Understanding Thermodynamics and Wave Motion Concepts

2.1 Thermodynamics: Concepts

Thermodynamics is a science that studies energy and its transformations.
A thermodynamic property is a system’s characteristic that does not depend on history. There are two types of thermodynamic properties: intensive and extensive properties. The intensive property is independent of the mass or size of the system (e.g., temperature, pressure, and density), whereas an extensive property is dependent (e.g., mass, weight, volume, and total energy).

A system is in thermal equilibrium when the temperature is uniform.
2.2 Zeroth Law of Thermodynamics

If two bodies are in thermal equilibrium with a third one, then all three are in thermal equilibrium with each other.

Remember that thermal equilibrium occurs when the temperature is uniform in the system.
Example 2.1

Consider three bodies, A, B, and C. If body A is in thermal equilibrium with body C, and body B is also in equilibrium with body C, then, according to the Zeroth Law of Thermodynamics, bodies A and B are also in thermal equilibrium.

2.3 First Law of Thermodynamics

In a thermodynamic process, the net heat (Q) added to a system minus the net work developed (W) is equal to the change in internal energy within. This law is the equivalent of the Energy Conservation Principle.

	Heat added to the system (+)		Work done by the system (+)
	Heat lost by the system (-)		Work done on the system (-)

Example 2.2

Consider the following data for a given system and calculate the change in internal energy.

First, you must check the measurement units, because you can use either calories or Joules (J), but the entire data should be presented in the same unit:

The change in internal energy would be the following:

2.4 Second Law of Thermodynamics

The Second Law tells us it is not possible to transform absolutely all the thermal energy into work. In other words, only part of the heat applied to a system becomes work; the other part dissipates into the region of lower temperature.

Entropy (S) is the measurement of that part of applied heat that does not transform into work.

Example 2.3

4357 J was needed to melt a piece of ice and maintain it at the same temperature (0ºC). Calculate the change in entropy.

First, convert the temperature from °C to K.

With the correct units, the change in entropy is calculated.

3.1 Waves: Characteristics and Concepts

A wave is a perturbation in a medium. There are two types of waves: mechanical and electromagnetic.

Mechanical waves require an elastic medium to propagate, for example, the motion of a spring, water, sound, etc., whereas electromagnetic waves can also travel through a vacuum.

Undulatory motion is defined as the propagation of a perturbation in a medium, namely the propagation of a wave.

The amplitude (A) of a wave consists of the maximum separation a wave can reach in relation to the reference axis. Wavelength () is the distance a pulse travels while performing a complete oscillation. Period (T) is the time that it takes for the pulse to perform a complete oscillation, and frequency (f) is the number of oscillations that a pulse performs in a given time.

3.2 Simple Harmonic Motion: Hooke’s Law

Simple harmonic motion is a periodic motion on the same trajectory; such motion depends on time. The most common example is an object that oscillates on the end of a spring.

Hooke’s Law states that the amount of change in length of an elastic material is directly proportional to the applied force.

Example 3.1

An object is hung from a spring that obeys Hook’s Law; the spring has an elastic constant of 17.3 N/cm and stretches to 27 cm. What would the object’s mass be?

3.3 Simple Pendulum

A simple pendulum is a system that consists of an object, with a mass that can be ignored, hanging from the edge of a cord that does not stretch.

When the angle is less than 15°, sin is approximately equal to. On the other hand, the arc x equals L, and the equation for its force would be as follows:

Period of a simple pendulum:

Example 3.2

On Earth, a simple pendulum has a 3.2 s period. If the same pendulum is placed on Venus, and its period changes to 2.7 s, what would the value of gravity be on Venus?

The length L is the same on both planets, so we work out the value of both formulas and equal: