# How can the electric potential be zero at a point where the electric field isn't, if that field can give a test charge kinetic energy?

Say we have 2 charges set up like above. Why is $$V_3=0$$ ?

I can understand mathematically that the scalars fields cancel out, but I don't understand it physically. If I place a positive charge in between the 2 charges, it has electric potential energy because it is feels a force towards the negative charge and repelled by the positive and gains kinetic energy. However because the Electric Potential is 0, thus the electrical potential energy is 0.

How can $$V_3 =0V$$ if a charge placed in between the two charges will gain kinetic energy?

• Which charge is at a higher potential? – Cinaed Simson Jul 17 '19 at 5:56
• "it has electric potential energy because it is feels a force" ---no, this does not follow. – Andrew Steane Jul 17 '19 at 13:22
• If you have a bowling ball on a hillside at an altitude equal to sea level, the ball will still roll downhill. It's not the value of the potential that causes a force on the charge, it's the gradient. – The Photon Jul 17 '19 at 16:23
• This is like asking "how can the sea level be zero when you can still go underwater?" – Dmitry Grigoryev Jul 18 '19 at 8:46

If the charge comes from a point of $$V_3=0$$ and gains kinetic energy, then it is because the value of potential energy at the new point is lower. Then potential energy can be converted into kinetic energy. Don't worry about the actual potential energy values - only the differences in value matter.

Think of having a box on the floor. You might say that there is zero (gravitational) potential energy associated with it. But that is only because you chose to consider the floor as the reference.

• Lift the box to a shelf, and there is positive potential energy stored.
• Put it in a hole in the floor, and there is negative potential energy stored.

The values don't matter. What matters is only that some values are smaller than others. Because the box will always want to fall towards lower values. It will fall from the high shelf to the floor at zero potential energy. And it will fall from the floor at zero to the hole at negative potential energy. It always wants to move towards lower values - the actual value doesn't matter.

You are free to pick whichever point you want as the zero-value reference. It doesn't matter, only the difference between points matters.

The same is the case for electrical potential energies. You could place a positive charge at the shown equipotential line and say that zero (electrical) potential energy is stored. Then surely, the charge will want to move towards the neighbour locations where the potential energy stored is less than zero. That somebody chose the potential energy values at this particular equipotential line to be zero, doesn't matter. It could have been anything else.

This tendency to move towards lower values of potential energy is what the field lines show. At all points on the equipotential line, there are field lines showing the direction that the charge wants to move along.

In general, you should forget about the actual values of potential energies and only care about the differences in the value between points. This is why voltage is the main parameter in these cases; voltage is the difference in electrical potential between two points. Just pick whichever reference that makes it easier to work with in your specific scenarios.

• ohh ok, I think my confusion arose from the fact I've always associated electric potential energy (EPE) with how much energy the charge would have when it was released. I'm starting to think EPE by itself isn't a very useful number then... (you need atleast another EPE for it to be useful) – John Hon Jul 17 '19 at 13:32
• @JohnHon the isoline at 0 tells you that moving a test particle along that line of equal distance between the two particles requires no work — but that's true of any isoline, the one labeled 0 isn't very special. – hobbs Jul 17 '19 at 15:46
• @JohnHon A typical misunderstanding, yes. It makes intuitive sense that stuff "has an amount of energy" - so, when zero, no more energy is left. This is true for kinetic energy. But not for potential energies; they are by their very nature - by definition - dependent on an arbitrarily chosen reference point. It is not enough to say that potential energy is stored because a ball is on a high shelf, but only because it is on a high shelf compared to another shelf. The value alone is quite useless, yes, whereas two numbers give you a difference, which is a useful parameter. – Steeven Jul 17 '19 at 19:14
• @JohnHon It can be useful if you define the zero point in some universal manner; for example, for gravitational potential energy, we usually have the zero point as the maximal potential (which makes any potential energy within the system negative); if you did the same with EPE, it would behave the way you expect it to - zero would be at some arbitrary point "very far away", and as you get closer to the point charge, the potential energy would get more and more negative. I'm not sure if it would be quite as useful as for gravity, though, given that electric charges can be negative... – Luaan Jul 18 '19 at 6:07
• @Steeven I would say it's not even really "true" for kinetic energy. For kinetic energy, it's also still based on some arbitrary frame of reference which the motion is relative to. – JMac Jul 18 '19 at 11:55

If I place a positive charge in between the 2 charges, it has electric potential energy because it is feels a force towards the negative charge and repelled by the positive and gains kinetic energy.

