Physics

Physics

1. Magnets and electromagnets

   • Magnets and magnetic materials
     • Magnets will attract and repel other magnets
     • Magnetic materials are attracted to magnets
     • Magnetic materials do not attract or repel each other
     • Non-magnetic materials are not attracted to magnets or magnetic materials
     • Needle and cork compass - domains
       • The regions of mini magnets in the needle (domains) are all facing in different directions - no alignment. By rubbing the magnet over the needle it gets them all facing one way and so they all work together and the needle is then pulled by magnetic fields.
         • You can have some alignment with domains or it can be saturated where all the domains are lined up and you have maximum magnetism
     • Some materials are 'hard' and some are 'soft'
       • Hard materials can be magnetised but they can hold their magnetism when the magnet is removed
       • Soft materials are easily magnetised and easily lose magnetism
     • Magnetic fields
       • Plotting compass
         • sensitive and gives direction but slow
       • Iron filings
         • quick shape but no direction and less sensitive
       • Field lines can be pushed to one side but they cannot cross over
       • Field lines always travel from north to south
   • Electromagnets
     • Solenoid
       • Coil of wire
     • Electromagnetism
       • The field becomes weaker further away from the wire and we show this by lines being spaced further apart
       • How can you make an electromagnet stronger?
         • Use more coils of wire
         • Use a higher electric current
         • Use a core made from a magnetic material
     • Application of electromagnets
       • Relays
         • Often used as a safety switch to allow a circuit with a small current flowing through it to switch on (/off) a circuit that will have a larger current flowing through it.
           • When you let go both switches go off
       • Electric bells
         • When someone presses the switch an electric current can flow around the circuit
           • When the current flows in the coil of wire it becomes a magnet and attracts the armature, when this moves towards the electromagnet the end of it hits the gong and it makes a sound
             • Now that the armature has moved there Is a gap between the contacts and so the elctromagnet stops being magnetic and the armature springs back

2. See book for diagrams!

3. Sound

   • A sound is a vibration. It vibrates against the air as it passes through, it reaches our ears and vibrates our ears
     • Sound can travel through air, water, it travels through everything but at different speeds but it cannot travel through vacuums where there is an absence of particles
   • How a loudspeaker works
     • It vibrates and makes a noise. The force goes push, pull, push, pull etc. This vibrates air which travels to our ears
       • To make it louder it will vibrate in and out further and will move more air (amplitude)
       • To make a higher pitched sound it vibrates more often and more quickly which creates a higher frequency and a higher pitch
   • Soundwaves
     • Longitudinal wave - vibration in the direction of wave travel, made up of compression and rarefaction pulses
       • Rarefaction - sound waves spread out
       • Compression - sound waves close together
   • The speed of sound
     • speed = distance/ time
       • distance = speed x time
       • time = distance/ speed
     • The speed of sound in different materials
       • wood - 4000 m/s
       • air - 330 m/s
       • metal - 5000 m/s
       • soft tissue/ water - 1500 m/s
       • vacuum - 0 m/s
       • cork - 500 m/s
   • Osclliscopes
     • Frequency, f = number of a cycles in 1 second
       • unit = Hz
       • f = 1/T
     • T = time for 1 cycle in seconds
     • Oscilloscope traces
       • Frequency - gaps between each "bump"
       • Amplitude - half the height of a "bump"
       • One wave - down and up (2 "bumps")
         • Wave time for these 2 "bumps"
     • Speed of sound using an oscilliscope
       • When the microphone is moved away from the loudspeaker (measure distance) the trace gets shorter (quieter sound) and the trace shifts to the right ie whatever makes the peak arrives a t a later time - we measure this
     • Cathode Ray Oscilloscope (CRO)
   • How we hear
     • The vibrations in the air travel down our auditory (ear) canal as sound waves
       • These vibrations reach or tympanic membrane (ear drum) which transfer the vibrations to our middle ear bones (hammer, anvil and stirrup - ossicles) which amplify the sound (x 20)
         • The vibrations reach the cochlea where fluid moves hairs of different lengths (longer respond to lower frequencies)
           • This gets sent to our brain via our auditory nerve
     • Our semi-circular canals in our middle ear help with balance
     • Our Eustation tube which is connected to our throat balance pressure
     • The human range of hearing
       • 20 Hertz to 20,000 Hertz (20KHz)

4. Heat transfer

   • Conduction
     • When particles are heated they vibrate more. They "bump" into adjacent particles and they do the same and the heat travels through the substance
     • Solids (especially metals - copper, brass and then steel because they also have "free electrons" that can easily jump from particle to particle) are good conductors of heat because their particles are close together
     • Bad conductors can be called insulators
     • Conductors can keep heat in and stop heat getting in
   • Convection
     • The main way that heat travels through liquids and gases
       • Hot air rises over cold air which gets pushed down
         • This is because it is warmer and so vibrated more, taking up more space and is therefore less dense than its surroundings
     • It happens when one part of a liquid or gas is heated more than the rest
       • Causes wind an ocean currents
     • Radiators
       • Should really be called convectors!
         • Heats the air nearby and so some is colder, the warmer, less dense air rises and the colder therefore sinks and takes its place and is then heated
   • Radiation
     • Infa-red radiation
     • Heat behaves like light. This means it can be reflected off a shiny surface or focused through a lens
     • Matt, dark surfaces
       • Emit heat (gives out heat)
       • Absorb heat
     • Shiny, light surfaces
       • Good reflectors of heat
         • Can reflect heat away (so things keep cool) or back to the body (or what needs to be kept warm)
       • Not good at emitting (giving out) heat
   • Expansion
     • When particles are heated they vibrate more and so take up more space
       • Gas expands the most - lots of space between particles
   • Thermos flask
     • Cap
       • Stops convection - heat being lost to its surroundings
     • Outer case
       • Traps a layer of air - insulator, protects glass
     • Vacuum
       • No particles - prevents conduction, convection and evaporation
     • Silvered glass walls
       • Reflects the heat back to keep it warmer
     • Insulated support
       • Protects the glass