|MadSci Network: Physics|
Hello Pankaz, The ampere is one of the 7 SI base units, along with metre, kilogram, second, kelvin, candela and mole. The ampere is the current, which if flowing in two infinitely long, straight, parallel wires, one metre apart in a vacuum, produces a force between the wires of 2 x 10^7 newtons per metre length. The coulomb is then the charge transported per second by a current of one ampere. To be more precise
S I DEFINITION OF THE COULOMB There is nothing cyclic about 1C = 1As. That is a definition. 1A = 1C/s is a consequence of that definition but it is not the definition of the ampere. S I DEFINING EQUATION FOR QUANTITY of CHARGE There is nothing cyclic about Q = integral I dt. I = dQ/dt is a consequence of that definition but is not the definition of I. CONCEPT OF CHARGE This is another matter. To me the concept of charge is more fundamental than that of current. The concept of charge has to be built up by experience. In summary it is that property of matter that gives rise to the forces we call electric forces. I regard current as rate of flow of charge. The problem is that it is easier to measure currents than charges. Hence SI makes current a fundamental quantity starts with a definition of I, the intensity of a current, and the ampere, the unit of measure of the intensity. DEFINITION OF INTENSITY OF CURRENT AND THE AMPERE This is complicated; which is why the average text book keeps it dark. The general equation for the force between two current elements in a vacuum is: dF = (mu-vacuum) I1 I2 [ ds1 x ( ds2 x r ) / 4 (pi) r^2] where ds1 = vector length of current element number one and r = displacement between elements. Putting (mu-vacuum) = 4 (pi) x 10^-7 newtons/ ampere^2 defines the ampere. The schoolboy definition uses a calculation based on the above for the force/length, (that is F/L) between two infinitely long thin straight parallel wires, a distance r apart, in a vacuum, carrying identical currents. The schoolboy defining equation is I = [F r / L ( 2 x 10^-7)(newtons/ampere^2) ]. This makes one ampere the current which when passing through each of two long, straight, thin, parallel wires, one metre apart in a vacuum, gives rise to an attractive force between them of 2 x 10^-7 N per m of their length. ALTERNATIVE SYSTEM (not SI) However one can define Q, the Quantity charge using a similar thought experiment to that used to define the I, the Intensity of a current. In such a system Q is defined by the magnetic force between moving point charges F = (mu-vacuum) Q1 Q2 v1 x (v2 x r^ )/ 4 ƒÎ r^2 [v1 x (v2 x r^)is a vector product and r^ is a unit vector from one charge to the other.] With identical charges Q = [ 4 (pi) r^2 F / (mu-vacuum) Q1 Q2 v1 x (v2 x r^ )]^0.5 And the I = dQ/dt Now of course, if you are asking - what is the nature of charge? - that's a completely different question and one for which the answer would be a candidate for a Nobel Prize! Hope this helps Keith
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