14. Entropic Forces

Question 1

What is a macrostate?

Answer 1

A definition of the system and its contents, including all of the possible configurations of the variables.

Question 2

What is a microstate?

Answer 2

One particular state and the values of each variable within this state. It is one of the many configurations that make up a macrostate.

Question 3

Describe the macrostate and the microstate for water molecules in a box

Answer 3

Macrostate: the size of the box and the number of particles.
Microstate: the position and velocity of all the particles.

Question 4

Give the equation for entropy

Answer 4

S = entropy
kB = Boltzmann’s constant
W = number of available microstates corresponding to a given macrostate

Question 5

How are entropy and the number of available microstates related?

Answer 5

Entropy is defined by the number of available microstates.

Question 6

Give the equation for the free energy of a system

Answer 6

U = free energy
∆H = enthalpy
T = temperature
∆S = entropy

Question 7

Give the equation for the entropy change required to produce a force

Answer 7

F = force
T = temperature
∆S = entropy
D = displacement

Question 8

What are entropic forces?

Answer 8

Forces that configure a system into the microstate with the highest possible entropy.

Question 9

Give an example of an entropic force

Answer 9

When surfaces have long molecules attached to them, these molecules will push away other surfaces so that they have the largest number of configurations for movement. This pushing force is repulsive.