particles
Summary
A particle is an idealized object treated as a single point with mass (and sometimes charge) but no size or rotation. Most testable: a charged particle moving perpendicular to a magnetic field follows a circle, r = mv/(qB), at constant speed.
Detail
Meaning: In MCAT physics, a particle is an idealized object treated as a single point with mass (and sometimes charge) but no size or rotation. This lets Newton's laws and energy conservation be applied using one position and one velocity. A block, ball or ion is modeled this way when its size does not affect the answer. In atomic and nuclear topics, "particle" also names subatomic bodies such as electrons, protons and alpha particles.
Equation: qvB = mv²/r, so r = mv/(qB), for a charge moving perpendicular to a uniform magnetic field. q is charge (C), v is speed (m/s), B is magnetic field (T), m is mass (kg), r is path radius (m).
On the MCAT: Passages send charged particles through electric or magnetic fields, as in a mass spectrometer. Use the right-hand rule for the direction of the force (reversed for a negative charge) and compare radii: larger mass or speed gives a larger r, and larger charge or field gives a smaller r.
Common confusion: A magnetic field does not speed the particle up. Its force is perpendicular to the velocity, so it does no work and changes only the direction of motion. An electric force, F = qE, can change the speed. A neutral particle feels neither force.
Sources
- Physics (Cutnell and Johnson)
- Fundamentals of Physics (Halliday, Resnick and Walker)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.