U
Ultrasonic
Sound waves too high in frequency to be heard by the human ear; frequencies above 20,000Hz.
Ultraviolet
Ultraviolet (UV) radiation is invisible. It is a form of electromagnetic radiation, with a wavelength just shorter than that of visible violet light.
Unbalanced forces
When a number of forces act on a body and the resultant force is not zero, then the forces are said to be unbalanced.
Uniform Acceleration
When the velocity of a body increases by equal amounts in equal intervals of time it is said to have uniform acceleration.
Uniform circular motion
Circular motion in which the magnitude of the velocity vector remains constant.
Uniform Speed
When a body travels equal distances in equal intervals of time then it is said to have uniform speed.
Uniform Velocity
When a body travels along a straight line in particular direction and covers equal distances in equal intervals of time it is said to have uniform velocity.
Universal law of gravitation
Every object in the universe is attracted to every other object with a force directly proportional to the product of their masses and inversely proportional to the square of the distance between the centers of the two masses.
Unpolarized light
Light consisting of transverse waves vibrating in all conceivable random directions.
Unstable equilibrium
One in which any deviation of the object from its equilibrium position results in a force pushing it even farther away.
V
Van der Waals force
General term for weak attractive intermolecular forces.
Vapor
The gaseous state of a substance that is normally in the liquid state.
Variable Resistor
Variable resistors are resistors whose resistance can be altered, usually by means of a rotating or sliding contact. They can be used as potential dividers or rheostats.
Vector Quantity
A quantity, which needs both magnitude and direction to describe it, is called a vector quantity. Such a physical quantity should also follow the vector law of addition.
Velocity
Distance traveled by a body in a particular direction per unit time is called its velocity. It can also be defined as the displacement of the body per unit time. It is a vector quantity. The SI units of velocity are m / s.
Vibration
A back and forth motion that repeats itself.
Virtual image
An image where light rays appear to originate from a mirror or lens; this image cannot be projected on a screen.
Voltage drop
The electric potential difference across a resistor or other part of a circuit that consumes power.
Volt
The volt is the SI unit of voltage. In fact 1 V = 1 JC-1 (i.e. a charge of one coulomb will gain or lose one joule of potential energy when moved through a potential difference of one volt).
Voltmeter
A device for measuring voltage differences.
Voltage
Electrical potential energy per unit charge that will be possessed by a charged particle at a certain point in space.
Voltage Divider
A simple circuit branch consisting of two resistors connected in series. The voltage across the terminals of the branch is shared between the two resistors, with the bigger resistor getting the biggest share.
Voltage Gain
Voltage gain has no units of its own. It expresses how many times greater the output signal voltage is (from some component) compared to the input voltage. A closely related concept is that of power gain. Voltage Gain = Output Voltage / Input Voltage.
Vector
A quantity that has both an amount (magnitude) and a direction in space.