# The meaning of floating potential

1. Jan 23, 2012

### Studiot

Since there are two competing and almost opposite definitions for the term 'floating potential' as applied to an electric circuit or arrangement, I would be interested to learn the view of PF members on the subject.

2. Jan 24, 2012

### I like Serena

Hey Studiot!

What are these opposite definitions of floating potential?

3. Jan 24, 2012

### Studiot

The term ‘floating’ potential was originally tightly defined by Electrical Engineers, Physicists and Mathematicians before there was a science of Electronics.

A science dictionary definition is

“The potential appearing on a single electrode, when all other potentials on other electrodes are held constant”

With this definition it is essential that the electrode in question is connected to the other electrodes and the potential appearing on it is the result of the impedances between the electrodes and their potential differences.

Electrical engineers use the reciprocal of impedance, called admittance, in the matrix circuit equation (network analysis - method of node currents)

I=YV

Where I is the vector of all currents, Y is the admittance matrix and V the vector of all voltages at circuit nodes.

All nodes are considered floating.

Given a table of the network admittances (impedances) the equation may be solved
for the voltages in terms of the currents or vice versa.

This definition simply means that a circuit node that is floating may take on any value that is dictated by the circuit parameters.

Unfortunately electronic engineers began using the term floating in what seems to be the opposite sense – That of ‘disconnected’.
Alternatively the term is used when the node voltage is not referenced to ground.

Thus memory makers use the floating-gate MOSFET transistor which has an extra gate not connected to any other part of the circuit.

Thus a battery or transformer supply is called floating if neither side is connected to ground.

4. Jan 24, 2012

### I like Serena

Interesting.

I've always interpreted "floating" as "not fixed" or "drifting", which would match your second definition.

5. Jan 24, 2012

### Antiphon

The modern meaning is that the floating potential is the voltage assumed by a node that has all connecting admittances zero. In other words, circuit theory is inapplicable and static electric field theory must be used to compute the voltage.

I beleive the original meaning was the same as this but I might be wrong. I beleive the first definition you gave is consistent with what I describe as the modern usage.

6. Jan 25, 2012

### I like Serena

Isn't that the second definition?