Basic Physics Tutorials

To explain the irreversible thermodynamic processes, the scientists formulated the second law of thermodynamics. The second law of thermodynamics explains what processes can occur in the universe and what processes cannot happen. One scientist named R. J. E. Clausius (1822-1888) made the following statement:

Naturally, heat moves from high-temperature objects to low-temperature objects; naturally, heat does not proceed from low-temperature object to high-temperature object (Second law of thermodynamics—Clausius’s statement).

Clausius’s statement is one of the special statements of the second law of thermodynamics. It is called special statement because it only applies to one process just, related to heat transfer. Since this statement is not related to other processes, we need a more general statement. The development of a general statement of the second law of thermodynamics is based on the study about heat engine. Therefore, we discuss heat engine first.

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Article Thermodynamic processes : Isothermal Adiabatic Isochoric Isobaric

There are four thermodynamic processes, namely Isothermal, isochoric, isobaric and adiabatic processes.

Isothermal Process (constant temperature)

In an isothermal process, system temperature is kept constant. Theoretically, the analyzed system is an ideal gas. Ideal gas temperature is directly proportional to ideal internal gas energy (U = 3/2 n R T). T does not change, so U also does not change. Thus, if applied to the isothermal process, the first law of the thermodynamic equation becomes:

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Thermodynamic process

Heat (Q) is the energy that moves from one object to another because of the temperature difference. About systems and environments, heat is energy moving from system to environment or energy moving from environment to system, due to the temperature difference. If the system temperature is higher than the ambient temperature, heat will flow from the system to the environment. If the ambient temperature is higher than the system temperature, then heat flows from the environment to the system.

Heat (Q) is energy that moves due to the temperature difference, whereas work (W) is related to energy transfer through work. For example, if the system does work on the environment, then energy moves from system to environment. Conversely, if the environment does work on the system, then energy moves from environment to system.

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The conservation of kinetic energy law is not applicable in inelastic collisions. The conservation of momentum law is applicable in inelastic collisions if only no external force acts on the two colliding objects. In an inelastic collision, two objects stick together or are attached to each other after the collision.

Example question 1.

Two objects are of the same mass, namely 1 kg. Object 1 moves on a flat plane at a speed of 10 m/s and collides with object two which is at rest. After the collision, the two objects stick together. What is the speed of the two objects after the collision?

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In partially elastic collisions, the law of conservation of momentum is applicable, while the conservation of kinetic energy law is not applicable. At the time a collision takes place, some kinetic energy is converted to sound energy, heat energy, and internal energy. The use of the word elastic signifies that after the collision, the two objects do not stick together but bounce off.

An example of partially elastic collision is the one-dimensional collision of two marbles or two pool balls.

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Law of conservation of linear momentum states that if there is no external force acting on two colliding objects, the momentum of the objects before the collision is equal to the momentum of the objects after the collision.

p₁ + p₂ = p₁ ’ + p₂ ’ ………………….. Equation 1.4

m₁ v₁ + m₂ v₂ = m₁ v₁ ’ + m₂ v₂ ’

If after collision both objects stick together,

m₁ v₁ + m₂ v₂ = (m₁ + m₂ ) v’

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A collision of two objects is called a perfectly elastic collision if the momentum or kinetic energy of each object before the collision is equal to the momentum and kinetic energy of each object after the collision. In other words, the conservation of momentum law and conservation of kinetic energy law are applicable in perfectly elastic collisions. The use of the word elastic signifies that after the collision, the two objects do not stick together or are not attached to each other but bounce off. The momentum of each object is conserved.

The momentum of each object is conserved.

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The work-kinetic energy theorem states that the net work or the work done by the net force is equal to the change in kinetic energy.

W_net = KE_t – KE_o = 1⁄2 m(v_t² – v_o²)

W_net = There are two types of forces, namely conservative force, and non-conservative force. Thus, net work can be considered to be comprised of the work done by a conservative force and the work done by a non-conservative force.

W_c + W_nc = ΔKE

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Article Work done by conservative forces Potential energy

Observe an object which moves vertically upwards and then return to its initial position after reaching a maximum height. When the object is moving vertically upwards, weight does negative work on the object. When the object is moving upwards, the object’s height increases. Therefore, the object’s gravitational potential energy increases as well. It can be concluded that the negative work done by weight is equal to the increase in the object’s gravitational potential energy (PE).

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1. Conservative Force

1.1 Weight (w)

Observe an object which moves vertically upwards until reaching a maximum height before moving downwards towards its initial position. When moving vertically upwards by h, the weight is opposite in direction from displacement. Thus, the weight does negative work on the object. 

W = w h (cos 180^o) = – w h = – m g h

After reaching a maximum height, the object moves downwards towards its initial position by h. When moving downwards, the weight is in the same direction as the displacement. Because it is in the same direction as displacement, the weight does positive work.

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