Internal Energy
Definition and Components
The term internal energy refers to the total energy contained within a system, arising from the motion and interactions of its constituent particles. It is constituted by two primary components: intermolecular potential energy and random kinetic energy of molecules.
- Intermolecular Potential Energy: Every molecule within a system interacts with others via forces of attraction or repulsion. The energy associated with these interactions contributes to the intermolecular potential energy of the system. It depends on the nature, intensity, and distance of interactions between molecules.
- Random Kinetic Energy: Molecules are in perpetual motion, and the energy associated with this random, incessant movement is the random kinetic energy. It’s influenced by the temperature of the system, with higher temperatures leading to increased molecular speeds and thus, heightened kinetic energy.
Internal Energy
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Temperature and Thermal Energy Transfer
The dynamics of energy flow within or between systems is primarily governed by temperature differences. The direction of thermal energy transfer is unequivocally from a region of higher temperature to one of lower temperature until a point of thermal equilibrium is achieved.
- Temperature Gradient: This term is used to describe the rate of change in temperature observed between two distinct points or regions. The temperature gradient is fundamental in driving the flow of thermal energy.
- Thermal Equilibrium: A state where the temperature is uniform throughout a system signifies that thermal equilibrium has been attained. In this state, there is no net exchange of thermal energy.
Phase Changes
Particle Behaviour and Energy Changes
The transitions of matter from one state to another—such as from solid to liquid or liquid to gas—are intriguing processes governed by particle behaviour and inherent energy changes that interestingly, occur at a constant temperature.
- Melting and Freezing: The process of melting ensues when a solid structure absorbs thermal energy, instigating increased vibrations among its molecules. When the energy is sufficient to overcome the intermolecular forces holding the molecules together, the solid transitions into a liquid. Freezing, on the contrary, unfolds as a liquid loses thermal energy. The molecules decelerate, and intensified intermolecular forces facilitate their bonding into a solid structure.
- Boiling and Condensing: Boiling is marked by a liquid’s absorption of energy, escalating molecular motions to a point where a transition into the gaseous state occurs. Conversely, in condensation, a gas releases energy, leading to a reduction in molecular motion and a subsequent transition into the liquid state as intermolecular forces predominate.
- Evaporation: A surface phenomenon, evaporation is characterised by molecules at a liquid’s surface gaining enough energy to transition into a gas. This process can occur at temperatures below the boiling point and is influenced by factors such as surface area and air pressure.
Phase changes
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Constant Temperature during Phase Changes
A distinguishing feature of phase changes is the constant temperature during the transformation. Energy input or output is utilised in modifying the intermolecular forces and not in changing the temperature.
- Latent Heat: This term refers to the energy absorbed or released during a phase change at a constant temperature. It underscores the energy required to alter the phase of a substance without a corresponding change in temperature.
Graphical representation of Latent heat in phase change
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Key Terms
Melting
Melting is a phase transition where a solid absorbs thermal energy and transforms into a liquid. This process occurs at a specific temperature called the melting point, where the internal energy is adequate to overcome the forces binding the solid structure.
Freezing
Freezing denotes the phase change from liquid to solid. It occurs when a liquid loses thermal energy, leading to a deceleration of molecular motion. The intensification of intermolecular forces facilitates the formation of a solid structure.
Boiling
This phase transition from liquid to gas is precipitated by the absorption of thermal energy. It occurs at a defined boiling point where the internal energy suffices to overcome intermolecular forces, facilitating the liquid-to-gas transformation.
Condensing
Condensing is the reverse process of boiling, where a gas loses internal energy and transforms into a liquid. This phase change is facilitated by the intensification of intermolecular forces as the molecular motion decelerates.
Evaporation
Evaporation is characterised by the gradual escape of molecules from a liquid’s surface into the gaseous phase. This process is facilitated when surface molecules acquire sufficient kinetic energy to overcome intermolecular forces, occurring even at temperatures below the boiling point.
In sum, a nuanced understanding of internal energy and phase changes serves as a gateway to a profound comprehension of thermal physics. The complex interplay of molecular interactions, energy transformations, and phase transitions unveils a world where forces and energies are in constant flux, painting a dynamic and intricate portrait of the thermal behaviour of matter. These foundational concepts pave the way for more advanced explorations into the world of thermodynamics, where every phase change, every flux of energy, echoes the intricate symphony of forces that define the very essence of the material world.
FAQ
If a substance is not undergoing a phase change, an increase in internal energy typically results in a temperature rise. The added energy increases the kinetic energy of the particles, causing them to move more rapidly. In solids, this increased movement leads to more vigorous vibrations of particles around their fixed positions. In liquids and gases, it results in faster translational movements of particles. This does not alter the phase of the substance but affects properties dependent on kinetic energy and temperature, such as thermal expansion, electrical conductivity, and viscosity.
Latent heat is measured by calculating the amount of energy absorbed or released during a phase change, without a change in temperature. The formula Q = mL is often used, where Q is the heat energy, m is the mass of the substance, and L is the latent heat constant, specific to each material and phase change type (fusion or vaporisation). Factors affecting its value include the nature of the material, atmospheric pressure, and impurities or mixtures, as they can alter the intermolecular forces and hence the energy required for phase changes.
The concept of internal energy is pivotal in real-life applications such as heating systems and refrigeration. For heating systems, understanding the conversion of electrical energy into internal energy is essential. The device’s efficiency and effectiveness are dictated by how well it can transfer energy to increase the internal energy of the air or water being heated. In refrigeration, the principle involves removing internal energy from a space or substance to lower its temperature. The appliance works by absorbing internal energy and expelling it externally, thus understanding the mechanisms of energy transfer and how substances respond to changes in internal energy is crucial for optimising the performance of these appliances.
Internal energy cannot be negative because it is the sum of the kinetic and potential energy of the molecules in a system, both of which are always positive or zero. Kinetic energy is proportional to the square of the speed of the molecules, and since squaring any real number gives a positive result, kinetic energy is always positive. Potential energy, arising from intermolecular forces, can be zero or positive. As a result, the total internal energy, being the sum of these two components, is always a positive value or zero in the case of absolute zero temperature.
Different substances have different internal energies at the same temperature due to variations in their molecular structure and intermolecular forces. The internal energy is not only dependent on the random kinetic energy (which is similar for different substances at the same temperature) but also on the potential energy arising from intermolecular forces. Different substances have molecules with distinct structures, masses, and forces of attraction or repulsion between them. These variations lead to differences in potential energy, resulting in disparate internal energies even when substances are at the same temperature.
Practice Questions
The internal energy of a substance is intricately linked to its phase changes due to the combined effects of intermolecular potential energy and kinetic energy. During a phase change, the internal energy increases as the substance absorbs thermal energy. This absorbed energy, termed latent heat, is used to alter the intermolecular potential energy, overcoming forces that hold the particles together in their initial phase, without increasing the kinetic energy. Consequently, the temperature remains constant during phase changes as the absorbed energy is wholly utilised in changing the state of the substance, not increasing its temperature.
As the solid substance reaches its melting point, the particles begin to vibrate with increased energy due to the absorption of thermal energy. The enhanced kinetic energy increases the internal energy of the substance, causing a weakening in the intermolecular forces that hold the particles in a fixed, orderly structure. The absorbed energy, transforming the solid's internal energy landscape, facilitates the liberation of particles from their fixed positions. They gain freedom to move past each other, marking the phase transition from solid to a more disordered liquid state where particles have greater mobility and less order.
