Second law of Thermodynamics A gas expands to fill the available volume. A hot body cools to the temperature of its surroundings. A chemical reaction runs in one direction rather than another. The direction of change that does not require work to be done to bring the change about is called spontaneous direction of change. Internal energy lets us access whether a change is permissible. Only those changes occurs for which the internal energy of an isolated system remains constant.

Is it perhaps the internal energy that tends toward minimum for a spontaneous process? System Surroundings dU<0 dU>0 Perfect gas expands spontaneously into vaccum. dU=0

No process is possible in which the sole result is the absorption of heat from a reservoir and its complete conversion into work.

A ball bouncing on the floor A ball resting on the warm surface.

Entropy is measure of randomness/ chaosness. Total entropy of the system and its surroundings increases in the course of a spontaneous change.

My Hostel Room Blame Entropy, not Me

Thermodynamic definition of entropy.

Clausius inequality theorm

Entropy changes in a Reversible process

Heating or Cooling process

Isothermal process for an ideal gas

Adiabatic Processes for an ideal gas

Order/disorder number of ways in which a system can be configured. What is the probability of specific sequence of four coin flips (HTHH)? (1/16) What is the probability of observing 3 H and 1 T in any order ? (composition) (4/16) No. of different permutation for a sequence of N distinguishable objects (N !) For a collection of N objects with two categories. N!/n!(N-n)!

Maximizing multiplicity predict the most probable outcome. Tossing of 4 coins (4 heads and 0 tails) (composition) nWlnW A simple maximization principle predicts the most probable composition: 2 heads and 2 tails.

While the random process of flipping coins can result in any composition of heads and tails, the composition has strong tendency toward 50 % H and 50 % T.

Why do Particles spread out into largest possible volume ? Multiplicity increases as volume increases. Particles spread out into largest possible volume to maximize the multiplicity of the system. This is the force called pressure.

Relation between multiplicity and entropy

Why does energy Exchange ?

UA=3 UB=3 UA=5 UB=1

Temperature decides the tendency for energy exchange

Pressure is the force for Changing volume.