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I have a question on the derivation of the electric potential for a single charge located at the origin.

The electric potential is defined as $$V\left(\mathbf{r}\right)=-\int_{\mathcal{O}}^{\mathbf{r}}\mathbf{E}\cdot d\mathbf{l}$$ Where $\mathcal{O}$ is the reference position where $V$ is defined to be $0$.

When $\mathcal{O}$ is taken to be $\infty$, the integral becomes $$V\left(\mathbf{r}\right)=-\int_{\mathcal{\infty}}^{\mathbf{r}}\mathbf{E}\cdot d\mathbf{l}$$

When computing the electric potential due to a single charge, (were $r$ now represents the distance) $$V\left(\mathbf{r}\right)=-\int_{\mathcal{\infty}}^{\mathbf{r}}\frac{q}{4\pi\epsilon_0}\frac{1}{r'^2}\hat{\mathbf{r}}\cdot d\mathbf{l}=\frac{q}{4\pi\epsilon_0}\frac{1}{r}$$

Here $\hat{\mathbf{r}}\cdot d\mathbf{l}$ was taken to be 1, as in all the derivations for electric potential that I can find.

I don't understand why when integrating over a straight line from $\infty$ to $\mathbf{r}$, the sign of the dot product is positive. Intuitively, it seems like it should be $-1$ since the tangent vector to the curve is always pointing in the opposite direction to $\hat{\mathbf{r}}$.

But if the dot product was negative, the work done by the electric field would be positive when bringing two like charges together which doesn't make any physical sense. Is there an intuitive reason for why my reasoning above is incorrect? (Preferably in terms of the line integral itself, I understand why having a negative value for the potential is incorrect physically, but I don't understand why the math is incorrect).

Qmechanic
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QED
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2 Answers2

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In this answer I address with various levels of mathematical sophistication/rigour why the minus sign is already taken into account by the limits of integration. If you want to completely avoid such confusions, note that line integrals are defined along paths. So, we should denote a line integral of a vector field along a curve as \begin{align} \int_{\gamma}\mathbf{E}\cdot\,d\mathbf{l}, \end{align} where $\gamma:(t_1,t_2)\to\Bbb{R}^3$ is a smooth curve (actually $C^1$ is enough). Here, we allow $-\infty\leq t_1<t_2\leq \infty$, and if we allow for infinite values, then we should of course ensure that everything is convergent. Recall that by definition, this is equal to \begin{align} \int_{\gamma}\mathbf{E}\cdot d\mathbf{l}:=\int_{t_1}^{t_2}\left\langle\mathbf{E}(\gamma(t)), \gamma'(t)\right\rangle\,dt, \end{align} where $\langle\cdot,\cdot\rangle$ refers to inner/dot product. The parametrization of $\gamma$ contains all the information about directions.

For example, suppose we want the path $\gamma$ to "start at infinity and end at point $\mathbf{r}$" (for convenience assume $\mathbf{r}\neq 0$). Then, a radial parametrization of this is given by $\gamma:(0,1]\to\Bbb{R}^3$, $\gamma(t)=\frac{1}{t}\mathbf{r}$. Note that as $t\to 0^+$, we have $\|\gamma(t)\|\to \infty$, and at $t=1$, we have $\gamma(1)=\mathbf{r}$. So, this perfectly captures our idea of "starting at infinity and ending at $\mathbf{r}$". As a technical remark, note that there is no point $\infty\in\Bbb{R}^3$, which is why I only said $\lim\limits_{t\to 0^+}\|\gamma(t)\|=\infty$, I didn't say $\gamma(0)=\infty$, because that would be a nonsensical statement. Ok, now note that \begin{align} \gamma'(t)&=-\frac{1}{t^2}\mathbf{r}. \end{align} Notice how there is a minus sign here. This minus sign is precisely capturing the direction of traversal along the path.

