I–L
20.1 A–D
 
20.2 E–H
 
20.3 I–L
 
 
20.4 M–P
 
20.5 Q–T
 
20.6 U–Z
 
I–L
I
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 defined 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 reflection or refraction, the incident ray is the ray of light before it strikes the reflecting or refracting 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 refraction
The index of refraction n = c/v of a substance characterizes the speed of light in that substance, v. It also characterizes, by way of Snell's Law, the angle at which light refracts 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 reference frame
A reference frame in which Newton’s First Law is true. Two inertial reference 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 reference 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 defining a certain quantity, those quantities to which it's 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 therefore different masses. Atoms of the same element but with different numbers of neutrons are called isotopes of the same element.
J
Joule
The joule (J) is the unit of work and energy. A joule is 1 N · m or 1 kg · m2/s2.
K
Kelvin
A scale for measuring temperature, defined such that 0K is the lowest theoretical temperature a material can have. 273K = C.
Kepler’s First Law
The path of each planet around the sun is an ellipse with the sun at one focus.
Kepler’s Second Law
If a line is drawn from the sun to the planet, then the area swept out by this line in a given time interval is constant.
Kepler’s Third Law
Given the period, T, and semimajor axis, a, of a planet’s orbit, the ratio is the same for every planet.
Kinematic equations
The five equations used to solve problems in kinematics in one dimension with uniform acceleration.
Kinematics
Kinematics is the study and description of the motion of objects.
Kinetic energy
Energy associated with the state of motion. The translational kinetic energy of an object is given by the equation .
Kinetic friction
The force between two surfaces moving relative to one another. The frictional force is parallel to the plane of contact between the two objects and in the opposite direction of the sliding object’s motion.
Kinetic theory of gases
A rough approximation of how gases work, that is quite accurate in everyday conditions. According to the kinetic theory, gases are made up of tiny, round molecules that move about in accordance with Newton’s Laws, and collide with one another and other objects elastically. We can derive the ideal gas law from the kinetic theory.
L
Latent heat of fusion
The amount of heat necessary to transform a solid at a given temperature into a liquid of the same temperature, or the amount of heat needed to be removed from a liquid of a given temperature to transform it into a solid of the same temperature.
Latent heat of sublimation
The amount of heat necessary for a material undergoing sublimation to make a phase change from gas to solid or solid to gas, without a change in temperature.
Latent heat of transformation
The amount heat necessary to cause a substance to undergo a phase transition.
Latent heat of vaporization
The amount of heat necessary to transform a liquid at a given temperature into a gas of the same temperature, or the amount of heat needed to be taken away from a gas of a given temperature to transform it into a liquid of the same temperature.
Law of conservation of energy
Energy cannot be made or destroyed; energy can only be changed from one place to another or from one form to another.
Law of reflection
For a reflected light ray, . In other words, a ray of light reflects of a surface in the same plane as the incident ray and the normal, and at an angle to the normal that is equal to the angle between the incident ray and the normal.
Legs
The two shorter sides of a right triangle that meet at the right angle.
Lenz’s Law
States that the current induced in a circuit by a change in magnetic flux is in the direction that will oppose that change in flux. Using the right-hand rule, point your thumb in the opposite direction of the change in magnetic flux. The direction your fingers curl into a fist indicates the direction of the current.
Longitudinal waves
Waves that oscillate in the same direction as the propagation of the wave. Sound is carried by longitudinal waves, since the air molecules move back and forth in the same direction the sound travels.
Loudness
The square of the amplitude of a sound wave is called the sound’s loudness, or volume.
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