# Visualizing field lines with moving current

• Mr Boom
In summary, In order to calculate the electric field between two spheres with charges, one needs to calculate the potential, use a grid to find the field, or use a 2D simulation.

#### Mr Boom

Let's say I have a circle filled with positive charges. Some distance away I have an identical circle filled with negative charges. Since the distribution is uniform in the circles, I can just use the center of the circles as points and calculate the field lines between the two. So far so good. Now let's say I connect the first circle to the second by a resistor. If the sum of the charges are so large that it is essentially unchanged for some length of time (as in I have constant charges and constant current), how can I calculate the electric field?

Let's say I have a circle filled with positive charges. Some distance away I have an identical circle filled with negative charges. Since the distribution is uniform in the circles, I can just use the center of the circles as points and calculate the field lines between the two.
No, this just works with spheres (unless you take a modified electrodynamics in a two-dimensional world).

Now let's say I connect the first circle to the second by a resistor. If the sum of the charges are so large that it is essentially unchanged for some length of time (as in I have constant charges and constant current), how can I calculate the electric field?
Calculate the field along the connection with usual rules for circuits (constant current in the wire). Use some simulation tool for the electric field, as you probably do not find an analytic solution (except for special resistor profiles where the field is unchanged).

mfb said:
No, this just works with spheres (unless you take a modified electrodynamics in a two-dimensional world).

I was thinking of spheres, yes, but I was trying to just use a 2D example.

mfb said:
Calculate the field along the connection with usual rules for circuits (constant current in the wire). Use some simulation tool for the electric field, as you probably do not find an analytic solution (except for special resistor profiles where the field is unchanged).

This was my question. I'm wondering how the field lines look visually outside the resistor by allowing current to pass. I realize there will also be a magnetic component. Will the lines straighten or become more arched?

It depends on the resistor. Without the connection, the field strength is larger close to the spheres and smaller in the middle. If the resistor is a long wire with the same resistance per distance everywhere, you modify this - the field strength close to the spheres gets reduced, the field strength in the middle (and close to the resistor) increases a bit. This should make the field lines close to the resistor a bit "more parallel".

OK, that makes sense. I'd like to try to do this problem on my own. Any recommendations on how to start this program? I've plotted the static field lines and I need to superimpose the field due to the resistor?

I've plotted the static field lines and I need to superimpose the field due to the resistor?
That won't work.

You can calculate it with a grid, for example, and I would solve for the potential first:
$\phi(x)=0$ for sphere 1, $\phi(x)=1$ for sphere 2, $\Delta \phi = 0$ in free space, and $\phi(x)=f(x)$ with some function f(x) at the resistor. Instead of a 3-dimensional simulation, it is possible to use the symmetry of the problem (if the connection is symmetric) in a 2-dimensional simulation to reduce the required computing power.

## 1. What are field lines and how are they related to current?

Field lines are imaginary lines that represent the direction and strength of a magnetic field. They are related to current because current is the flow of charged particles, and this flow creates a magnetic field with field lines that extend from the current-carrying wire.

## 2. How can field lines be visualized with moving current?

Field lines can be visualized by using a compass to trace the direction of the magnetic field created by the moving current. The compass needle will align itself with the field lines, providing a visual representation of their direction and strength.

## 3. Can field lines change with moving current?

Yes, field lines can change with moving current. As the current moves, the strength and direction of the magnetic field can change, causing the field lines to shift and curve.

## 4. How does the direction of the current affect the shape of the field lines?

The direction of the current affects the shape of the field lines. If the current is moving in a straight line, the field lines will form concentric circles around the current. If the current is moving in a curved path, the field lines will also curve and follow the path of the current.

## 5. How can visualizing field lines with moving current be useful in scientific research?

Visualizing field lines with moving current can be useful in scientific research because it allows us to understand and study the behavior of magnetic fields. This can be applied to various fields such as electrical engineering, astrophysics, and geophysics. It also helps us to better understand the relationship between current and magnetic fields, which is essential for many technological advancements.