G
Gamma decay
A form of radioactivity where an excited atom releases a photon of gamma radiation, thereby returning to a lower energy state. The atomic structure itself does not change in the course of gamma radiation.
Gamma ray
An electromagnetic wave of very high frequency.
Gold foil experiment
An experiment by Ernest Rutherford that proved for the first time that atoms have nuclei.
Gravitational constant
The constant of proportionality in Newton’s Law of Gravitation. It reflects the proportion of the gravitational force and , the product of two particles’ masses divided by the square of the bodies’ separation. N · m2/kg2.
Gravitational Potential Energy
The energy associated with the configuration of bodies attracted to each other by the gravitational force. It is a measure of the amount of work necessary to get the two bodies from a chosen point of reference to their present position. This point of reference is usually chosen to be a point of infinite distance, giving the equation . Objects of mass m that are a height h above the surface of the earth have a gravitational potential energy of .
Ground state
In the Bohr model of the atom, the state in which an electron has the least energy and orbits closest to the nucleus.
H
Half-life
The amount of time it takes for one-half of a radioactive sample to decay.
Harmonic series
The series of standing waves supported by a string with both ends tied down. The first member of the series, called the fundamental, has two nodes at the ends and one anti-node in the middle. The higher harmonics are generated by placing an integral number of nodes at even intervals over the length of the string. The harmonic series is very important in music.
Heat
A transfer of thermal energy. We don’t speak about systems “having” heat, but about their “transferring” heat, much in the way that dynamical systems don’t “have” work, but rather “do” work.
Heat engine
A machine that operates by taking heat from a hot place, doing some work with that heat, and then exhausting the rest of the heat into a cool place. The internal combustion engine of a car is an example of a heat engine.
Heat transfer
A transfer of thermal energy from one system to another.
Hertz (Hz)
The units of frequency, defined as inverse-seconds (1 Hz = 1 s–1). “Hertz” can be used interchangeably with “cycles per second.”
Hooke’s Law
For an oscillating spring, the restoring force exerted by the spring is directly proportional to the displacement. That is, the more the spring is displaced, the stronger the force that will pull toward the equilibrium position. This law is expressed mathematically as F = –kx, where F is the restoring force and x is the displacement. The constant of proportionality, –k, is the spring constant.
Hypotenuse
The longest side of a right triangle, opposite to the right angle.
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 w
