Chapter 7: Problem 22
A particle with charge \(e\) and mass \(m\) is projected with linear momentum \(p\) at right angles to a uniform magnetic field \(B\). What are the magnetic dipole moment and angular momentum possessed by the particle as a result of its subsequent motion?
Short Answer
Expert verified
Question: Determine the angular momentum and the magnetic dipole moment of a charged particle projected with linear momentum at a right angle to a uniform magnetic field.
Answer: The angular momentum of the particle is p^2/(qB), and the magnetic dipole moment is (pv)/(2B).
Step by step solution
01
Calculate the magnetic force on the particle
Using the formula of the magnetic force on a moving charged particle, we can find the magnetic force acting on the particle.
F = qvB (since the force is orthogonal to both the magnetic field and velocity)
As the particle moves in a circle due to the influence of the magnetic field, we can equate the magnetic force with the centripetal force, making F = mv^2/r.
02
Find the radius and velocity of the circular path
From the previous step, we have:
qvB = mv^2/r. Rearrange the equation to find r and v.
r = mv/qB (radius of the circular path)
v = p/m (velocity from given linear momentum)
03
Calculate the angular momentum of the particle
The angular momentum of the particle (L) can be calculated using the formula L = mvr. Substitute v = p/m and the expression for r (see step 2) into the formula:
L = m(p/m)(mv/qB)
L = p^2/(qB)
The angular momentum of the particle is p^2/(qB).
04
Calculate the frequency of the circular motion
We will need the frequency of the circular motion to find the magnetic dipole moment. To find the frequency (f), we can use the formula f = qB/(2πm).
05
Calculate the magnetic dipole moment
The magnetic dipole moment (μ) can be calculated using the formula μ = IA, where I is the current and A is the area of the loop created by the particle's orbit. For a charged particle moving in a circle, the current is given by I = qf (charge multiplied by the frequency), and the area A = πr^2.
Substitute the expressions for f and r derived above into these formulas:
I = q(qB/(2πm))
A = π(mv/qB)^2
Now, we can find the magnetic dipole moment:
μ = IA
μ = q(qB/(2πm))π(mv/qB)^2
μ = (pv)/(2B)
So, the magnetic dipole moment of the particle is (pv)/(2B).
In summary, the angular momentum of the particle is p^2/(qB), and the magnetic dipole moment is (pv)/(2B).
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Magnetic Force
When we dive into the world of electromagnetic physics, one of the fundamental concepts we encounter is the magnetic force. This force is a result of a charged particle moving through a magnetic field. In simple terms, if you have a charged particle, like an electron, moving through a magnetic field, it will experience a force that is perpendicular to both the field direction and the particle's velocity.
For a particle with charge (\( e \) and mass (\( m \) moving at a velocity (\( v \) in a magnetic field (\( B \) the magnetic force exerted is (\( F = evB \) when it moves perpendicular to the magnetic field. This force is always at right angles to the motion of the particle, which means it does no work on the particle and hence doesn’t change the particle’s speed but can only change its direction.
Understanding this force is crucial because it explains the circular path taken by the charged particle when subjected to a magnetic field. Think of it like a tetherball tied to a pole; as you hit it, the ball moves in a circle around the pole. Similarly, the magnetic force acts as a 'central force' keeping the charged particle moving in a circular path.
For a particle with charge (\( e \) and mass (\( m \) moving at a velocity (\( v \) in a magnetic field (\( B \) the magnetic force exerted is (\( F = evB \) when it moves perpendicular to the magnetic field. This force is always at right angles to the motion of the particle, which means it does no work on the particle and hence doesn’t change the particle’s speed but can only change its direction.
Understanding this force is crucial because it explains the circular path taken by the charged particle when subjected to a magnetic field. Think of it like a tetherball tied to a pole; as you hit it, the ball moves in a circle around the pole. Similarly, the magnetic force acts as a 'central force' keeping the charged particle moving in a circular path.
Angular Momentum
Angular momentum in classical mechanics is a measure of the momentum of an object rotating around a point. Think of it like momentum, but for spinning or revolving objects. It’s what keeps a spinning top from immediately falling over or planets from deviating off their orbits around the sun.
For our charged particle moving in a circular path within a magnetic field, we can determine its angular momentum (\( L \) using the formula (\( L = mvr \) where (\( m \) is the mass, (\( v \) is the velocity, and (\( r \) is the radius of the circular path. This value remains constant as long as there is no external torque acting on the system. In the step-by-step solution provided, angular momentum is found using the relationship between the charge (\( e \) the magnetic field (\( B \) linear momentum (\( p \) and this was expressed as (\( L = p^2/(eB) \) clearly showing its direct dependence on these properties.
For our charged particle moving in a circular path within a magnetic field, we can determine its angular momentum (\( L \) using the formula (\( L = mvr \) where (\( m \) is the mass, (\( v \) is the velocity, and (\( r \) is the radius of the circular path. This value remains constant as long as there is no external torque acting on the system. In the step-by-step solution provided, angular momentum is found using the relationship between the charge (\( e \) the magnetic field (\( B \) linear momentum (\( p \) and this was expressed as (\( L = p^2/(eB) \) clearly showing its direct dependence on these properties.
Circular Motion in Magnetic Field
Circular motion is a movement of an object along the circumference of a circle or rotation along a circular path. It's characterized by constant changes in direction which implies that an object in circular motion is always accelerating toward the center of the circle, a type of acceleration called 'centripetal'.
In the context of a charged particle in a magnetic field, this circular motion arises because the magnetic force provides the centripetal force necessary to keep the particle moving in a circle. This is nicely encapsulated in the solution's step, showing the equality (\( evB = mv^2/r \) which seamlessly unites the magnetic and centripetal forces. The radius of the circular path (\( r \) can be derived, illustrating how the magnetic field influences the size of the particle's orbit.
In the context of a charged particle in a magnetic field, this circular motion arises because the magnetic force provides the centripetal force necessary to keep the particle moving in a circle. This is nicely encapsulated in the solution's step, showing the equality (\( evB = mv^2/r \) which seamlessly unites the magnetic and centripetal forces. The radius of the circular path (\( r \) can be derived, illustrating how the magnetic field influences the size of the particle's orbit.
Charge-to-Mass Ratio
The charge-to-mass ratio (\( q/m \) is a key concept in electromagnetism that has profound implications in understanding the behavior of charged particles in electric and magnetic fields. It’s essentially a way of comparing how much charge a particle has relative to its mass.
This ratio helps determine the curvature of the path for a charged particle in a magnetic field. A larger ratio means that the particle will curve more sharply - it's like having a car with really good power steering versus a truck with poor steering. In the problem at hand, we can observe the importance of this ratio when calculating the velocity (\( v \) and the radius of the path (\( r \) of the charged particle. The expressions (\( r = mv/qB \) and (\( v = p/m \) highlight the inverse relationship between the charge-to-mass ratio and the radius of circular motion. A high charge-to-mass ratio translates to a smaller radius, indicating a tighter circular path.
This ratio helps determine the curvature of the path for a charged particle in a magnetic field. A larger ratio means that the particle will curve more sharply - it's like having a car with really good power steering versus a truck with poor steering. In the problem at hand, we can observe the importance of this ratio when calculating the velocity (\( v \) and the radius of the path (\( r \) of the charged particle. The expressions (\( r = mv/qB \) and (\( v = p/m \) highlight the inverse relationship between the charge-to-mass ratio and the radius of circular motion. A high charge-to-mass ratio translates to a smaller radius, indicating a tighter circular path.