It's true that a positive test charge feels a force towards the negative charge since a positive charge 'rolls downhill' in the (electric) potential, and the $$V = 0$$ line is not the lowest point in the potential. Take a look at this surface plot of an electric dipole potential:

Image credit

How can $$V_3=0V$$ if a charge placed in between the two charges will gain kinetic energy?

The potential is negative in the 'depression' - the charge has minimum potential energy at the bottom of the depression.

Now, you might wonder why the zero in potential isn't assigned to the bottom of the depression (since a global change in the potential leaves the physical electrical field unchanged). Such a global change would require the potential 'at infinity' to be non-zero. However, it seems more logical to set the potential energy of a test charge, 'infinitely' far away from the dipole, to be zero.

There seems to be a basic misunderstanding between the electric potential energy of a system of charges and the electric potential at a point due to a system of charges.

To make things easier let's assume that the zero of electric potential is at infinity and that the electric potential energy of a system of charges is zero when all the charges are infinitely far away from one another.

The diagram is a good one in that shows an important feature in that the electric field lines (yellow) are at right angles to the equipotential lines (dashed).
This means that when a test charge is moved along an equipotential line the direction of the force (the direction of the tangent to the electric field line) on that test charge is at right angles to the direction of motion to the charge and so no work needs to be done moving the test charge.
This means that moving a test charge from infinity (at zero potential) along the dashed line labelled $$V_3$$ requires no work to be done.
So dashed line labelled $$V_3$$ is at zero potential.

Now does that test charge contribute to the potential energy of the system of charges?
In this special case it does not but also in general a test charge does not contribute to the potential energy of a system of charges because the definition of an electric field at a point is $$\vec E = \lim\limits _{q\to 0} \frac{\vec F}{q}$$ where $$\vec F$$ is the force on test charge $$q$$ as it tends to zero.

Going back and now considering the potential energy of the system of charges.
The work done to assemble the $$+q$$ and $$-$$ charges from infinity to a separation $$r$$ is $$- \frac{kq^2}{r^2}$$ and this is the potential energy of that system of two charges.
If one brings in another charge $$Q$$ along line $$V_3$$ and places it exactly between the othe rtwo charges the potential energy of the system of three charges is still $$- \frac{kq^2}{r^2}$$.
This might seem rather strange but perhaps better understood if one assembled the tree charges in a different way.
First just have charge $$+q$$ present and bring up charge $$Q$$ from infinity to be $$\frac r2$$ away from it.
The work done to do this is $$\frac{2kQq}{r}$$.
Now bring up charge $$-q$$ from infinity to be $$r$$ away from charge $$+q$$ with charge $$Q$$ midway between them.
The work done to do this is $$-\frac{kq^2}{r} - \frac{2kQq}{r}$$.
So the total work done to assemble the three charges is $$\frac{2kQq}{r}-\frac{kq^2}{r} - \frac{2kQq}{r}= - \frac{kq^2}{r}$$ which is the potential energy of the system of three charges is the same as before.

Note that the equipotential line $$V_3$$ is a special case and bringing a charge $$Q$$ to a position on equipotential line $$V_2$$ and the potential energy of the system of three charges will now be $$QV_2 - \frac{kq^2}{r^2}$$.

The potentials here are defined relative to infinity, meaning it takes zero net work to bring a test charge from infinity to the point with potential $$V_3$$. This is easy to see explicitly: if the point charge is initially very far away but on the plane of symmetry, the force on it is always perpendicular to the plane, so it takes no work to move the particle along this plane. This is the physical meaning of the value of the electric potential: the potential energy (relative to infinity in this case) per unit charge.

This of course doesn't contradict the fact that a positive test charge released from this point will start to move toward lower potential, gaining the difference in potential energy as kinetic energy in the process. How a charge moves doesn't depend on the value of the potential at a given point, but rather its gradient, i.e. how it changes. In this sense, only differences in potential matter, and the value of the electric potential is only unique up to an additive constant.

When the charge is repelled by the positive charge, its position changes to a point not on the centre line. As a result, there will be a change in potential which is the reason for the increase in kinetic energy.