# Particle in a magnetic field -- question

• Cyclone Charlie
In summary, the conversation discusses a calculation using the equation R=mv/qb to determine the diameter of a particle. The answer obtained is 4.1e-4 m, while the reference site gives an answer of 3.95e+10 m, which is deemed unrealistic. The speaker believes that the reference site must have made a typo, and the correct answer is 4.1e-4 m.
Cyclone Charlie
Homework Statement
Question: An alpha particle (a He nucleus, containing two protons
and two neutrons and having a mass of 6.64 * 10-27 kg) traveling
horizontally at 35.6 km/s enters a uniform, vertical, 1.80-T magnetic field.
(a) What is the diameter of the path followed by this alpha particle?
Relevant Equations
R=mv/qb
I went with R=mv/qb, thus -> 6.64e-27*35.6e3/2*1.6e-19*1.8, and got 4.1e-4 m (metres), so diameter is 2R, 8.2e-4 m, as an answer, the reference site gives 3.95e+10 m as the answer, who's right here?

Last edited:
Hi Cyclone Charlie and welcome to PF.

Cyclone Charlie said:
the reference site gives 3.95e+10 as the answer, who's right here?
3.95e+10 what? Apples, bananas, oranges? Without units a number is meaningless unless it is dimensionless, which in this case it cannot be. Same with your answer of 4.1e-4, meaningless. It is not possible to tell who is right and who is not.

Sorry Kuruman, forgot the units, the answer for the diameter is in metres.

OK, do you think that the answer of 4 e+10 m for the diameter is realistic? That's about one-quarter the Sun-Earth distance. I cannot speak for your reference and how they got the absurd answer, but your answer is correct. Theirs must be a typo.

## 1. What is a particle in a magnetic field?

A particle in a magnetic field refers to a charged particle that is subjected to the forces exerted by a magnetic field. This force causes the particle to move in a circular or helical motion around the magnetic field lines.

## 2. How does a magnetic field affect a particle?

A magnetic field can exert a force on a charged particle due to the interaction between the magnetic field lines and the particle's magnetic moment. This force can cause the particle to accelerate or change direction.

## 3. What determines the path of a particle in a magnetic field?

The path of a particle in a magnetic field is determined by the strength and direction of the magnetic field, as well as the charge and velocity of the particle. These factors determine the magnitude and direction of the magnetic force acting on the particle.

## 4. How is the motion of a particle in a magnetic field described mathematically?

The motion of a particle in a magnetic field can be described by the Lorentz force equation, which relates the magnetic force on the particle to its charge, velocity, and the strength of the magnetic field. The equation is F = qvB, where F is the force, q is the charge, v is the velocity, and B is the magnetic field.

## 5. What are some applications of the concept of a particle in a magnetic field?

The concept of a particle in a magnetic field has many practical applications, including in particle accelerators, magnetic resonance imaging (MRI) technology, and magnetic separation techniques. It is also crucial in understanding the behavior of charged particles in space, such as in the Earth's magnetic field and the solar wind.

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