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.