Null (physics)
Encyclopedia
In physics a null is a point in a field
where the field quantity is zero as the result of two or more opposing quantities completely cancelling each other. The field may be scalar
, vector
or tensor
in nature. Common situations where nulls arise are in the polar patterns of microphone
s and antennae
, and nulls caused by reflections of waves.
A common polar pattern for microphones is the cardioid
. This has a single direction in which the microphone does not respond to impinging sound waves. Highly directional (shotgun) microphones have more complex polar patterns. These microphones have a large, narrow lobe in the main direction of sound reception but also a smaller lobe in the opposite direction and usually also several other smaller lobes. This pattern is achieved by wave cancellation inside the body of the microphone. Between each of these lobes is a null direction where no sound at all is detected.
. This has a figure-of-eight polar pattern with two nulls on opposite sides. Highly directional antennae, such as the Yagi have polar patterns very similar to highly directional microphones and for similar reasons. That is, they have multiple small lobes off the main direction with nulls between them.
s can be caused by a wave being reflected back through the transmission medium in which it arrived. If the incident and reflected waves are transmitted without loss then there will be points along the transmission path where the incident and reflected waves exactly cancel each other due to them being in antiphase.
Standing waves are found in wind musical instruments. Wind instruments consist of a tube which acts as an acoustic transmission line in which standing waves are set up. Open ended tubes must have zero air pressure change at the end of the tube so this point is a pressure null. Closed tubes must have zero air velocity at the end of the tube so this point is a velocity null. There may further nulls along the tube depending on the vibration mode that has been set up by the instrument player. Higher modes have more nulls.
Standing waves also occur on electrical transmission line
s. A line with a non-resistive
termination will cause a reflection
of the signal from the termination with the same amplitude as the incident wave. These waves will cancel periodically along the line causing nulls every half wavelength
. The distance of the first null from the termination depends on the nature of the terminating impedance
. Nulls on transmission lines are very sharp, in contrast to the peaks which are broad and flat. This makes the nulls easier to measure. An instrument in electronics for measuring the positions of nulls on a line is the slotted line. This instrument can also be used to measure VSWR. With the measurement of VSWR and the null positions the value of the terminating impedance's magnitude and phase can be calculated.
used to accurately measure the resistance of an unknown component against a calibrated variable resistance by comparing the voltages across each. The central branch of the bridge has the voltage under test on one side and the calibrated voltage on the other: when they are equal the voltage across the central branch is nulled out. The measurement is carried out by placing an instrument to detect voltage or current in the central branch. An advantage of the bridge method is that this instrument does not need to be calibrated since it is only required detect nulls, not to actually measure a voltage. The accuracy of the measurement will, however, depend on the sensitivity of the instrument since this will affect the resolution of the bridge.
Field (physics)
In physics, a field is a physical quantity associated with each point of spacetime. A field can be classified as a scalar field, a vector field, a spinor field, or a tensor field according to whether the value of the field at each point is a scalar, a vector, a spinor or, more generally, a tensor,...
where the field quantity is zero as the result of two or more opposing quantities completely cancelling each other. The field may be scalar
Scalar field
In mathematics and physics, a scalar field associates a scalar value to every point in a space. The scalar may either be a mathematical number, or a physical quantity. Scalar fields are required to be coordinate-independent, meaning that any two observers using the same units will agree on the...
, vector
Vector field
In vector calculus, a vector field is an assignmentof a vector to each point in a subset of Euclidean space. A vector field in the plane for instance can be visualized as an arrow, with a given magnitude and direction, attached to each point in the plane...
or tensor
Tensor field
In mathematics, physics and engineering, a tensor field assigns a tensor to each point of a mathematical space . Tensor fields are used in differential geometry, algebraic geometry, general relativity, in the analysis of stress and strain in materials, and in numerous applications in the physical...
in nature. Common situations where nulls arise are in the polar patterns of microphone
Microphone
A microphone is an acoustic-to-electric transducer or sensor that converts sound into an electrical signal. In 1877, Emile Berliner invented the first microphone used as a telephone voice transmitter...
s and antennae
Antenna (radio)
An antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver...
, and nulls caused by reflections of waves.
