An investigation is designed to determine the size of the generated current when the strength of a magnet is varied.
Which is the independent variable for this investigation
a) Speed of the Magnet
b) Strength of the magnet *
c) Size of the generated current
d) Distance between the coil and the magnet
In an experiment, the independent variable is the quantity that is deliberately changed, in a predetermined fashion, in order to measure the changes in another (or several other) quantities of interest. These quantities are referred to as the dependent variables.
In order to draw correct conclusions, it is almost always vital to maintain the constancy of every other aspect of the experiment fixed ‘variables’ – otherwise it is impossible to separate the results of changes in the independent variable from changes in other quantities.
For example, in the investigation described in the question, one may design the experiment with the magnet moving past the coil at a fixed speed. In this case, we can change the strength of the magnet (by swapping magnets of different strength in and out of the apparatus) and watch how the current induced in the coil varies depending on the magnet. We need to be careful to keep the speed of the magnet constant, as changes in the speed a magnet moves will affect the current. Similarly, we need to keep the trajectory of the magnet fixed – a magnet that passes closer to the coil will induce a different current to a magnet that passes further away, even if the magnets are of identical strength.