year 9 science revision

physics

Density calculations

Density is the mass per unit volume. It can be measured in several ways

The most accurate way to calculate the density of any solid liquid or gas is to divide its mass in kilograms by its volume (length x width x height) in cubic meters

The unit for density is kg/m3. The density of water is approximately 1000kg/m3 and the density of air is approximately 1.2 kg/m3.

If solid objects are placed in water and they sink they have a density greater than water. The reverse is also true.

If several liquids that dont mix are placed in the same container the least dense one will rise to the top and the densest one will sink to the bottom. This is also the same with gases but they are often harder to see as they tend to mix with each other very easily

Density practical

-Density can be measured for regular solids, irregular solids and liquids

-You could get different results to somebody else doing the same experiment because to wont measure then the same all the time or they added the extra drips from before they placed the object in

Changes of state

Some substances can change directly from sold to gas or from gas to solid without becoming a liquid in between. This is called sublimation.

Energy must be transferred by heating to a substance for these changes of state to happen. During these changes the particles gain energy. The energy is used to break some of the bonds between particles during melting and overcome the remaining forces of attraction between particles during evaporating or boiling. In evaporation particles leave a liquid from its surface only. In boiling bubbles of gas start to form throughout the liquid. They rise to the surface and escape to the surrounding forming gas.

Internal energy

When the material is heated or cooled two changes could happen to the particles within the material.

-Chemical bonds between the particles may form break or stretch. There is a chemical potential store of energy in the material

-The material will heat up or cool down as the particles within it gain or lose speed. There is a change in the thermal store of energy within the material.

-When energy is given to raise the temperature, particles speed up and gain kinetic energy.

-When the substance melts or boils, energy is put in to breaking the bonds that are holding particles together, which increases the potential energy.

-The conservation of energy means that, assuming no energy is lost to the environment, any energy transferred to a material will be distributed between the chemical store and the thermal store of the internal energy.

-Whether the energy breaks bonds, increases the speed of the particles to stretch bonds, or just increases the speed of the particles depends on the temperature and state of the material.

Internal energy is linked to the temperature of matter but the two are very different things:

Internal energy is a measure of the total energy of all the particles in the object or substance. This includes the kinetic energy of the particles and chemical potential energy of the bonds between them.

Temperature is a measure of the average speed of the particles. This is based on the kinetic energy of individual particles.