Coulomb's law and the Biot–Savart law, have no factor of 4π attached to r in the denominator. (The quantity 4π appears because 4πr is the surface area of the sphere of radius r, which reflects the geometry of the configuration. For details, see the articles Relation between Gauss's law and Coulomb's law and Inverse-square law.) Unit of charge A major difference between the Gaussian system and the ISQ is in the respective definitions of the quantity charge. In the ISQ, a separate base dimension, electric current, with the associated SI unit, the ampere, is associated with electromagnetic phenomena, with the consequence that a unit of electrical charge (1 coulomb = 1 ampere × 1 second) is a physical quantity that cannot be expressed purely in terms of the mechanical units (kilogram, metre, second). On the other hand, in the Gaussian system, the unit of electric charge (the statcoulomb, statC) can be written entirely as a dimensional combination of the non-electrical base units (gram, centimetre, second), as: = . For example, Coulomb's law in Gaussian units has no constant: where F is the repulsive force between two electrical charges, Q and Q are the two charges in question, and r is the distance separating them. If Q and Q are expressed in statC and r in cm, then the unit of F that is coherent with these units is the dyne. The same law in the ISQ is: where ε0 is the vacuum permittivity, a quantity that is not dimensionless: it has dimension (charge)2 (time)2 (mass)−1 (length)−3. Without ε0, the equation would be dimensionally inconsistent with the quantities as defined in the ISQ, whereas the quantity ε0 does not appear in Gaussian equations. This is an example of how some dimensional physical constants can be eliminated from the expressions of physical law by the choice of definition of quantities. In the ISQ, 1/ε0 converts or scales flux density, D, to the corresponding electric field, E (the latter has dimension of force per charge), while in the Gaussian system, electric flux density is the same quantity as electric field strength in free space aside from a dimensionless constant factor. In the Gaussian system, the speed of light c appears directly in electromagnetic formulas like Maxwell's equations (see below), whereas in the ISQ it appears via the product . Units for magnetism In the Gaussian system, unlike the ISQ, the electric field E