What happens to these balloons when you open the valve? | Veritasium

The Gist

When you open a valve between a large balloon and a small balloon, air flows from the small balloon into the large balloon, making the large balloon even bigger. This happens because a smaller balloon is actually at a higher internal pressure due to the tight, uncoiled polymer chains in the stretched rubber.

Quick Overview

Air flows from the smaller balloon into the larger balloon when a connecting valve is opened, making the big balloon grow even larger because smaller balloons possess higher internal pressure. Veritasium demonstrates this counterintuitive physics phenomenon to random people on the street before explaining the mechanics of rubber elasticity. The higher stiffness of a small balloon stems from tangled polymer chains that require more force to stretch initially, but they relax as inflation increases.

Key Points: A valve connects a large balloon and a small balloon filled with air. Multiple pedestrians guess that nothing will happen, that the sizes will swap, or that the larger balloon will deflate into the smaller one. When the valve opens, the big balloon gets visibly bigger while the small balloon shrinks entirely. The smaller balloon holds a higher internal pressure than the larger balloon. Polymer chains inside the rubber are tangled up tight when a balloon is small, making the material much stiffer. As the balloon inflates, those polymer chains uncoil and relax, which results in a lower internal pressure for the larger balloon. Blowing up a balloon is hardest at the very beginning and becomes noticeably easier as it expands.

Context: Veritasium host Derek Muller tests physical misconceptions on the street by asking pedestrians to predict counterintuitive mechanical outcomes. Fluid dynamics and material science often produce results that contradict everyday intuition, such as pressure gradients in elastic membranes.

Detailed Analysis

The video investigates the pressure dynamics of connected balloons of different sizes. Derek Muller approaches people on the street, asks them to predict what will happen when a valve connecting a large and a small balloon is opened, and records their incorrect hypotheses. When the valve is actually opened, the large balloon grows larger at the expense of the small balloon. Derek explains that this occurs because small balloons are at a higher internal pressure due to tightly tangled polymer chains in the rubber, which relax and uncoil as inflation increases, lowering the internal pressure of the larger balloon.