### Magnetism: quantities, units and relationships

18) revision & exam advice plus tutorials i.e., +5 mag difference is equivalent to x fainter. – (mag Magnitudes, like flux, depend on distance, i.e. The radiation from the area you observe is spread over a sphere with the radius R. So you calculate the flux at a distance of R. This will eliminate the distance. Magnitude System(s). Relationship to other photometric calibrations: Photometry - Advice. Reliability: For total flux: Petrosian or optimal model magnitudes.

However, in order to properly extend the magnitude scale further into the infrared, this peculiarity of Vega should not affect the definition of the magnitude scale. On this basis the spectral irradiance usually expressed in janskys for the zero magnitude point, as a function of wavelength, can be computed.

With the modern magnitude systems, brightness over a very wide range is specified according to the logarithmic definition detailed below, using this zero reference. In practice such apparent magnitudes do not exceed 30 for detectable measurements.

The brightness of Vega is exceeded by four stars in the night sky at visible wavelengths and more at infrared wavelengths as well as the bright planets Venus, Mars, and Jupiter, and these must be described by negative magnitudes. Negative magnitudes for other very bright astronomical objects can be found in the table below. Astronomers have developed other photometric zeropoint systems as alternatives to the Vega system.

The most widely used is the AB magnitude system, [6] in which photometric zeropoints are based on a hypothetical reference spectrum having constant flux per unit frequency intervalrather than using a stellar spectrum or blackbody curve as the reference.

The AB magnitude zeropoint is defined such that an object's AB and Vega-based magnitudes will be approximately equal in the V filter band. This nebula has an apparent magnitude of 8. Normally, if you put several volts across any randomly arranged bit of wire then what will happen will be a flash and a bang; the current will follow Ohm's Law and unless the wire is very long and thin there won't be enough resistance to prevent fireworks.

### Apparent magnitude - Wikipedia

It's a different story when the wire is wound into a coil. If the current increases then we get flux build up which induces a voltage of its own. The sign of this induced voltage is always such that the voltage will be positive if the current into the coil increases. We say that the induced voltage will oppose the externally applied voltage which made the current change Lenz's law. This creates a limit to the rate of rise of the current and prevents at least temporarily the melt-down we get without coiling.

In this situation it is helpful to build a mental picture by imagining that, rather than having an idealised inductor Figure FSA ayou replace it with a model which includes a very small series resistance - as all practical inductors must have Figure FSA b.

## Magnetism: quantities, units and relationships

It should now be clear that the value of vL must be only the tiniest bit less than vG. In the standard example toroid core assume you have no flux to start with. After applying twenty volts for three microseconds figure FYV.