charge carriers in metals. The negative charge carriers (electrons) in metals are loosely held to the metal atoms, are mobile, and contribute to the conductivity of the metal. The positive ions of the metal are stationary, and do not transport any charge.
Why do the analog folks equate pressure (force per unit area) with voltage. I would think they would choose force for voltage. After all, the electric field does exert a force on the conducting electrons. Voltage is not a force. A force has direction, voltage does not have a direction. Voltage is measure of potential energy difference. Electric fields are the source of force on a charge.
Correct, but I was suggesting the analogy folks use force for voltage instead of pressure. I never said that voltage was force.
The prof equated physical height with voltage, which I thought was not necessary. Voltage is really an energy density, and is not a difficult concept to understand.Voltage is not an energy density. It is a measure of potential energy per unit of charge (Joules/Coulomb), or the amount of energy it takes to move 1C of charge against a 1 N/C E-field. It is a scalar quantity, i.e. it has no direction. This can be understood by remembering that Work (or energy) = force * distance, so for a coulomb of charge in an E-field the change in potential energy of the charge = E-field * distance moved, or in SI units: V = (Newtons/Coulomb)*meters = N*m/C = J/C = Volts.
I said that voltage was the energy density per unit charge. See reply #4 of this thread. I agree with, and have known the facts in the above paragraph since the year one.
In the water pipe analogy: (I use ? to represent the nabla symbol, i.e. the upside down delta)
Yes, the symbol for the gradient in vector calculus.
Pressure Gradient (?P) is in units of Newtons/meters^3; it is analogous to E-field which is in Newtons per unit of charge. These are a vector quantities.
Use whatever analogy you want. Agreed, the E-field is a vector quantity. Its field strength is measured in units of newtons/charge or volts/meter. That is old news to me.
Pressure (P) is a measure of potential energy per unit volume of a fluid (joules/meter^3), or the amount of energy it takes to move 1 cubic meter of fluid against a pressure gradient of 1N/m^3. This is a scalar quantity just like the analogous Volts. Again, Energy = Force * distance, so for a cubic meter of fluid we have P = ?P * distance, or in SI units (N/m^3)*m = N/m^2 = N*m/m^3 = J/m^3. (in the second term you see the more familiar definition of pressure as force per unit area: a Pascal in SI units).
If the "fluid" were a gas, I would agree with you. But a liquid is incompressible, so so no work is done. It is like applying a large force against a immovable object. No work is done in that case, either. The energy transfer to compress/expand a gas is well known in stoichiometic chemistry as PV. No change in volume, no energy exchanged.
Summary of the analogy:
E-field (N/C) => Pressure Gradient (N/m^3)
Volts (J/C) => Pressure (J/m^3)
Say what you want about those analogies, I never had any use for them.
Ratch
Hopelessly Pedantic