Example Questions About the Second Law of Thermodynamics, Especially Heat Engines

 In the previous article, we have learned about the 1st law of thermodynamics for 11th-grade high school seniors. In this article, we will share some examples of questions about the second law of thermodynamics, especially heat engines for 11th grade, and also be accompanied by a discussion. A heat engine is a machine that converts heat into mechanical energy. One of the heat engines is a car engine. The car engine converts heat or heat from the combustion of gasoline or matter into mechanical energy that can move the car.

Not all heat energy can be converted into mechanical energy. Usually, the rest of the heat energy is converted into mechanical energy as well as energy other than mechanical energy or residual energy. If you want to know the stages of work done on a heat engine. Take a look at the stages of work done by a heat engine.

 There are 4 stages of work carried out by a heat engine, namely as follows:

1. Piston in a fixed or constant volume. Then pressure P! increased by burning to P2. pressure will increase with the constant volume on the piston

2. At the contrasting pressure, P3 = P2, the salt volume is from V1 to V3, so the machine is working.

3. The piston becomes a constant volume so that the pressure decreases to the initial pressure or P1. the pressure will decrease at a constant volume so that there is no pressure on the gas.

4. The gas is compressed with constant pressure and heat is removed so that the heat becomes its original volume.

The explanation above can be summarized in the following formula

Q in = Q1 + Q2

Q out = Q3 + Q4

Then the work that can be done is W = Q in - Q out

Here is the sound of the second law of thermodynamics

“It is impossible for a heat engine that works cyclically to not produce other effects other than absorbing heat from the reservoir. And do an equivalent amount of work.”

From the sound of the second law of thermodynamics. It can be concluded that the heat engine in addition to generating power also produces exhaust heat energy.

The efficiency of a heat engine is the ratio of the work done to the heat absorbed in the reservoir: the efficiency equation is as follows:

Efficiency will be achieved 100% if Q out (Qd) = 0. or no heat is removed, so the absorbed heat is converted into mechanical energy.

Example questions about the 2nd law of thermodynamics

1. A heat engine does 240 joules of work if the heat input to the engine is 440 joule how much heat is lost in the heat engine?

Discussion: as has been explained if the work on the Carnot engine that the work on Carnot engine is the result of reducing the incoming energy minus the energy output as follows:

W = Q in - Q out

W = 440 m- 240 = 200 joules

2. From question number one above, the heat efficiency of the Carnot engine is...

discussion:  it can be seen that efficiency is the ratio of work and heat absorbed, so the following efficiency formula is

= 1- (Q out : Q in )

= 1 - (240 : 440)

= 1 - 0.54 = 0.45 or 45%

Then the efficiency of the Carnot engine is 45%

3. Consider the following heat engine:

If the heat engine has 120 joules of work and the energy entering the heat engine is 400 joule, what is the efficiency of the engine:

Discussion: This question is the same as questions number 1 and 2, namely looking for engine efficiency, so if you know the amount of heat entered by the engine's work, the answer is

= W/Qin = 120/400 = 0.3 or 30%