Hydraulic Analogy for Counter-EMF ?

[Rolliet]

I'm trying to get my mind around counter-emf. Is there a good analogy for counter EMF in either hydrology or mechanical examples ?
I've read the textbook descriptions.

Likewise, I wonder if there are analogies for the phenomenon of Hysteresis.

 

[jim hardy]

Motor or transformer?

In my simple mind it's just the Lorentz force QV cross B

 

[Rolliet]

motor

 

[jim hardy]

We had this question come up some time ago

check this thread
https://www.physicsforums.com/showthread.php?t=624785

second page has some diagrams, hopefully the discussion in that thread will help make it intuitive for you.

What's emf? It's a force on a charge, be it magnetic chemical or electrostatic in origin..
What's current? Charge in motion.

back emf is kinda unavoidable. it comes from the same thing that makes the motor turn, namely the force on a charge that's moving in a magnetic field.

Charge traveling forward inside the conductor feels a force perpendicular to both the magnetic field and the direction of its motion. That mutual perpendicularity is called a "Vector Cross Product", though the name isn't really important. It pushes the charge sideways which makes the motor try to turn.

When the motor starts to turn, the wire itself now has motion relative to the magnetic field, and THAT motion creates another "vector cross product" which pushes the charge backward against its forward motion along the conductor. That opposes the applied voltage, trying to reverse current flow. So it's called 'back emf' or counter emf.

And it's really quite a nice thing , it makes motors and transformers practical.

old jim

 

[meBigGuy]

A motor sort of includes a "built in" generator that tries to reduce the current into the motor to zero when no work is being done. Without that back-emf generator the motor would appear as a low impedance short circuit (as it does when starting up).

 

[sophiecentaur]

One shouldn't lose any sleep over finding analogies, imo. If one fits readily then use it (carefully), otherwise, the fact that it is hard to think of would imply that it is not a good parallel and likely to confuse you and others.
The nearest thing to an analogy for a back emf is, I would say, the reaction force that you get when trying to accelerate a mass. The analog to the mass, being the Inductance of the circuit. No inductance, no back emf so, potentially, infinite current into zero resistance: no mass, infinite acceleration with no friction.

 

[Windadct]

As for a basic inductor - creating the "counter EMF" - in a hydraulic analogy I consider the mass of the fluid, or more accurately the momentum. You can not instantaneously change the rate flow of a fluid with mass - for example wen you shut off a valve - you generate a "water hammer" - this is analogous to inductive kickback - creating a pressure ( force) wave against the flow. To change the flow rate takes ( or returns energy) however steady state flow uses no energy - other then resistance, the mass / momentum is not a factor.

 

[sophiecentaur]

Water hammer must have a capacitative component too, I think. This is what I meant about analogies and their associated problems. It's only when there is a totally common mathematical form describing the reality and the analogy that it is even vaguely bringing in an analogy.

 

[Windadct]

Well I would disagree with IT must - when we consider an ideal inductance - we do not consider it's "real world" capacitance - we can design perfectly fine real world circuits ( not all of them) without considering parasitics. However - the water hammer effect IS suppressed by a "capacitive" component is the water snubber, or water hammer arrestor. Another way is if the if the pipe has elastic properties like a hose the shock wave is absorbed however these are very "lossy" so they absorb the energy, but dissipate it. It absorbs the shock (pressure) wave generated by the valve closure.
I can understand the argument with analogies - however they are not meant as perfect mathematical alternates. They are a 10,000 foot perspective.

Lastly oh there is some math ... F = m dV/dt Mass ... V = L dI/dT -

So in my view - adding inductance is like adding mass to the system - yes this disagrees with a pure interpretation - if you "visualize" a small mass moving at high velocity -- stopping it has similar effect to stopping a large current flowing though a small inductance ( my solenoid valve controlled humidifier can be heard in the whole house). I deal with 300A stopping in say 77 nS - in a device with 15nH of parasitics - while I can explain that the chip will see 60V over the DC Bus voltage to the EE's in the room - convincing management that device A with 15nH (60V) is considerably better than device B with 25nH (100V) - is difficult...