∙Bubble chamber, I. Certain types of bubble chambers are...

\bullet Bubble chamber, I. Certain types of bubble chambers are filled with liquid hydrogen. When a particle (such as an electron or a proton) passes through the liquid, it leaves a track of bubbles, which can be photographed to show the path of the particle. The apparatus is immersed in a known magnetic field, which causes the particle to curve. Figure 20.77 is a trace of a bubble chamber image showing the path of an electron. (a) How could you determine the sign of the charge of a particle from a photograph of its path? (b) How can physicists determine the momentum and the speed of this electron by using measurements made on the photograph, given that the magnetic field is known and is perpendicular to the plane of the figure? (c) The electron is obviously spiraling into smaller and smaller circles. What properties of the electron must be changing to cause this behavior? Why does this happen? (d) What would be the path of a neutron in a bubble chamber? Why?

Key Concepts

Behavior of Neutral Particles in Magnetic Fields

Neutral particles, like neutrons, do not carry an electrical charge and therefore do not experience a Lorentz force in a magnetic field. As a result, their trajectories remain unaffected by the magnetic field, leading them to travel in straight lines through devices like bubble chambers.

Energy Loss Mechanisms in Charged Particles

Charged particles lose energy as they traverse a medium through processes such as ionization and radiation emission. In the context of bubble chambers, this energy loss manifests as a gradual decrease in momentum, causing the radius of the particle's circular path to reduce over time. This spiraling inward behavior is a direct consequence of the particle’s diminishing kinetic energy.

Lorentz Force

The Lorentz force is the force exerted on a charged particle moving through a magnetic field. Its magnitude and direction are given by the cross product of the velocity and the magnetic field vectors, which allows the determination of the curvature of the particle’s path. By applying rules such as the right-hand rule (or its equivalent for negative charges), one can deduce the sign of the charge from the direction in which the particle curves.

Cyclotron Motion and Momentum Determination

Charged particles in a magnetic field undergo circular (cyclotron) motion when their velocity is perpendicular to the field. The relationship p = qBr, where p is momentum, q is the charge, B is the magnetic field strength, and r is the radius of curvature, allows scientists to calculate both the momentum and speed of the particle by carefully measuring the radius of its curved path in experimental setups like bubble chambers.

Transcript

00:01 So for part a, the direction of the curvature, rather, we can look at the path of a particle and see the charge.

00:13 And the question, rather part a of the question is asking us how we do that.

00:19 So the direction of the curvature of the path tells us the direction of the force on the charge.

00:43 Now, if this is the same as the right -hand direction, if this is the same as the right -hand rule direction, the particle is positive.

01:05 Q is greater than zero kulums.

01:08 Again, this would be the right -hand direction for velocity and magnitude vectors.

01:14 So again, for we can say velocity and magnitude.

01:19 However, if it's opposite, we of course know that the charge is negative.

01:28 So the charge would be less than zero korems.

01:33 For part b, it's asking us, well, what would, like, what could, rather, what would we be able to find the velocity from a photograph? so we can say that mv equals the absolute value of the charge times r times b.

01:53 Now here r would be the radius of the curvature and b would be the again the magnitude of the magnetic field.

02:01 However from the photograph, so from photograph r can be measured.

02:09 The radius of the curvature of the path can be measured.

02:16 And from the photograph, if we have this and we have the charge and the magnetic field, we can find the momentum...

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