Confusion over Voltage

  • Thread starter Jimmy87
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  • #1
Jimmy87
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Hi, please could someone guide me to a correct definition of voltage. I have looked through previous posts and there seems to be conflicting definitions. My school book and some posts on the forum say voltage "pushes" the current. However, I can't see how this could possibly be true for the following reasons:

1)Voltage is just telling you the energy change for a charge between two points it isn't a physical thing so how can it push charge? If you say that voltage pushes charge this would be like saying that gravitational potential energy pushes mass which it doesn't, the gravitational field pushes mass.

2) I think I'm right in saying that electric fields exert forces on charges so if you have a p.d. across a circuit then an electric field will materialize within the wire and it is this field that "pushes" the charges. Is that right?

Other posts support what I think but as I initially stated some say that the voltage pushes charge. Please could someone clarify. Many thanks.
 

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  • #2
jbriggs444
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1)Voltage is just telling you the energy change for a charge between two points it isn't a physical thing so how can it push charge? If you say that voltage pushes charge this would be like saying that gravitational potential energy pushes mass which it doesn't, the gravitational field pushes mass.

The distinction between gravitational potential and gravitational force is the same as the distinction between electrical potential (voltage) and electrical force.

See http://en.wikipedia.org/wiki/Scalar_potential for a more technical version of the following.

In terms of vector calculus, gravitational force can be considered as a "vector field". It is the vector force that a unit mass would experience, expressed as a function of position. In general, a vector field assigns a vector value to each point in space.

The gravitational potential (if it exists) is the calculated as "path integral" of the force required to move a unit mass from a defined starting point to a particular position. [You can think of a path integral as figuring out how much work it takes to roll a rock up a mountain along a particular path] The resulting potential is a "scalar field". That is, it gives the energy that it would take to move a unit mass from the starting point to a target position. In general, a scalar field assigns a scalar value to each point in space.

Not all vector fields have the property that the path integral will be the same regardless of path. For those that do one can talk about the "potential" associated with the original vector field.

The "gradient" of a scalar field is defined as the rate of change of potential with respect to position. [You can think of this as measuring how steeply the mountain slopes at a particular point and in which direction]. Taking the gradient of a scalar field is the inverse to taking the potential of a vector field. It returns the original vector field.

Back to the question at hand...

In electrical terms, the voltage at each point in space constitutes a scalar field (like gravitational potential energy) its gradient is the electrical force field (like gravitational force).

It is not that Voltage pushes electrons. It is the difference in voltage that pushes electrons.

Or, more properly, it is the difference in voltage that describes how electrons are pushed. Searching for a physical thing that does the pushing is probably not a good idea. Maybe it is Coulomb repulsion. Maybe it is magnetic force. Maybe it is chemicals in a cell. Maybe it is temperature in a thermocouple. What matters is that there is a model that describes what happens. Ultimately there is no "what's really going on". It's all just models.
 
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  • #3
DrZoidberg
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You are absolutely correct. It is incorrect terminology to state that voltage(or a voltage difference) pushes charge. Charge is usually pushed/pulled by an electric field. Although there are also other effects that can move charge, e.g. the chemical reactions in a battery. And yes in a circuit there is a field inside the wire. Otherwise the electrons wouldn't flow.
 
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  • #4
mikeph
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Sounds like we had the same teacher.
 
  • #5
UltrafastPED
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The electric field and the voltage are related:

E=-∇V

For example, see http://hyperphysics.phy-astr.gsu.edu/hbase/electric/efromv.html#c2

So yes, you can say that the change in voltage moves the current: if the voltage is the same at both ends of a wire there is no current. If there is a current, then the voltage is different at the two ends.

In electrical engineering it is more convenient to work with the voltage: we control this directly.
 
  • #6
cabraham
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Voltage does not move current. The vtage difference between the ends os a wire is not what moves the charge through said wire. The power source, battery, generator, photodiode, etc. Provides energy to move charge. Charges moving through the wire collide with lattice io.s and dron
 

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