Amount-like size

A quantity-like quantity is a physical quantity for which one can specify:

• a density (how much of the size is in a given volume?) and
• a stream (at what rate does size penetrate a system boundary?).

The quantity-like size can thus be illustrated as a substance in the broadest sense that is contained in a system and can be exchanged between systems.

Examples of quantity-like quantities are mass , amount of substance , electrical charge , energy , etc. If one also allows vector quantity-like quantities, then u belong. a. also the impulse to do so.
As part of the Karlsruhe physics course , entropy is also counted among the quantity-like quantities, although it is not a conserved quantity .

Connection with extensive quantities

Set-like quantities are extensive and most of them have a conservation law and a continuity equation .

But not all extensive quantities are set. Thus, the volume in the narrower sense is not set , although it is an extensive quantity: it makes no sense to specify a "density" for the volume (this would in any case be 1). However, if the volume is taken as a measure of a fluid , then the volume can also be understood as a quantity and a volume flow rate can be defined (e.g. 3 liters of water per second), although the volume itself, without being bound to matter, cannot can flow.

Relation to specific quantities

If one divides a quantity-like quantity by a quantity that quantifies the system quantity, one arrives at the corresponding intensive quantity :

• a density relates the quantity-like quantity to the volume
• a specific quantity relates the quantity-like quantity to the mass
• a molar quantity relates the quantity-like quantity to the amount of substance in the system.

Mathematical description

If it is indicated by how much of the quantity has penetrated an oriented surface from the point in time to the point in time , then is ${\displaystyle X(t)}$ ${\displaystyle t_{0}=0}$ ${\displaystyle t}$

${\displaystyle I_{X}(t)={\frac {\mathrm {d} X(t)}{\mathrm {d} t}}}$

the current strength of the quantity-like quantity . (If is a vector quantity , then is also a vector). ${\displaystyle X}$ ${\displaystyle X}$ ${\displaystyle I_{X}}$

A positive sign of means either that a positive amount is moving in the direction of the surface orientation or a negative amount is moving in the opposite direction. A well-known example is the technical direction of current : there, a positive current strength in the counting direction means that negatively charged electrons may move in the opposite direction. ${\displaystyle I_{X}}$

Individual evidence

1. ↑ F. Herrmann: What is a quantity-like quantity? In: Practice of Natural Sciences - Physics in School. 2006, 1/55, p. 44ff, (pdf ( memento of the original from March 3, 2017 in the Internet Archive ) Info: The archive link has been inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice . , accessed on March 3, 2017) 
2. ↑ ^{a b} R. Mayer: Physical quantities in general, momentum and energy in particular. Books on Demand, Norderstedt 2010. (google book search , accessed on March 3, 2017)