E
Echo
A reflected sound that can be distinguished from the original sound, which usually arrives 0.1 sec or more after the original sound.
Efficiency
Efficiency is defined as 'Useful Energy Out' / 'Total Energy In'. It is a fraction, sometimes written as a percentage.
Elastic strain
An adjustment to stress in which materials recover their original shape after a stress is released.
Electric circuit
Consists of a voltage source that maintains an electrical potential, a continuous conducting path for a current to follow, and a device where work is done by the electrical potential; a switch in the circuit is used to complete or interrupt the conducting path.
Electric current
Electric current is the flow (movement) of electric charge. The SI unit of electric current is the Ampere (A), which is equal to a flow of one Coulomb of charge per second. Electric current is measured using an ammeter.
Electric dipole
An object that has an imbalance between positive charge on one side and negative charge on the other; an object that will experience a torque in an electric field.
Electric field lines
A map of an electric field representing the direction of the force that a test charge would experience; the direction of an electric field shown by lines of force.
Electric generator
A mechanical device that uses wire loops rotating in a magnetic field to produce electromagnetic induction in order to generate electricity.
Electric potential energy
Potential energy due to the position of a charge near other charges.
Electrical conductors
Materials that have electrons that are free to move throughout the material; for example, metals.
Electrical energy
A form of energy from electromagnetic interactions; one of five forms of energy-mechanical, chemical, radiant, electrical, and nuclear.
Electrical force
A fundamental force that results from the interaction of electrical charge and is billions and billions of times stronger than the gravitational force.
Electrical insulators
Electrical nonconductors, or materials that obstruct the flow of electric current.
Electrical nonconductors
Materials that have electrons that are not moved easily within the material-for example, rubber; electrical nonconductors are also called electrical insulators.
Electrical resistance
The property of opposing or reducing electric current.
Electrolyte
Water solution of ionic substances that conducts an electric current.
Electromagnet
An electromagnet is a solenoid with an iron core inserted into it. If a current flows in the coil, a magnetic field is generated. All the randomly oriented domains of the iron core then align in the presence of the field of the solenoid. Thus, the core greatly enhances the strength of the electromagnet.
Electromagnetic induction
Process in which current is induced by moving a loop of wire in a magnetic field or by changing the magnetic field.
Electromagnetic Spectrum
This includes Gamma, X-rays, Ultraviolet, Visible, Infra-red, Microwave and Radio - in order of increasing wavelength.
Electromagnetic waves
The waves which are due to oscillating electrical and magnetic fields and do not need any material medium for their propagation are called electromagnetic waves. These waves can, however, travel through material medium also. Light waves, radio waves are examples of electromagnetic waves. All electromagnetic waves travel in vacuum with a speed of 3×10 8 m/s.
Electron configuration
The arrangement of electrons in orbits and sub-orbits about the nucleus of an atom.
Electron current
Opposite to conventional current; that is, considers electric current to consist of a drift of negative charges that flows from the negative terminal to the positive terminal of a battery.
Electron pair
A pair of electrons with different spin quantum numbers that may occupy an orbital.
Electron volt
The energy gained by an electron moving across a potential difference of one volt; equivalent to 1.60 x 10^-19 Joules.
Electron
Electrons are sub-atomic particles. They carry a negative electrical charge (qe=-1.602 x 10-19 C).
They have a mass of me = 9.110 x 10-31 kg.
Beams of electrons are used in TV sets where they are called cathode rays.
Electro negativity.
The comparative ability of atoms of an element to attract bonding electrons.
Electronic System
An electronic system can simply be considered as consisting of three parts: input, process and output. Input devices include microphones and LDRs, thermistors and switches. Process sections can be made from one or more transistors, perhaps built into logic gates or computer chips. The output section will consist of one or more output devices, for example, a buzzer, LED, lamp, CRT.
Electrostatic charge
An accumulated electric charge on an object from a surplus or deficiency of electrons.
Element
A pure chemical substance that cannot be broken down into anything simpler by chemical or physical means.
Energy
The capacity of a body to do work is called its energy. Energy is a scalar quantity. The SI unit of energy is Joule.Energy can not be created or destroyed, but can be changed from one form to another.
Escape Velocity
The minimum velocity with which an object must be thrown upwards so as to overcome the gravitational pull and escape into space, is called escape velocity (V esc ). The escape velocity depends upon the mass and radius of the planet/star. It does not depend upon the mass of the body thrown up.
Equilibrium
A state in which an object's momentum and angular momentum are constant.
Evaporation
Process of more molecules leaving a liquid for the gaseous state than returning from the gas to the liquid. It can occur at any given temperature from the surface of a liquid. Evaporation takes place only from the surface of the liquid. Evaporation causes cooling. Evaporation is faster if the surface of the liquid is large, the temperature is higher and the surrounding atmosphere does not contain a large amount of vapor of the liquid.
Eyepiece Lens
The lens on a telescope (or microscope) which the observer places his or her eye next to. The eyepiece's sole funciton is to magnify the image.