proton
Summary
A proton is a positively charged particle in the nucleus, with charge +1.60 × 10⁻¹⁹ C and mass about 1.67 × 10⁻²⁷ kg. Its count is the atomic number Z. Most testable: it has the same charge size as an electron but about 1836 times the mass, so the same force gives it far less acceleration.
Detail
Meaning: A proton is a subatomic particle in the nucleus. Its charge is +e = +1.60 × 10⁻¹⁹ C, equal in size and opposite in sign to the electron's, and its mass is about 1.67 × 10⁻²⁷ kg, roughly 1836 times the electron's. The number of protons is the atomic number Z, which defines the element. The strong nuclear force holds protons together in the nucleus despite their electric repulsion. In chemistry, H⁺ is also called a proton, since it is a hydrogen atom without its electron.
Equation: F = eE, where F is the electric force on the proton (N), e is the elementary charge (C), and E is the electric field (N/C, equivalently V/m).
On the MCAT: Passages place protons in electric or magnetic fields and ask for direction or comparison. In an electric field the force on a proton points along E (an electron goes the opposite way). In a magnetic field, F = qvB sinθ is perpendicular to the velocity, so the path curves, circular if v is perpendicular to B. Nuclear questions use A = Z + N, beta-plus decay (a proton becomes a neutron) and mass defect.
Common confusion: Equal charge magnitude does not mean equal motion. A proton and an electron in the same field feel forces of equal size, but a = F/m, so the electron accelerates about 1800 times more. Compare masses before comparing accelerations.
Sources
- University Physics
- Physics
- Fundamentals of Physics
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.