The Science Teacher, December 2005 | Go to article overview

Q What does "spin" refer to in particle physics? Why is this concept necessary?

Danny, Student

Scottsburg High School

Scottsburg, Indiana

A To answer this question, let's start with some classical mechanic concepts. Angular momentum is the rotational analog of (linear) momentum. An everyday object that is spinning has angular momentum. If we attach electric charge to that spinning object, the circulating charge acts like a loop of current, and produces a magnetic dipole--a tiny electromagnet.

Because an electron has angular momentum and a magnetic dipole, it is natural to talk of its spin. Natural, but somewhat misleading, because on such a small scale, quantum mechanics, rather than classical mechanics, describes what happens. Like the energy of electrons in an atom, the spin of a fundamental particle is quantized: only discrete values are allowed (+1/2 and -1/2). Bear in mind, electrons do not behave like tennis balls.

Spin is a necessary concept because it is measurable. If we apply an external magnetic field, the energy of an electron is increased or decreased depending on the orientation of its dipole to the field and its spin. Spin also provides particles with an extra quantum number. The Pauli exclusion principle forbids electrons to have the same quantum numbers. Therefore, for any energy level in the atom, there can be as many as two electrons with opposite spin. Thus, spin accounts for the coexistence of twice as many electrons, which has very considerable consequences for the periodic table and chemistry.

Q My physics teacher tells me that when I go around a sharp curve in my car, there is no force causing me to move away from the center of curvature. So what is happening to make me feel as if I am sliding toward the outside?

Physics student

Meyers Park High School

Charlotte, North Carolina

(Courtesy of Teacher: Wayne Fisher)

A Let's imagine that the car is a convertible and I am watching you from above. Before the curve, the car is coasting and you and it are traveling in a straight line, with no horizontal forces acting on you or the car. Now the car turns to the left. …

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