Question

how to calculate the magnitude of current

Answer 1

Current, measured in Amperes (A), has the symbol "I" ('eye') and is equal to the charge (Coulombs, C) per unit time (Seconds, s). In other words, it is the number of electrons passing a point per second. 

I = Q/t 

Where Q is charge and t is time

Current is also equal to the potential difference (voltage, V) across a component over the resistance (ohms) of the component

I = V/R