Thermal Physics

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Definitions

TEMPERATURE:

• determines the direction of the flow of thermal energy between two bodies in thermal contact. if an object is hotter than another, heat energy will flow from the hotter object to the colder one.
• measure of the average random kinetic energy of the particles in a substance (not travelling at same speed or direction).
• the hotter the temperature, the faster the speed of the particles.

THERMAL EQUILIBRIUM:

• two bodies in thermal contact will eventually reach the same temperature.

HEAT:

• amount of thermal energy

INTERNAL ENERGY:

• total kinetic + potential energies of the particles in a substance
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Definitions Pt. 2

SPECIFIC HEAT CAPACITY (C):

• the energy required per unit mass per unit temperature rise.

SPECIFIC LATENT HEAT OF FUSION/VAPORIZATION (Lf/Lv):

• the amount of energy per unit mass to change a solid to a liquid at a constant temperature and pressure (SI UNIT = J/KG)
• the amount of energy per unit mass to change a liquid to a vapour at constant temperature and pressure (SI UNIT = J/KG)

PRESSURE:

• the force exerted per unit area (Pascals).

EVAPORATION:

• the process by which faster-moving molecules escape from the surface of a liquid, resulting in a cooling of the liquid.
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Definitions Pt. 3

MOLE:

• contains the same number of atoms as in 12 grams of carbon-12 (grams).

• the number of particles in a mole A=6.02x1023

MOLAR MASS:

• number of moles of a substance, found dy dividing the mass of substance by its relative atomic mass
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Evaporation

• if a molecule at the surface is moving fast enough it may escape the liquid.
• the average speed of the remaining molecules is now lower, the temperature drops (less kinetic energy).
• takes place at any temperature.

INCREASING THE RATE OF EVAPORATION:

• increase temperature, giving molecules more energy to escape.
• increase surface area, more molecules are at the surface of the liquid therefore able to escape.
• increase air flow over surface, molecules are carried away before they can fall back into liquid.
• decrease humidity, high water vapour gradient, no water molecules from the atmosphere can enter the liquid.
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Boiling

• only occurs when vapour is produced in the body of the liquid, only happening at boiling point of the liquid.
• saturated vapour pressure = external pressure
• bubbles are vapour not air
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Kinetic Theory of Gases

uses simple model, 'ideal gas' which follows the following assumptions:

• no intermolecular forces between molecules (aka NO POTENTIAL ENERGY).
• moves with distribution of speeds, moving randomly in different dimensions.
• volume of individual gas particles is very small compared to the volumes of the gas.
• collisions between the particles and the walls of the container and the particles themselvs are ELASTIC (no kinetic energy lost) therefore total kinetic energy is conserved in an elastic collision.
• the duration of a collision is small compared to the time between collsions.

collisions of the particles with the sides of a container give rise to a force, which average of billions of collisions per second macroscopically is measured as the PRESSURE of the gas, every collsion has a change of momentum.

Pressure = Force/Area

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Behaviour of Gases

we heat a gas at a constant volume, the pressure increases

• increased average kinetic energy of the particles means there are more collsions with the container walls in a period of time (usually per second) and the collsions involve a greater change in momentum.

we heat a gas at constant pressure, the volume increases

• increasing the volume reduces the chance of particles collding with the container walls, opposing the effect of the particles increased kinetic energy

we compress (reduce the volume) a gas at constant temperature, the pressure increases

• a smaller volume increases the likehood of a particle colliding with the container walls.
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Formulas

Specific Heat Capacity = Energy/(Mass x Change in Temperature)

C [J/Kg/ºC] = E [J] /(m [Kg] x ∆T [ºC]) OR E = m c ∆T

Specific Latent Heat of Fusion/Vapourisation = Energy/Mass

Lf/Lv [J/Kg] = E [J] / m [Kg]

Pressure = Force/Area

P [p] = F [N] / A [m2]

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