# Particles and waves

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1. What is a particle?

A particle has the attributes of mass, momentum, and thus kinetic energy. A particle has a definite location in space, and thus is discrete and countable. A particle may carry an electric charge.

2. ## What is a wave?

   A wave is a periodic variation of some quantity as a function of location. For example, the wave motion of a vibrating guitar string is defined by the displacement of the string from its center as a function of distance along the string. A sound wave consists of variations in the pressure with location. A wave is characterized by its wavelength l and frequency v, which are connected by the relation in which u is the velocity of propagation of the disturbance in the medium. The velocity of sound in the air is 330 m s-1. What is the wavelength of A440 on the piano keyboard?

   Two other attributes of waves are the amplitude (the height of the wave crests with respect to the base line) and the phase, which measures the position of a crest with respect to some fixed point. The square of the amplitude gives the intensity of the wave: the energy transmitted per unit time).

   A unique property of waves is their ability to combine constructively or destructively, depending on the relative phases of the combining waves.