The direction of the electric field at any point on the z axis is directly proportional to the direction of the positive z-axis. This is only true if the z-axis is parallel to the force causing the electric field, or if there are no other axes perpendicular to the z-axis.
When there are other axes perpendicular to the z-axis, then the electric field at any point on the z axis depends on which axis is dominating at that point. If the x and y axes are dominating, then the electric field will be in a different direction than if only the z axis is dominating.
Calculating the direction of the electric field on any given axis can be tricky, but here we will give you some tips on how to do it.
Negatively charged particles move toward the z axis
To understand this concept, you first have to understand the electric field. The electric field is the set of forces that cause charged particles to be forced in a particular direction.
These forces are based on the influence that a charge has on other charges. Charges that have a positive charge will be pushed or pulled in a certain direction when placed in a electric field. This is because they experience a force as a result of the field.
The strength of the electric field is measured by how much force it exerts on a positive charge. The higher the electric field, the stronger it is and the more likely positive charges will be forced in its direction.
The strength of the electric field at any point on the z axis is zero. There is no force pulling charges either up or down on the z axis.
The electric field points in the direction of the greatest increase in positive charges
At any point on the z-axis, the electric field points in the direction of the greatest increase in positive charges. This is called the normal direction, and it is also the direction of maximum displacement.
We can imagine this concept by thinking about how rain falls down from the sky. The water flows down from higher places to lower places, so we say that water has a tendency to go down.
The same idea applies to charges: There is a tendency for positive charges to move or gather in such a way that they form peaks, or highs, of charge density. These peaks flow toward lower points, or negative charges.
When we draw these normal directions on a diagram, they look like arrows pointing down the z-axis. We can also call this arrow a flux line.
The electric field points in the direction of the greatest increase in negative charges
The direction of the electric field at any point on the z axis, or on the surface of the Earth, can be determined by just looking.
Just look at where the negative charges are increasing, and that is the direction of the electric field. At any other point in space, you would just need to look where the negative charges are increasing to find this direction.
You can see this in action in this video. The man pulling off this experiment is an astronaut with over 20 years experience. He explains how to identify the direction of the electric field on Earth using this simple trick.
This fact is very useful in determining whether an electromagnetic wave will induce a voltage in a receiver or not. If the field points away from the receiver, it will not receive a signal!
Experiment with different positions and angles to learn more about how this works.
The z-axis is the direction of the perpendicular intersection of an electric field line with the surface of an object
The direction of the electric field at any point on the z axis is always out of the surface of an object. The field is strongest in the direction of the z axis, and weaker in all other directions.
The strength of the electric field on the z-axis is dependent on how many lines of force intersect the z-axis. The more lines that intersect the z-axis, the stronger the electric field will be on it.
We describe this as having a strong z-component of an electric field. Field lines that are closer together have a stronger z-component, and thus produce a stronger electric field on the z-axis.
How do we determine which way the electric field is moving on any given point on the Z axis? You can’t! The only way to tell if an electric field is stationary or not is by looking at its effects.