B Difference Between Potential and Potential Energy

[Doc Al]

At the high school and undegrad level potential and potential energy are not the same, and thus should be carefully distinguished.

Absolutely. A student who cannot distinguish potential from potential energy will have trouble with simple problems. Such as: Calculate the KE a given charge gains when it falls through a given potential difference.

The two concepts are intimately related but are not the same. Especially at the lower levels.

 

[weirdoguy]

Why is it important to make this distinction between the energy and the energy per unit charge?

Because for high-school students physics is confusing enough

 

[weirdoguy]

I have not even mentioned the electric potential

You tend to forget that there are different levels of physics. For a high school teacher that is obvious - there are two potentials considered in high-school: gravitational and electric. No gradients and other fancy stuff. And yes, you can teach electrostatics without using gradients and vector calculus explicitly, just look at some of the physics olimpiad problems.

 

[vanhees71]

Maybe it's another convention you use. A potential of a vector field is a scalar field, whose gradient is this field. In the context of the question posed in #1 it can only be the potential of a force. In #3 I've chosen the most simple case of the motion of a particle in an external force field, i.e., you have
$$\vec{F}(\vec{x})=-\vec{\nabla} V(\vec{x}).$$
In high school, of course, you don't have vector calculus at your disposal and that's why it's usually simplified to the motion along a straight line, i.e., you have
$$F(x)=-V'(x),$$
and also here one calls $V$ the potential of the force.

There is no way to understand electrostatics without the use of vector calculus. In high school we usually used the integral form and only very symmetric situations to calculate electrostatic fields. Of course also the potential of the electric field was introduced using the application of the motion of a charge in this field. Then of course $V(\vec{x})=q \Phi(\vec{x})$, where $q$ is the charge of the particle under consideration.

As I said, you make things more complicated in avoiding a minimum of math!

 

[fresh_42]

My 2 cents and personal opinion.

No gradients and other fancy stuff.

This is certainly the main opinion and the given fact at our schools.

However, it does not mean that it has to be the only valid point of view. Au contraire, it is the reason why STEM fields have the reputation they have and why students are forced to relearn concepts over and over again instead of treating them scientifically in the first place in my opinion. I do not like the underlying assumptions that students at school are not capable of learning things right. Such an assumption does not match my experiences.

Teach them right instead of teaching them twice!

We as a website are committed to the way subjects are learned at universities. There is a reasonable exception in the homework forums. I can not see that the homework standard should be applied everywhere else as some members try to achieve. (Evidence: this thread and https://www.physicsforums.com/threads/epsilon-delta-proof-and-algebraic-proof-of-limits.1016299/.) We should not lower our standards needlessly.

'Keep them stupid' is the reason students hate STEM. And those who don't are damned to relearn things over and over again, excused by "complicated math".

 

[robphy]

Let me take a slightly different approach to answer the question.

Like it or not,
physics is also taught in an algebra-based context (not just calculus-based).

Along the lines of @andrewkirk 's , @sophiecentaur 's, @weirdoguy 's and @Delta2 's early comments...

In an algebra-based course [and higher-level],
"potential energy" is associated with a conservative force (e.g. gravitation, Hooke's Law, Coulomb's Law).
In introductory physics-texts ("B"-level [as opposed to "I"-level),
"potential" almost certainly refers to "electric potential [in units of Volts]"
(since gravitational potential is rarely mentioned, except maybe as an afterthought to the electric potential).
A calculus-based text may make a passing reference to the mathematical notion of a "potential" as a generalization of the "electric potential" concept
[even though "potential" is more mathematically fundamental and "electric potential" is merely a special case].

In the algebra-based context,
one describes "electric force" as an interaction on a particular charge and
"electric potential energy" as a measure of the work done by the conservative force
in reconfiguring a charge distribution in a system.
By contrast [in algebra-based electrostatics],
the "electric field" $\vec E$ and later the "electric potential" $\Phi$ [since $V$ is already taken above for potential energy] are introduced as "fields" setup by the source charge.

I think the "source [charge]" vs "target charge" distinction is important to emphasize.

