Physics P1

MATTER, MODELS AND DENSITY

• A Simple Model of Atoms
• Matter is made of atoms.
• An atom can be pictured as a very small ball (Billiard Ball is a model of an atom.)
• Inside Atoms - Electrons
• All atoms have even smaller particles inside it.
• J.J. Thomson discovered electrons (Tiny particles with negative electric charge). He suggested that electrons are embedded in atoms. Knew a whole atom is electrically neutral - with no overall electric charge. Electrons must be balanced by a positive charge within each atom.
• Thomson suggested that the part of the to with positive electric charge was like the soft 'sponge' of a cake.
• The cake is Thomson's model of atom.
• Inside Atoms - Nuclei
• Scientists discovered small positively charged particles from some kinds of radioactive materials at very high speed (Alpha Particles)
• Ernest Rutherford, Hans geiger and Ernest Marsden - Fired these particles at thin sheets of gold. They believed that the alpha particles would shoot through the gold atoms. However, some bounced back (Alpha scattering)

Slide 2

• To cause the scattering atoms must contain something very dense and positively charged
• Rutherford had concluded that the positive electric charge must be concentrated into a very tiny space deep inside each atom. He called this the nucleus.
• Niels Bohr (Denmark), improved this nuclear model by describing the pathways of electrons around the nucleus.
• 
• Caption: Model of Atoms

Slide 3

• Mass and charge in Atoms
• Electric charge is shared equally between the electrons (negative) and the nucleus (positive). But the nucleus has nearly all of the mass
• Electric charge can cause forces between atoms. The forces hold atoms together to make solids and liquids.
• Some atoms can form groups held together by electric force. These groups are molecules.
• Water is made of molecules - two hydrogen (H) atoms and one oxygen (O) atom, Water = H2O
• Gases have a very weak force between particles. So gases spread out into the space around them.
• Gases have lower density in comparison to Solids and Liquids.
• Density is a ratio of mass to volume
• Density (kg/m3) = Mass (kg) / Volume (m3)

TEMPERATURE AND STATE

• Change of State
• Melting (Solid to Liquid), Sublimating (Solid to Gas) and boiling (Liquid to Gas), require the material to gain energy
• Condensing (Gas to liquid) and freezing (Liquid to Solid), the material loses energy.
• When there is change of state there is no change in the structures. However, their energy and speed and the distance apart and force between them can change.
• Changes of state are not chemical changes. They are physical changes (reversible)
• Mass of material does not change during a change of state. Mass is conserved.
• Volume and density usually change during a change of state.
• 

Slide 5

• Specific Latent Heat
• Energy needed to melt a substance is different for all substances.
• These values are called the specific latent heat . A substance has a different value of specific latent heat for the change of state from liquid to gas. This is called the specific latent heat for vaporisation.
• Thermal energy for a change in state = mass x specific latent heat
• Specific Heat Capacity
• Energy is also needed to raise the temperature of a material.
• The specific heat capacity of a material is the energy needed to raise the temperature of 1kg by 1℃ (Or 1K)
• Change in thermal energy = mass x specific heat capacity x change in temperature

PRESSURE

• Gas Pressure
• A gas in a box exerts pressure on all of the walls because it's molecules bombard the walls.
• Most gases have atoms in small groups or molecules. This includes oxygen and hydrogen, where each molecule is a pair of atoms.
• Pressure is force acting on each square area.
• Pressure is measured in pascals (Pa). Pressure (Pa) = Force (N) / Area (m2)
• Gas Pressure, Volume and Temperature
• The faster the particles in a gas move, the bigger the pressure. Pressure is related to temperature
• Pressure also depends on volume. As the particles become closer and the walls are hit more by the gas molecules
• Applying pressure to a gas can reduce its volume (Gas is compressed)
• Reducing pressure on the gas can make it expand
• Space and Atmosphere
• Space is a vacuum.
• High in the Earth's atmosphere, the gas particles are far apart, so they exert little pressure.
• Lower in the atmosphere the particles are closer togeter and make more collisions.
• Gravitational field strength (g), Is close to 10N/Kg at the Earth's surface. Meaning every mass of 1KG experiences a force of gravity (Weight) of 10 newtons (N)

Slide 7

• Upthrust, Floating and Sinking
• An object experiences more pressure on its lower surface that on its upper surface.
• Pressure difference = Height x density x g
• This pressure difference creates an upward force on the object (Upthrust)
• If the weight of the object is greater than the upthrust, it will sink.
• If the weight of the object is smaller than the upthrust, it will rise to the surface.
• 
• Caption: Balanced force (Upthrust of water: 40,000 N Weight of boat: 40,000 N)