# Electron or Proton gun (+/- charge) from magnetic field?

• d3nat
In summary, the conversation discusses the use of a compass and the right hand rule to determine the direction of a magnetic field caused by a moving charge. The person concludes that the alien is using a protongun based on the direction of the needle on the compass when the alien shoots his rifle vertically at the sky. However, the person is reminded to consider the location of the window ledge and the possibility that their original positive charge assumption may need to be reversed.
d3nat

## Homework Statement

The news just announced that aliens armed with either electron- or proton-guns have invaded the city. A quick glance reveals that there is indeed an alien on the street just below my top-floor, north-facing window. When the alien shoots his rifle vertically at the sky, the needle of the compass I keep on the window ledge points to the left. Which of the two kinds of gun is the alien using? (Explain your answer, if you like with the help of a sketch).

## The Attempt at a Solution

I have this sketched on paper, but I'll just describe it because I can't attach.

I have a square notated N E S W.
In the middle, I have a circle with a dot to signify that the gun is pointing up.
According to the right hand rule, my magnetic field is rotating counterclockwise.

A compass always points in the direction of a magnetic field.
And if I'm correct, doesn't positive electric charge produce a magnetic field which is orientated at a right angle to charges direction of motion.

So it should be a protongun?

d3nat said:

## Homework Statement

The news just announced that aliens armed with either electron- or proton-guns have invaded the city. A quick glance reveals that there is indeed an alien on the street just below my top-floor, north-facing window. When the alien shoots his rifle vertically at the sky, the needle of the compass I keep on the window ledge points to the left. Which of the two kinds of gun is the alien using? (Explain your answer, if you like with the help of a sketch).

## The Attempt at a Solution

I have this sketched on paper, but I'll just describe it because I can't attach.

I have a square notated N E S W.
In the middle, I have a circle with a dot to signify that the gun is pointing up.
According to the right hand rule, my magnetic field is rotating counterclockwise.
Yes, everything is right so far, *assuming* that the charge moving up is positive. Keep in mind though, at this point, the charge polarity is just an assumption. But you have to start somewhere; you can correct that assumption later if necessary.
A compass always points in the direction of a magnetic field.
Yes, it points in the same direction that the B field is pointing.
And if I'm correct, doesn't positive electric charge produce a magnetic field which is orientated at a right angle to charges direction of motion.
Ummm, yes, that is technically correct. But it's also correct for a negative charge too. You can use the right hand rule to determine the direction of a magnetic field caused by a moving, positive charge. You'll need to reverse the direction of the field if the charge is negative (all else being the same). But regardless of whether the charge is positive or negative, the B field is always perpendicular to the moving charge's direction of motion.
So it should be a protongun?

Don't forget to draw the window ledge on your diagram. The window ledge is not at the same location as the center dot indicating the gun pointing up: It's somewhere below the dot (on the paper). Orient the figure such that the window ledge is between you and the center dot. You know that the compass needle points toward the left. Is your original positive charge assumption correct or does the assumption need to be reversed?

## 1. What is an Electron or Proton gun?

An Electron or Proton gun is a device that is used to accelerate and direct charged particles, specifically electrons or protons, towards a target. It typically consists of an electron or proton source, an accelerating electrode, and a focusing electrode.

## 2. How does a magnetic field affect the trajectory of charged particles in an Electron or Proton gun?

A magnetic field can be used to control the path of charged particles in an Electron or Proton gun. When a charged particle enters a magnetic field, it experiences a force perpendicular to both its direction of motion and the magnetic field. This force causes the particle to follow a curved path, allowing it to be directed towards a specific target.

## 3. What is the difference between a positive and negative charge in an Electron or Proton gun?

The difference between a positive and negative charge in an Electron or Proton gun lies in the direction of their paths. A positive charge will curve in one direction, while a negative charge will curve in the opposite direction due to the direction of the magnetic field and the force acting on the charged particle.

## 4. Can an Electron or Proton gun be used for different applications?

Yes, Electron or Proton guns have various applications in different fields. In scientific research, they are commonly used in particle accelerators to study the properties of matter. They can also be used in medical treatments, such as radiation therapy for cancer treatment, and in industrial processes, such as ion implantation in semiconductor manufacturing.

## 5. How is the strength of the magnetic field in an Electron or Proton gun determined?

The strength of the magnetic field in an Electron or Proton gun is typically determined by the strength of the current passing through the electromagnetic coils that generate the field. The higher the current, the stronger the magnetic field, and the more control the gun has over the charged particles' paths.

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