When the [target] charge is placed at a particular location in space,
then we obtain the "electric force" on that target charge due to the field set up by the sources
$$\vec F_{\mbox{on $q_{target}$}}=q_{target} E_{\mbox{due to sources}}$$
and the "electric potential energy" of that target charge in that field [i.e. a measure of the work done if that charge were brought from infinity]
$$V_{\mbox{of $q_{target}$}}=q_{target} \Phi_{\mbox{due to sources}}$$
(We assume that the sources are setup once and for all... and the target charge is a test charge in the field of the sources).

So, the "electric field" and "electric potential" describe a vector field and scalar field set up by the sources.
The "electric force" and "electric potential energy" describe an interaction involving a test charge and the sources [mediated by the fields that produced the sources].
This may be good enough for the algebra-based course.

Many times we have to meet the students where they are [in their preparation].

Yes, there is calculus that relates the "electric field" and the "electric potential",
but calculus is not the explicit route taken in an algebra-based physics course to establish that relationship.
If this feature is that important, then the algebra-based class is not the appropriate class for the student.

 

[vanhees71]

What is the electric potential good for, if there is no relation to the electric field, which is describing an observable phenomenon in Nature? I've no clue, how you connect the potential with the electric field without calculus (or at least using derivatives).

Concerning "algebra-only physics", I've once given a lecture for a colleague in a non-calculus mechanics lecture. Of course, also there they used time derivatives, but it was not allowed to call it so. They just used difference quotients and then (of course without calling it so) took the limit $\Delta t \rightarrow 0$. I found this very difficult, at least more difficult than using calculus, which is pretty intuitive on this level.

 

[robphy]

What is the electric potential good for, if there is no relation to the electric field, which is describing an observable phenomenon in Nature? I've no clue, how you connect the potential with the electric field without calculus (or at least using derivatives).

I try to use “slope” and sketches to suggest the relation. But I don’t expect the students in that algebra-based class to evaluate a derivative operation.

Sometimes it does have to be “then a miracle occurs”. (I might suggest that those interested should study a more advanced level for details.)

The point is that the students gets glimpses of what is going on in order to solve simple problems… toward getting to what they need to know (according to those who set up the course sequence and curriculum).

We want to promote the field concept from a point charge…. But also want to evaluate voltage differences around a circuit and energies associated with charging a capacitor. Sure it’s great to fill in all the details (I know I want to… but I know I can’t expect many students to follow it all.)

“Connecting the dots” could be calculus and the operation of evaluating a derivative … but it also could be graphical sketching, or plotting with Desmos, or numerical calculation by hand or by writing a program, or other descriptive words, or analogies with another system.

While I am often the one interested in the details, many students are often more interested in how this is “useful” to their lives or livelihood. That’s just the reality on the ground.

 

[fresh_42]

This thread is getting off-topic.

While I am often the one interested in the details, many students are often more interested in how this is “useful” to their lives or livelihood. That’s just the reality on the ground.

Two answers:

Let's not make an organizational question (which amount of science in which classes and a distinction between mandatory and optional) a matter of content! This is the first significant failure in the current systems. Few need integration, but for those who actually need it, the dozens of examples are a waste of time.

I call them disco-accidents. They are reported in the newspapers on Mondays, after the weekend. Young men often brag about their cars or nonexisting experience to drive and end their and the lives of their friends wrapped around a tree. Basic knowledge about physics could prevent a lot of those tragedies. This cannot be mentioned too often.

 

[sophiecentaur]

In an algebra-based course [and higher-level],
"potential energy" is associated with a conservative force (e.g. gravitation, Hooke's Law, Coulomb's Law).

If this feature is that important, then the algebra-based class is not the appropriate class for the student.

The only reason for bringing calculus into things is where the models are non linear. The SUVAT equations all assume uniform acceleration and so does Hooke's law calculation. I look back at my early Physics and still resent the fact that it was all based on linearity; I had to suss that out for myself; so many 'examples' unashamedly involved motor cars with no caveats. I can't have ben the only student who didn't worry about it. (Or perhaps I was regularly looking out of the window whenever it was made clear.)
Triangles are quite good enough until kids have been given calculus.

