Physics for Engineers | Lesson 10.4 | 2nd Law of Thermodynamics

A brief recap of the first law of thermodynamics, which relates internal energy change to heat absorbed/released and work done. The four thermodynamic processes—adiabatic (no heat flow), isochoric (constant volume), isothermal (constant temperature), and isobaric (constant pressure)—are also reviewed.

The second law of thermodynamics deals with natural, irreversible processes where heat flows spontaneously from high to low temperatures. Reversible processes, which are not natural, can proceed in two directions. The concept of entropy, defined as the disorder or randomness of a system, is introduced as a key component of the second law.

The three main statements of the second law are discussed: 1) The engine statement: It's impossible for any heat engine to convert heat completely into mechanical work. 2) The refrigerator statement: It's impossible to transfer heat from a cooler to a hotter region without doing work. 3) The entropy statement: It's impossible for any system to have decreasing entropy; natural processes always tend towards disorder.

A heat engine transforms heat partially into mechanical energy. The human body and automobile engines are examples. A heat engine absorbs heat from a hotter region, does work, and releases some heat to a colder region. Its thermal efficiency, a measure of how well it converts heat to work, is the ratio of work output to heat input. The second law dictates that no heat engine can achieve 100% efficiency.

Gasoline engines operate on the Otto cycle, which involves adiabatic compression, isochoric heat absorption, adiabatic expansion, and isochoric heat release. Their theoretical efficiency is about 56%, but real engines achieve around 35%. Diesel engines, which follow a diesel cycle (replacing isochoric heat absorption with an isobaric process), are more efficient, with theoretical efficiencies up to 70%.

A refrigerator is a heat engine operating in reverse, where work is done to move heat from a cooler to a hotter region. Refrigerators (and air conditioners) absorb heat from inside and release it outside. Their efficiency is measured by the coefficient of performance (COP), which is the ratio of heat absorbed from the cold region to the work input.

The Carnot cycle is an idealized, hypothetical cycle consisting of isothermal expansion, adiabatic expansion, isothermal compression, and adiabatic compression. A Carnot engine has the maximum possible efficiency, which depends only on the temperatures of the hot and cold regions. A Carnot refrigerator is a Carnot engine in reverse, and its COP also depends solely on these temperatures.

The entropy statement reiterates that systems naturally tend towards increasing disorder. Examples include an exploding firecracker where neatly packed chemicals disperse, and ink mixing with water to spread equally, increasing the system's entropy.

This example revisits a previous problem, incorporating human body efficiency (25%) as a heat engine. Given 180,000 food calories, it calculates the height a 60 kg person needs to climb to burn those calories, demonstrating that efficiency drastically reduces the required effort compared to 100% efficiency.

A gasoline engine in a truck takes in 10,000 Joules of heat and delivers 2,000 Joules of work per cycle. Calculations are performed to determine: a) the thermal efficiency (20%), b) the heat discarded per cycle (8,000 Joules), c) the power output in watts and horsepower if it cycles 25 times per second, and d) the mass of gasoline burned per cycle and per second/hour.

A Carnot engine takes 2,000 Joules of heat from a 500 Kelvin reservoir and discards some heat to a 350 Kelvin reservoir. The problem calculates a) the efficiency (42.9%), b) the work done (857 Joules), and c) the heat discarded to the cold reservoir (1,143 Joules).

A freezer with a COP of 2.40 converts 1.80 kg of water at 25°C to ice at -5°C in one hour. The problem calculates: a) the total heat that must be removed (a calorimetry problem, leading to negative 809 kJ), b) the electrical energy (work) consumed by the freezer during this hour (337 kJ), and c) the wasted heat delivered to the room (1146 kJ).

This lesson concludes Chapter 10 on thermodynamics. The next chapter will introduce the fourth fundamental quantity: electric charge, beginning with electrostatics.