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Lab 3: The Motion of Microscopic Objects
The competition between Brownian motion and directed forces:
How large a force is needed to transition from random to directed motion?
Brownian motion is ubiquitous in living systems at a molecular and cellular level and is distinct from the types of motion in students’ everyday experiences. Machines on microscopic scales, such as molecular motors or ion pumps in living systems, constantly compete against and take advantage of the thermal forces that create random Brownian motion. In this two-week lab, students use beads of different sizes suspended in solution to explore the crossover from random to directed motion as an external force is applied. By tilting the microscope and thus inclining the slide, students can gather and analyze videos to determine the motion of the different sized beads under a condition where the gravitational force is affecting bead motion in addition to thermal forces. Students can directly observe the crossover from random motion where thermal forces dominate to directed motion where gravity dominates and analyze how it depends on the size of the bead. Due to differences in how displacements scale with time, even small directed forces lead to directed motion on long enough timescales; thus students also examine the effect of the time frame over which the bead is followed.
Pre-requisite readings:
Week 1:
Observing Brownian motion and directed motion in microscopic systems
Files for Students:
File for TAs:
Media files:
Week 2:
When are forces strong enough for directed motion to overcome Brownian motion?
File for TAs:
Kim Moore, Spring 2014
Matt Severson, Fall 2018
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