 

[vanhees71]

The only reason for bringing calculus into things is where the models are non linear. The SUVAT equations all assume uniform acceleration and so does Hooke's law calculation. I look back at my early Physics and still resent the fact that it was all based on linearity; I had to suss that out for myself; so many 'examples' unashamedly involved motor cars with no caveats. I can't have ben the only student who didn't worry about it. (Or perhaps I was regularly looking out of the window whenever it was made clear.)
Triangles are quite good enough until kids have been given calculus.

This is utter nonsense. You cannot even define velocity and acceleration without using derivatives of the position vector with respect to time. For classical mechanics you need at least linear algebra of the Euclidean affine space and derivatives as well as integrals to start. Even the simple case of constant acceleration (a good model for the motion of a mass point close to Earth, where the force is $\vec{F}=m \vec{g}$ with $\vec{g}$ the gravitational field of the Earth, which can be approximated as constant for motions close to its surface) needs very basic integration to get what you call "SUVAT equations". In many socalled "calculus-free textbooks" they somehow manage it to treat these basic integrals with some tricks without explicitly doing the integrals. It's utterly confusing, and it ends with the sad result that students rote learn these "SUVAT equations" and apply them to all kinds of problems without understanding them. I don't know, how these books treat the somewhat more complicated problem of a force that's linear to displacement as in Hooke's Law. It must be even more confusing.

 

[robphy]

The only reason...

This is utter ...

FYI: You misattributed the quote to me.

 

[vanhees71]

Ugh? How could this happen? I corrected the quote by hand now:

The only reason for bringing calculus into things is where the models are non linear. The SUVAT equations all assume uniform acceleration and so does Hooke's law calculation. I look back at my early Physics and still resent the fact that it was all based on linearity; I had to suss that out for myself; so many 'examples' unashamedly involved motor cars with no caveats. I can't have ben the only student who didn't worry about it. (Or perhaps I was regularly looking out of the window whenever it was made clear.)

 

[Delta2]

In my primary high school years (ages 13,14,15) I did physics without calculus. It wasn't that bad and indeed the worst math you could get was a system of two linear equations with two unknowns. Oh and the only non linear formula I remember from those years was $s=\frac{1}{2}gt^2$.

But yeah the quality of Greek state education (free education) was below mediocre back then, and I think it is at most mediocre even now. That's why everyone starting at primary high school years and intensifying at secondary high school (ages 16-18) is going to "Frontistirio" and pays in order to learn and understand something. (Google frontistiria in greece).

 

[sophiecentaur]

This is utter nonsense. You cannot even define velocity and acceleration without using derivatives of the position vector with respect to time.

Funnily enough, the introduction to Differential Calculus would be nearly impossible without starting with straight lines and triangles and deriving the limit of the slope of a curve as the intervals reduce to zero.

By your argument, you shouldn't try to teach any Science to a non-Mathematician. With that attitude, you'd lose some very useful potential Scientists to 'the other side' by the time they got to 16 years of age. There's some brilliant and engaging stuff available on the Arts side and thank god enough clever young people go in that direction. In any case, we're arguing about a false dichotomy (I like that term).

The original P vs PE question has no particular field of application - it's a general thing. P implies Energy and the units will always agree, somewhere in there.

Ugh? How could this happen? I corrected the quote by hand now:

Gremlins in the works! I didn't; bother to fess up - I knew someone would re-point your finger.

 

[vanhees71]

Funnily enough, the introduction to Differential Calculus would be nearly impossible without starting with straight lines and triangles and deriving the limit of the slope of a curve as the intervals reduce to zero.

Exactly, and what's the merit not to call this a "derivative" and to work out the basic rules, how to work with them? To the contrary to avoid this step makes the physics more complicated rather than simpler as advocated by the anti-math lobby.

By your argument, you shouldn't try to teach any Science to a non-Mathematician. With that attitude, you'd lose some very useful potential Scientists to 'the other side' by the time they got to 16 years of age. There's some brilliant and engaging stuff available on the Arts side and thank god enough clever young people go in that direction. In any case, we're arguing about a false dichotomy (I like that term).

