The direction of the natural flow of thermal energy between two objects is determined by the 'hotness' of each object.
Thermal energy naturally flows from hot to cold. The temperature of an object is a measure of how hot it is. In other words, if two objects are placed in thermal contact, then the temperature difference between the two objects will determine the direction of the natural transfer of thermal energy. Thermal energy is naturally transferred 'down' the temperature difference - from high temperature to low temperature.
Eventually, the two objects would be expected to reach the same temperature. When this happens, they are said to be in thermal equilibrium.
Heat is not a substance that flows from one object to another. What has happened is that thermal energy has been transferred. Thermal energy (heat) refers to the non-mechanical transfer of energy between a system and its surroundings.
In order to use Kelvin and Celsius, you do not need to understand the details of how either of these scales has been defined, but you do need to know the relation between them. Most everyday thermometers are marked with the celsius scale and temperature Is quoted in degrees Celsius (°C).
There is an easy relationship between a temperature 'Tas' measured on the Kelvin scale and the corresponding temperature 't' as measured on the Celsius scale. The approximate relationship is T (K) = (°C) + 273. This means that the 'size' of the units used on each scale is identical, but they have different zero points.
The Kelvin scale is an absolute thermodynamic temperature scale and a measurement on this scale is also called the absolute temperature. Zero Kelvin is called absolute zero.
For a given sample of gas, the pressure, the volume and the temperature are all related to one another.
The SI units of pressure are $N{m}^{-2}$ or Pa (Pascals). 1*Pa = 1*N
The temperature, t, of the gas is measured in °C or K
In order to investigate how these quantities are interrelated, we choose:
one quantity to be the independent variable (the thing we alter and measure)
another quantity to be the dependent variable (the second thing we measure).
The third quantity needs to be controlled (i.e. kept constant ).
The specific values that will be recorded also depend on the mass of gas being investigated and the type of gas being used so these need to be controlled as well.
When a substance changes phase, the temperature remains constant even though thermal energy is still being transferred.
Cooling Curve For Molten Lead (Idealized)
The amount of energy associated with the phase change is called the latent heat. The technical term for the change of phase from solid to liquid is fusion and the term for the change from liquid to gas is vaporization.
The energy given to the molecules does not increase their kinetic energy so it must be increasing their potential energy. Intermolecular bonds are being broken and this takes energy. When the substance freezes bonds are created and this process releases energy.
It is a very common mistake to think that the molecules must speed up during a phase change. The molecules in water vapour at 100 °C must be moving with the same average speed as the molecules in liquid water at 100 °C.
The specific latent heat of a substance is defined as the amount of energy per unit mass absorbed or released during a change of phase. In symbols,
Specific latent heat $L=\frac{Q}{M}(Jk{g}^{-1});Q=ML$
In the idealized situation of no energy loss, a constant rate of energy transfer into a solid substance would result in a constant rate of increase in temperature until the melting point is reached:
The specific heat capacity of the solid as the gradient of the line that corresponds to the liquid phase is greater than the gradient of the line that corresponds to the solid phase. A given amount of energy will cause a greater increase in temperature for the liquid when compared with the solid.
The two possible methods for measuring latent heats are very similar in principle to the methods for measuring specific heat capacities. A method for measuring the
Method 1: Electrical Circuit.
The amount of thermal energy provided to water at its boiling point is calculated using electrical energy = I t V. The mass vaporized needs to be recorded.
The specific latent heat $L=\frac{ItV}{({m}_{1}-{m}_{2})}$
Sources of experimental error
Loss of thermal energy from the apparatus.
Some water vapour will be lost before and after timing.
Method 2: Fusion Of Water
Providing we know the specific heat capacity of water, we can calculate the specific latent heat of fusion for water. For example, ice (at 0 °C) is added to warm water and the temperature of the resulting mix is measured.
If no energy is lost from the system then, energy lost by water cooling down = energy gained by the ice
Sources of experimental error
Loss (or gain) of thermal energy from the apparatus.
If the ice had not started at exactly zero, then there would be an additional term in the equation in order to account for the energy needed to warm the ice up to 0 °C.
The water clinging to the ice before the transfer.
The three ideal gas laws can be combined together to produce one mathematical relationship: $\frac{pV}{T}=$constant
This constant will depend on the mass and type of gas.
If we compare the value of this constant for different masses of different gases, it turns out to depend on the number of molecules that are in the gas - not their type.
In this case, we use the definition of the mole to state that for n moles of an ideal gas: $\frac{pV}{nT}=$a universal constant
The universal constant is called the molar gas constant R. The SI unit for R is J $mo{l}^{-1}{K}^{-1}$
R = 8.314 J $mo{l}^{-1}$ ${K}^{-1}$
The concepts of the mole, molar mass and the Avogadro constant are all introduced so as to be able to relate the mass of gas (an easily measurable quantity) to the number of molecules that are present in the gas.
It is the basic SI unit for 'amount of substance'. One mole of any substance is equal to the amount of that substance that contains the same number of particles as 0.012 kg of carbon-12. When writing the unit it is (slightly) shortened to the mol.
The mass of one mole of a substance is called the molar mass. A simple rule applies. If an element has a certain mass number, A, then the molar mass will be A grams.
This is the number of atoms in 0.012 kg of carbon-12. It is 6.02 x ${10}^{23}$.
An ideal gas is a one that follows the gas laws for all values of p, V and T and thus ideal gases cannot be liquefied. Real gases, however, can approximate to ideal behaviour providing that the intermolecular forces are small enough to be ignored. For this to apply, the pressure/density of the gas must be low and the temperature must be moderate.
Macroscopically, at constant pressure, the volume of a gas is proportional to its temperature in kelvin.
Microscopically this can be analysed as follows
A higher temperature means faster moving molecules.
Faster moving molecules hit the walls with a greater microscopic force
If the volume of the gas increases, then the rate at which these collisions take place on a unit area of the wall must go down.
The average force on a unit area of the wall can thus be the same. Thus the pressure remains the same.