Summary and key terms | B.1 Thermal energy transfers

B. The particulate nature of matter / B.1 Thermal energy transfers

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• Matter is anything that is made of particles with mass. Depending on the arrangement of the particles, how much energy they have and the strength of the intermolecular forces between them, they can be classified as either solids, liquids or gases.

• Internal energy is the total intermolecular potential energy plus the total random kinetic energy of the particles arising from their random motion.

• The density of a substance is a measure of how much mass it has per unit volume, measured in kilograms per cubic metre, kg m^–3. Solids have the highest density, while gases have the lowest.

• While we use either the degrees Fahrenheit, ^oF, or degrees Celsius, °C, to describe temperature in everyday life, scientists often use the absolute temperature or Kelvin scale. At absolute zero, or 0 kelvin, molecules of an ideal gas have theoretically zero energy, zero pressure and zero volume. We can convert from degrees Celsius to kelvin by adding 273. A temperature change of 1 °C is the same as a temperature change of 1 K.

• The absolute temperature (temperature in kelvin) of a substance is a measure of the average kinetic energy of its molecules.

• Specific heat capacity is the amount of energy required to increase the temperature of 1 kg of a substance by 1 K, measured in joules per kilogram kelvin, J kg^–1 K.

• When an object is heated, the thermal energy supplied is transformed to kinetic energy of the particles as well as being used to weaken the intermolecular forces, allowing particles to move further away from each other, thus increasing their potential energy.

• A change of phase occurs when a substance changes from one state (solid, liquid or gas) to another. This change of phase happens at a constant temperature because all of the energy supplied to the substance is used to increase the potential energy between particles.

• Specific latent heat is the amount of energy required to change the phase of 1 kg of a substance at constant temperature.

• Conduction is the transfer of thermal energy due to collisions between particles. For this reason, it happens best in solids. Metals are the best conductors of thermal energy because they have free electrons which can move through the metal, transferring thermal energy.

• The rate of thermal energy transferred by conduction depends on the thermal conductivity of the material, its cross-sectional area and length, and the difference in temperature between the hotter and colder parts of the material.

• Convection is the transfer of thermal energy due to the mass movement of particles and therefore happens best in liquids and gases.

• Thermal radiation is the transfer of energy by electromagnetic waves. All objects with a temperature above 0 K emit infrared radiation. Black, matt surfaces are the best emitters and absorbers of infrared radiation, while light coloured, shiny surfaces are the best reflectors.

• A black body is a theoretical object that absorbs all energy from all wavelengths of light from the electromagnetic spectrum. A black body in thermal equilibrium emits black body radiation containing all wavelengths of light from the electromagnetic spectrum. They do not exist in real life, but some objects, such as stars, are very close approximations to black bodies. Graphs called emission spectra illustrate the intensities of radiation that are emitted by black bodies at different wavelengths and Wien’s Law tells us that the absolute temperature of the star is inversely proportional to the wavelength at which the maximum intensity of radiation is emitted.

• The luminosity of a black body is the amount of energy it emits per second, measured in watts, W. It is directly proportional to the surface area of the star and also to the absolute temperature of the star raised to the fourth power, T⁴.

• The apparent brightness of a star is how bright it appears to an observer, or the amount of power per square metre received by an observer from the star, measured in watts per metre square, W m^–2. It is directly proportional to the star’s luminosity and inversely proportional to the square of the distance between the star and the observer.

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