Microphones
Cardioid
A cardioid is a plane curve traced by a point on the perimeter of a circle that is rolling around a fixed circle of the same radius. It is therefore a type of limaçon and can also be defined as an epicycloid having a single cusp...
. This has a single direction in which the microphone does not respond to impinging sound waves. Highly directional (shotgun) microphones have more complex polar patterns. These microphones have a large, narrow lobe in the main direction of sound reception but also a smaller lobe in the opposite direction and usually also several other smaller lobes. This pattern is achieved by wave cancellation inside the body of the microphone. Between each of these lobes is a null direction where no sound at all is detected.
Antennae
A common, and basic, radio antenna is the dipoleDipole antenna
A dipole antenna is a radio antenna that can be made of a simple wire, with a center-fed driven element. It consists of two metal conductors of rod or wire, oriented parallel and collinear with each other , with a small space between them. The radio frequency voltage is applied to the antenna at...
. This has a figure-of-eight polar pattern with two nulls on opposite sides. Highly directional antennae, such as the Yagi have polar patterns very similar to highly directional microphones and for similar reasons. That is, they have multiple small lobes off the main direction with nulls between them.
Standing waves
Standing waveStanding wave
In physics, a standing wave – also known as a stationary wave – is a wave that remains in a constant position.This phenomenon can occur because the medium is moving in the opposite direction to the wave, or it can arise in a stationary medium as a result of interference between two waves traveling...
s can be caused by a wave being reflected back through the transmission medium in which it arrived. If the incident and reflected waves are transmitted without loss then there will be points along the transmission path where the incident and reflected waves exactly cancel each other due to them being in antiphase.
Standing waves are found in wind musical instruments. Wind instruments consist of a tube which acts as an acoustic transmission line in which standing waves are set up. Open ended tubes must have zero air pressure change at the end of the tube so this point is a pressure null. Closed tubes must have zero air velocity at the end of the tube so this point is a velocity null. There may further nulls along the tube depending on the vibration mode that has been set up by the instrument player. Higher modes have more nulls.
Standing waves also occur on electrical transmission line
Transmission line
In communications and electronic engineering, a transmission line is a specialized cable designed to carry alternating current of radio frequency, that is, currents with a frequency high enough that its wave nature must be taken into account...
s. A line with a non-resistive
Electrical resistance
The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...
termination will cause a reflection
Reflections of signals on conducting lines
A signal travelling along an electrical transmission line will be partly, or wholly, reflected back in the opposite direction when the travelling signal encounters a discontinuity in the transmission parameters of the line, or at the far end of the line if the line is not correctly terminated in...
of the signal from the termination with the same amplitude as the incident wave. These waves will cancel periodically along the line causing nulls every half wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
. The distance of the first null from the termination depends on the nature of the terminating impedance
Electrical impedance
Electrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...
. Nulls on transmission lines are very sharp, in contrast to the peaks which are broad and flat. This makes the nulls easier to measure. An instrument in electronics for measuring the positions of nulls on a line is the slotted line. This instrument can also be used to measure VSWR. With the measurement of VSWR and the null positions the value of the terminating impedance's magnitude and phase can be calculated.
Measuring bridges
Nulls are used in electrical science to make many measurements. A frequently encountered technique is to adjust the voltage in one branch of a circuit until it nulls out a voltage in another branch. Commonly, a bridge circuit is used for this purpose. There are many varieties of measurement bridge, the most well known of these being the Wheatstone bridgeWheatstone bridge
A Wheatstone bridge is an electrical circuit used to measure an unknown electrical resistance by balancing two legs of a bridge circuit, one leg of which includes the unknown component. Its operation is similar to the original potentiometer. It was invented by Samuel Hunter Christie in 1833 and...
used to accurately measure the resistance of an unknown component against a calibrated variable resistance by comparing the voltages across each. The central branch of the bridge has the voltage under test on one side and the calibrated voltage on the other: when they are equal the voltage across the central branch is nulled out. The measurement is carried out by placing an instrument to detect voltage or current in the central branch. An advantage of the bridge method is that this instrument does not need to be calibrated since it is only required detect nulls, not to actually measure a voltage. The accuracy of the measurement will, however, depend on the sensitivity of the instrument since this will affect the resolution of the bridge.