p-type semiconductors – doped semiconductors in which the addition of impurities results in a predominance of positive carriers

pn junction – the junction of n-type and p-type semiconductors in which the impurity concentrations vary in such a way that the semiconductor crystal goes from n-type to p-type across a relatively narrow transition region (see figure 31.24)

Pascal – the SI unit of pressure; a Newton per square meter

Pascal’s principle – states that pressure applied at one point in an enclosed fluid is transmitted undiminished to every part of the fluid and to the walls of the container

Perfectly inelastic collision – a collision in which the colliding objects stick together after impact

Period – the time it takes to complete one oscillation or cycle

Periodic – occurring at regular intervals of time

Permittivity of free space – the quantity e₀ , a constant of proportionality in Coulomb's law

Phase angle – for two sinusoidal (sines or cosines) functions, the angle that one leads or lags the other

Phasor – a two-dimensional mathematical quantity that rotates with a constant angular frequency and represents oscillating voltage, or current, in an AC circuit ( see figure 21.13)

Photoelectric effect – the emission of electrons from a material as a result of light striking its surface

Photons – a quantized unit of electromagnetic radiation

Physics – the science dealing with the properties, changes, interactions, etc. of matter and energy

Pion – an elementary particle associated with the nuclear force field, larger than a muon, they occur in positive, negative, and uncharged forms all with the same mass (about 270 me) and zero spin

Planck’s constant– the constant of proportionality between the energy and frequency of an electromagnetic wave (see equation 27.4)

Plane of incidence – a plane perpendicular to the reflecting surface in which the normal, the incident ray, and the reflected ray all lie

Plane waves – a flat, small piece of the spherical wave front far from the source

Poiseuille’s law – describes the coefficient of viscosity of a fluid (see equation 10.10

Polarization – in wave behavior, the situation where the oscillations are confined to move in only one direction (or plane)

Polycrystalline solid – a solid, made-up of numerous individual crystalline subunits called grains, formed when the periodicity in the arrangement of atoms in the solid is disrupted (see figure 31.5)

Positron – a subatomic particle with the same mass as an electron but with one unit (e) of positive electric charge; also called beta plus

Potential energy – energy due to position; stored energy

Potential-energy diagram – a graph displaying the line of potential energy versus displacement

Power – the time rate of doing work

Pressure – the perpendicular force per unit area

Principle of equivalence – experiments performed in a uniformly accelerating reference frame, having a given acceleration with respect to an inertial frame, give exactly the same results as the identical experiments carried out in an inertial frame containing a uniform gravitational field

Principle of superposition – at any instant, the resulting wave is the algebraic sum of all the component waves

Principal quantum number – labels an electron's orbit and the allowed energy for the electron (see figure 27.13)

Probability density– the probability of finding a particle in a particular small volume of space (see equation 28.8)

Proper length – the length of an object measured by an observer at rest with respect to the object

Proper time – the time interval measured in a reference frame in which the observer is at rest with respect to the event being timed

Proton – elementary particle in the nucleus of an atom, carrying a positive charge

Pumping – in a laser, the process of supplying energy to the active medium in order to get an inverted population

Pure (intrinsic) semiconductors – a semiconductor whose behavior is not dominated by impurity atoms (see figure 31.21)