Ok, the case of moving a particle from infinity is slightly iffy to deal with mathematically because the parametrization isn't so obvious. Let us deal with another case. Say we fix $0\leq r_1<r_2<\infty$. Let $\mathbf{u}$ be a unit vector along some direction, let $\mathbf{r}_1=r_1\mathbf{u}$ and let $\mathbf{r}_2=r_2\mathbf{u}$. Suppose we wish to travel from point $\mathbf{r}_2$ to $\mathbf{r}_1$ (i.e decrease in radius). One way of parametrizing the path is $\gamma:[r_1,r_2]\to\Bbb{R}^3$, $\gamma(t)= (r_2+r_1-t)\mathbf{u}$. Notice that we have $\gamma(r_1)=\mathbf{r}_2$ and $\gamma(r_2)=\mathbf{r}_1$, i.e we start at the point $\mathbf{r}_2$ and end at the point $\mathbf{r}_1$, as desired. Now, notice that \begin{align} \gamma'(t)&=-\mathbf{u} \end{align} This minus sign is once again encapsulating the direction of travel (from a larger radius to a smaller radius). Hence, the integral of a vector field $\mathbf{E}$ in this case would be \begin{align} \int_{\gamma}\mathbf{E}\cdot \,d\mathbf{l}&:=\int_{r_1}^{r_2}\langle\mathbf{E}(\gamma(t)),\gamma'(t)\rangle\,dt\\ &=\int_{r_1}^{r_2}\langle\mathbf{E}((r_1+r_2-t)\mathbf{u}, \color{red}{-}\mathbf{u})\rangle\,dt \end{align} The red minus sign takes the direction into account. Also, one can switch the order of limits of integration at the cost of a minus sign and write this as \begin{align} \int_{r_2}^{r_1}\langle \mathbf{E}((r_1+r_2-t)\mathbf{u}), \mathbf{u}\rangle\,dt. \end{align}

The usual physics way of writing down the calculations is to just avoid explicitly writing out the parametrized curve $\gamma$; rather we simply indicate in the limits of integration the starting and ending points, such as $\int_{\text{start}}^{\text{end}}$ (this is only ok in the case of conservative force fields, because otherwise line integrals are path-dependent), and once you write this out as a one-dimensional integral, the signs are already captured by the placement of the limits of integration (remember that for Riemann integrals, if $b<a$ then we define the symbol $\int_a^b$ to mean $-\int_b^a$... this is the minus sign which you're overlooking). I'd suggest reading the link above to see a simple illustration of this in one-dimension.

