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　　Ideal gas law

　　An equation, PV = nRT, that relates the pressure, volume, temperature, and quantity of an ideal gas. An ideal gas is one that obeys the approximations laid out in the kinetic theory of gases.

　　Impulse

　　A vector quantity dined as the product of the force acting on a body multiplied by the time interval over which the force is exerted.

　　Incident ray

　　When dealing with rlection or rraction, the incident ray is the ray of light bore it strikes the rlecting or rracting surface.

　　Inclined plane

　　A wedge or a slide. The dynamics of objects sliding down inclined planes is a popular topic on SAT II Physics.

　　Index of rraction

　　The index of rraction n = c/v of a substance characterizes the speed of light in that substance, v. It also characterizes, by way of Snell&aposs Law, the angle at which light rracts in that substance.

　　Induced current

　　The current induced in a circuit by a change in magnetic flux.

　　Inelastic collision

　　A collision in which momentum is conserved but kinetic energy is not.

　　Inertia

　　The tendency of an object to remain at a constant velocity, or its resistance to being accelerated. Newton’s First Law is alternatively called the Law of Inertia because it describes this tendency.

　　Inertial rerence frame

　　A rerence frame in which Newton’s First Law is true. Two inertial rerence frames move at a constant velocity relative to one another. According to the first postulate of Einstein’s theory of special relativity, the laws of physics are the same in all inertial rerence frames.

　　Instantaneous velocity

　　The velocity at any given instant in time. To be contrasted with average velocity, which is a measure of the change in displacement over a given time interval.

　　Internal energy

　　The energy stored in a thermodynamic system.

　　Inversely proportional

　　Two quantities are inversely proportional if an increase in one results in a proportional decrease in the other, and a decrease in one results in a proportional increase in the other. In a formula dining a certain quantity, those quantities to which it&aposs inversely proportional will appear in the denominator.

　　Isolated system

　　A system that no external net force acts upon. Objects within the system may exert forces upon one another, but they cannot receive any impulse from outside forces. Momentum is conserved in isolated systems.

　　Isotope

　　Atoms of the same element may have different numbers of neutrons and therore different masses. Atoms of the same element but with different numbers of neutrons are called isotopes of the same element.

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