# Day 159: The Air Capacitor

In Honors Physics, we have finished elaborating the current model, identified its limitation with the pizza problem, and have started exploring the voltage model. We are now using the CASTLE materials including the modifications for Modeling Instruction. Presenting voltage as the difference in electric pressure resonates well with students. The concept is reinforced by drawing an analogy to air pressure. I introduce this analogy with the air capacitor:

The air capacitor is constructed from two peanut butter jars with rubber stoppers and fired glass tubes in each end. A balloon is stretched around one of the jars and then the jars are joined with packing tape such that the balloon acts as a membrane between the two halves of the air capacitor. The balloon serves as a visual indication of the pressure difference between the two halves of the apparatus. This apparatus reinforces a couple key aspects of the analogy:

• air moves from high pressure to normal pressure (blow into one half)
• air moves from normal pressure to low pressure (suck out of one half)
• in general, air moves when there is a difference in pressure
• pressurized air is stored in the capacitor when it cannot flow elsewhere (plug the ends of the air capacitor)
• the air that leaves one half of the air capacitor is not the same air that enters the other half; the air that leaves was already in the air capacitor

##circuits ##equipment