# In a circuit with a switch and a pull-down resistor, why does the high impedance of the input not discourage current from flowing into pin?

I'm a beginner to Arduino and circuits. I've spent the last couple hours hung up on the concept of how current flows through switches and pull-down resistors, having read every thread and watched every YouTube video I could find on the subject.

I understand that pull-down (and pull-up) resistors are necessary in order to prevent pins from floating. What confuses me is that, if the input pins effectively have a built-in 100 megohm resistance (according to this page), then how is it that a 10k ohm pull-down resistor is enough to dissuade current from flowing directly into ground even when the switch is closed?

Here is a diagram of my circuit:

Am I conflating the high-impedance state of input pins with resistance more generally?

Would really appreciate any clarification on this. Thanks!

• Huh? 5V across 10kohm is 500uA. Oct 4, 2017 at 1:37
• @IgnacioVazquez-Abrams Sorry, I'm not totally sure what you're getting at. Oct 4, 2017 at 2:10
• I'm not sure what your question means. Of course some current flows from the positive rail to ground when the button is closed. How much? Well... Oct 4, 2017 at 2:11
• @IgnacioVazquez-Abrams Ah, I'm wondering why all the current doesn't proceed across the resistor to ground when the switch is closed, given that there's a very high built-in resistance right before the pin. If I'm missing something really obvious, I apologize. You may be overestimating my knowledge here. Oct 4, 2017 at 2:14
• Ohm's Law Oct 4, 2017 at 2:15