To solve this problem we will consider the definition of Minimum Energy, as the change between the minimum voltage and its distance. Mathematically it can be written as

Rearranging to find the Voltage,

Replacing,


Therefore the minimum voltage needed between your finger and the doorknob to generate this spark is 28.5kV
Your grade will probably go down to a D 68% or little higher than that
No the only thing that affects it is how it is built
A type of material that tries to prevent the flow of thermal energy is called an insulator.