SATII物理考试常用术语

　　the following list defines all of the bold-faced words you encountered as you read this book.

　　a

　　absolute zero

　　the lowest theoretical temperature a material can have, where the molecules that make up the material have no kinetic energy. absolute zero is reached at 0 k or –273º c.

　　acceleration

　　a vector quantity defined as the rate of change of the velocity vector with time.

　　activity

　　in radioactive substances, the number of nuclei that decay per second. activity, a, will be larger in large samples of radioactive material, since there will be more nuclei.

　　alpha decay

　　a form of radioactive decay where a heavy element emits an alpha particle and some energy, thus transforming into a lighter, more stable, element.

　　alpha particle

　　a particle, which consists of two protons and two neutrons. it is identical to the nucleus of a helium atom and is ejected by heavy particles undergoing alpha decay.

　　amplitude

　　in reference to oscillation, amplitude is the maximum displacement of the oscillator from its equilibrium position. amplitude tells how far an oscillator is swinging back and forth. in periodic motion, amplitude is the maximum displacement in each cycle of a system in periodic motion. the precise definition of amplitude depends on the particular situation: in the case of a stretched string it would be measured in meters, whereas for sound waves it would be measured in units of pressure.