peek-a-boo
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  • I understand how the $dr$ picks up a negative sign when going the right way (i.e. from a lesser parameter value to a larger one) Could you clarify on why the sign is positive when 'reversing direction' i.e. making the path $t\hat{\mathbf{r}}$ and letting go from infinity to $r$ (rather than the other way), which makes $dr$ positive? (I think I understand it from a physical standpoint, just not a mathematical one) – QED Jan 01 '22 at 04:36
  • @QED I'm not sure what you mean; you seem to be mixing up signs. $\gamma:(0,\infty)\to\Bbb{R}^3$ defined as $\gamma(t)=t\mathbf{r}$ starts near the origin and goes to $\infty$, hence we do expect it to have a positive sign in the derivative $\gamma'(t)=\mathbf{r}$. On the other hand, $\gamma:(0,1)\to\Bbb{R}^3$ defined as $\gamma(t)=\frac{1}{t}\mathbf{r}$ starts at infinity and ends at $\mathbf{r}$, which is why there's the minus sign $\gamma'(t)=-\frac{1}{t^2}\mathbf{r}$. – peek-a-boo Jan 01 '22 at 04:49
  • also, did you take a look at the other answer where I give you an explicit calculation? (I strongly suggest you study that example very carefully) – peek-a-boo Jan 01 '22 at 04:51
  • Can you define the curve $\gamma:(\infty,0)\to \mathbb{R}^3$ as $\gamma(t)=t\mathbf{r}$ where $t$ ranges from $\infty$ to $0$ rather than the other way around? – QED Jan 01 '22 at 05:21
  • @QED No, intervals are written $(a,b)$ where $a<b$. and a parametrized curve is defined as a mapping $\gamma:[t_1,t_2]\to\Bbb{R}^3$, where $t_1\leq t_2$ and $\gamma(t_1)$ is called the initial/starting point and $\gamma(t_2)$ is called the terminal/final/end point. So, writing $\gamma(t)=t\mathbf{r}$ where $t\in [0,\infty)$ means the starting point is the origin and the ending point is infinity. If you want to start at infinity you need to multiply by a function of $t$ which blows up as $t$ approaches the initial parameter (hence my choice $\frac{1}{t}$). – peek-a-boo Jan 01 '22 at 05:27
  • As I wrote in the answer here, dealing with infinity is annoying. Even in the finite case, dealing with $\mathbf{r}_1=r_1\mathbf{u}$ and $\mathbf{r}_2=r_2\mathbf{u}$ with $0\leq r_1<r_2<\infty$, we write intervals as $[r_1,r_2]$, and $\gamma(t)=(r_1+r_2-t)\mathbf{u}$ describes the straight-line path travelled at unit speed from $\gamma(r_1)=\mathbf{r}_2$ to $\gamma(r_2)=\mathbf{r}_1$, which again has $\gamma'(t)=-\mathbf{u}$ with the minus sign. – peek-a-boo Jan 01 '22 at 05:31
  • Ah so writing the integral as $\int_{\infty}^{r}dr'$ can't be interpreted as simply defining the curve in terms of r' which ranges from $\infty$ to $r$ and integrating right? – QED Jan 01 '22 at 05:33
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    You can always take the path $\gamma(t)=t\mathbf{r}$, where $t\in (0,\infty)$, and then rather than considering $\gamma$, consider the negative of that integral $-\int_{\gamma}$, which after rewriting as a one-dimensional riemann integral would yield the limits $-\int_0^{\infty}$, but then by convention, you can write this as $\int_{\infty}^0$. – peek-a-boo Jan 01 '22 at 05:35
  • Alright, thank you for all the help – QED Jan 01 '22 at 05:36
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Express $\hat{\mathbf{r}}$ and the infinitesimal displacement $d\vec{\mathbf{l}}$ in polar-coordinates, without regard to the limits on the integral. Then, the $\hat{\mathbf{r}}$-component of $d\vec{\mathbf{l}}$ is $dr$. Let the limits handle the how the path is traversed.

(This is based on my answer to similar questions on
https://www.physicsforums.com/threads/dot-product-in-the-gravitational-potential-energy-formula.815230/
and
Sign conundrum while deriving electrostatic potential
and
Positive work along path )

robphy
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  • I understand that $d\mathbf{l}$ can be written as $d\mathbf{l}=dr\hat{\mathbf{r}}+rd\theta\hat{\boldsymbol{\theta}}+r\sin(\theta)d\phi\hat{\boldsymbol{\phi}}$, but doesn't $dr$ depend on the path and how it's parameterized (as $dr=\frac{dr}{dt}dt$)? The definition of electrostatic potential is a line integral, which seems to allow for a specific path to be chosen (even if all paths should give the same answer) – QED Jan 01 '22 at 02:49
  • @QED No, $dr$ describes the increment in the $r$-coordinate (along the positive $\hat r$-axis. – robphy Jan 01 '22 at 02:51
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    The wikipedia page says something different: https://en.wikipedia.org/wiki/Line_integral as it describes the line integral of $\mathbf{F}\cdot d\mathbf{l}$ as the single variable integral of $\mathbf{F}(\mathbf{l}(t))\cdot \frac{d\mathbf{l}}{dt}dt$. Also, if $dr$ always represents the same increment, how could line integrals depend on the path for non-conservative fields? – QED Jan 01 '22 at 02:59