That's utter nonsense again. You don't need to be a mathematician to understand basic calculus on the level used in physics. My impression is that we loose a lot of very clever people exactly because one tries to make "things simpler than possible" in the STEM area, i.e., particularly the very clever pupils get the impression that these subjects are something ununderstandable and thus build up an interest for other subjects.

The original P vs PE question has no particular field of application - it's a general thing. P implies Energy and the units will always agree, somewhere in there.

I've still no idea, what the difference should be. Any vector field may have a potential, also forces, and that's the potential going into the total energy of a mechanical system and is thus named "potential energy".

Gremlins in the works! I didn't; bother to fess up - I knew someone would re-point your finger.

?

 

[sophiecentaur]

That's utter nonsense again. You don't need to be a mathematician to understand basic calculus on the level used in physics.

Only nonsense if your personal view is that way. Who is a mathematician? Someone who can count his sheep reliably or someone who can do tensor calculus?
I had no calculus until the first year of A level Maths but I knew Suvat for O level. Are you saying I should not have ben taught Suvat at all?

Any vector field may have a potential

What are the units of that potential?

 

[vanhees71]

It has of course the dimension of the vector field times length since $\vec{V}=-\vec{\nabla} \Phi$.

 

[sophiecentaur]

It has of course the dimension of the vector field times length since $\vec{V}=-\vec{\nabla} \Phi$.

. . . . . and the Potential Energy? We are trying to establish the relationship between two quantities.
I think the answer must be that Potential Energy will only apply for fields where Work is involved i.e. a subset of Vector Fields.

 

[vanhees71]

An energy has the dimension of an energy, what else? Once more: The potential energy in point-particle mechanics is the potential of a force. That's what I said already in my first posting in this thread...

 

[sophiecentaur]

An energy has the dimension of an energy, what else? Once more: The potential energy in point-particle mechanics is the potential of a force. That's what I said already in my first posting in this thread...

I still don't get what you are saying about any distinction between P and PE.
Are you prepared to accept that the (potential) Energy in a charged battery is not the Voltage (i.e. potential), but the Watt hours or that the (potential) energy in a suspended mass in a clock is mgh and not gh? How can those pairs be the same?

 

[vanhees71]

Sigh. Is it so difficult to understand what I write? So once more: The potential of the force is the same as potential energy. What else should potential energy be?

A voltage is the difference of a potential of the electromagnetic field. That's not the same as potential energy, which is clear already from the dimensions. In a battery you have no voltage though but an electromotive force, but that's another issue. The energy of a charged battery is in form of internal/chemical energy.

The potential energy in a suspended mass in a clock is given by the potential of the gravitational force on this mass, which is $V=m g z$ (with the $z$ axis pointing "upwards", i.e., against $\vec{g}=-g \vec{e}_z$), while $\Phi=g z$ is the potential of the gravitational field. The force is $\vec{F}=-\vec{\nabla} V=-m g \vec{e}_z=m \vec{g}$ and the gravitational field is $\vec{g}=-\vec{\nabla} \Phi=-g \vec{e}_z=\vec{g}$. Of course, $V=m \Phi$.

I still don't understand what your point really is!

 

[sophiecentaur]

The potential energy in a suspended mass in a clock is given by the potential of the gravitational force on this mass, which is V=mgz (with the z axis pointing "upwards", i.e., against G→), while Φ=gz is the potential of the gravitational field.

"Sigh" are you really saying anything different from me?You say that the Potential Energy depends on the mass and that the Potential is independent of mass. That's what I am saying. What are you arguing about? How did I say it wrong?

 

[vanhees71]

I'm also puzzled, what we discuss about. The only point is that you keep saying "the potential". You have to state which potential you mean, that of the force or the gravitational field. They differ by a factor of $m$ in this example.

 

[sophiecentaur]

The only time I use the term "Potential" is when referring to a unit mass / charge. I think I've been consistent over this thread at least. When using the term "Field", I assume that it's force per unit Mass / Charge.
In " the Sun's Gravitational Potential Well" diagrams, I wouldn't expect the y-axis to be different, depending on what mass of planet we're discussing.

 

[vanhees71]

Well, it would have helped to clearly indicate the potential of what vector field you are talking. That's all I'm saying.