H
Half-life
The half-life of a radioactive material is the time taken for its activity level to drop by a half. Half-life is measured using any unit of time (seconds, hours).
Heat
Heat is a form of energy, which makes a body hot or cold. Heat is measured by the temperature-effect it produces in any material body. The SI unit of heat is Joule( J).
Heat Energy
Heat is a form of energy and is not to be confused with temperature. There is no such thing as 'cold energy'. At the molecular level, heat energy is transferred from particle to particle by vibration (this is called conduction). A particle vibrating violently can be said to have a lot of heat energy. As well as by conduction, heat can also be transferred by convection or radiation.
Heater
The operation of a bar heater is very similar indeed to a filament lamp. The difference is in the thickness of the 'filament' or element of the heater. Curiously, heaters operate at a lower temperature than lamps, but produce far more heat - as infra-red radiation.
Heat Pump
This is a device which can move heat energy from a colder object to a hotter one, but requiring an external energy source to do it. A fridge is an example of a heat pump. Due to the second law of thermodynamics, a heat pump can never be 100% efficient.
Heat death of the Universe
Time at which no work will be able to be done because all heat energy in the Universe will be evenly distributed (so that no further increase in entropy is possible).
Heisenberg uncertainty principle
You cannot measure both the exact momentum and the exact position of a subatomic particle at the same time-when the more exact of the two is known, the less certain you are of the value of the other.
Hertz
Unit of frequency; equivalent to one cycle per second.
Horsepower
Measurement of power defined as a power rating of 550 ft-lb/sec.
Hypothesis
A tentative explanation of a phenomenon that is compatible with the data and provides a framework for understanding and describing that phenomenon.
I
Ice-point
It is the melting point of pure melting ice under 1 Atmospheric pressure. The ice point is taken as the lower fixed point ( 0 deg C or 32 deg F ) for temperature scales.
Image
A place where an object appears to be, because the rays diffusely reflected from any given point on the object have been bent so that they come back together and then spread out again from the image point, or spread apart as if they had originated from the image.
Impulse
The impulse acting on a body is equal to the product of the force acting on the body and the time for which it acts. If the force is variable, the impulse is the integral of Fdt from t0 to t1. The impulse of a force acting for a given time interval is equal to change in momentum produced over that interval. J=m(v-u), assuming that the mass m remains constant while the velocity changes from v to u. The SI units of impulse are kg m/s.
Impulsive force
The force which acts on a body for a very short time but produces a large change in the momentum of the body is called an impulsive force.
Incandescent
Matter emitting visible light as a result of high temperature for example, a light bulb, a flame from any burning source, and the sun are all incandescent sources because of high temperature.
Incident ray
Line representing the direction of motion of incoming light approaching a boundary.
Independence
The lack of any relationship between two random events.
Index of refraction
The ratio of the speed of light in a vacuum to the speed of light in a material.
Induction
The production of an electric field by a changing magnetic field, or vice-versa.
Inert
A chemical element is inert if it does not react with anything. Examples are helium, neon, argon, krypton, xenon and radon. Inert gases can be used in filament lamps to protect the filament from destruction due corrosion.
Inertia
The property of matter that causes it to resist any change in its state of rest or of uniform motion. There are three kinds of inertia- inertia of rest, inertia of motion and inertia of direction. The mass of a body is a measure of its inertia.The SI unit for inertia is the kilogram.
Inertial frame
A frame of reference that is not accelerating, one in which Newton's first law is true.
Infrasonic
Sound waves having too low a frequency to be heard by the human ear; sound having a frequency of less than 20 Hz.
Infrared
Infrared radiation is invisible. It is a form of electromagnetic radiation, with a wavelength just longer than that of visible red light. Infrared light is use in telecommunications (via optical fibres). It is used in medicine to diagnose illness (thermograms) and accelerate healing in physiotherapy. Infrared radiation can be thought of as 'heat rays'.
Input Device
The first part of an electronic system, an input device typically processes requests from a human user. For example in a CD player, the 'ON' button and volume controls (potentiometer) are all input devices. Other input devices include switches, thermistors, LDRs, photodiodes.
Instantaneous
This means an event which occurs over a very short (infinitesimal) period of time. It usually refers to speed, i.e. instantaneous speed as opposed to average speed.
Instantaneous Speed
The speed at any particular stage or time during a journey. Instantaneous speed can be calculated by measuring short distances and time intervals.
Insulator
A material which does not usually conduct electricity. Air, for example is an insulator. Insulators have a very high (ideally infinite) resistance. Given a suitably high voltage, insulators can be forced to conduct ,for example lightning during a thunderstorm.
Insulation
A nonconducting material designed to reduce the transfer of energy or electric current between two materials.
Intensity
A measure of the energy carried by a wave.
Interference
Phenomenon of light where the relative phase difference between two light waves produces light or dark spots, a result of light's wavelike nature.
Intermolecular forces
Forces of interaction between molecules.
Internal energy
Sum of all the potential energy and all the kinetic energy of all the molecules of an object.
Invariant
A quantity that does not change when transformed.
Inverse proportion
The relationship in which the value of one variable increases while the value of the second variable decreases at the same rate (in the same ratio).
Inverter (NOT gate)
A processing device within an electronic system, the NOT gate is a single input logic gate which 'inverts' its input. A logic 'HI' input will become a logic 'LOW' output and visa-versa. Here is the truth table:
A Z
0 1
1 0
Ion
An electrically charged atom or molecule.
Ionization
Process of forming ions from molecules.
Ionized
An atom or a particle that has a net charge because it has gained or lost electrons.
Isostasy
A balance or equilibrium between adjacent blocks of crust.
Isotope
One of the possible varieties of atoms of a given element, having a certain number of neutrons.