How to enter Cartesian equation

I am trying to get an idea of how to enter a simple Cartesian equation:

For example, I want to plot a circle using A²+B²=4

If I populate a range in column A how do I get column B values. I tried =sqrt(A1-4) but got an error.

I know this is basic algebra but I am at a loss as to how to enter a formula like the one in question.