Yr 11 Physics GCSE Revision Notes 2018

NB. YOU WILL ALSO NEED THE YEAR 9 AND THE YEAR 10 NOTES,

AND THE FORMULAE SHEET!

Formulae you will need to know are in bold type.

CONTENTS

1. KINETIC THEORY, PRESSURE, AND GASES
2. TURNING EFFECTS OF FORCES
GEARS
4. CIRCULAR MOTION
5. ASTRONOMY
6. LIGHT AND SOUND
7. INFRA RED RADIATION
8. MAGNETISM AND ELECTROMAGNETISM
9. ELECTROMAGNETIC INDUCTION

1. PRESSURE AND GASES

  basics of kinetic theory - there are 4 states of matter

 • In solids particles vibrate about fixed points. A temperature rise provides more KE for particles

    and causes bigger vibrations, and expansion occurs.

 • If melting temperature is reached, then heat energy supplied (latent heat of fusion) goes to weaken bonds (increasing particle freedom)

    NOT to raise the temperature 

 • In liquids particles have more energy and vibrate about variable points. Again a temperature rise provides more KE for particles

    and causes bigger vibrations, and expansion occurs

  • If boiling temperature is reached, then heat energy supplied (latent heat of vaporisation) goes to break bonds completely, NOT to raise the temperature

 • In gases particles are completely free, they move randomly in straight lines, but experience frequent collisions with the container walls.

    The higher the temperature of the gas, the higher the  KE of the particles, and the faster the particles move.

 • In the 4th state of matter, a plasma, temperature and particle KE is so high that ionisation occurs. Individual gas atoms

     get broken up into a plasma of positive and negative ions.

 • pressure (in Pa)  =  Force (in N)  / Area (in m2),      p  =  F / A

 • In liquids the pressure at a particular depth acts equally in all directions

 • In liquids the pressure increases with depth, so in this HEP system the turbines

 are placed as low as possible:

  • Pressure Difference (Pa)  =  Height (m)  X  Density (kg/m3) X  g (N/kg),      p  =  h ρ g

  • a manometer is used to measure the mains gas pressure, eg in the example below

   the gas pressure = 4cm of water (= 0.04m X 1,000 kg/m3 X 9.8 N/kg  =  390 Pa), this is the

   pressure above external air pressure, so total gas pressure = A.P. + 390 Pa = 101,000 Pa + 390 Pa = 101,390 Pa.

  • Gases exert pressure because their molecules continually bombard the walls of  the container

    - experimental evidence: Brownian motion

  • Boyle's Law explains how changing the pressure (only) changes the volume

    ‘pressure x Volume  =  constant (for a constant mass of gas at constant temperature)

             p1  X  V1  =  p2  X  V2

  eg. doubling the pressure would halve the volume.

  •  as temperature increases average KE of particles increases, so collisions are more forceful

    and more frequent, so the pressure increases.

  •  the lowest attainable temperature is called absolute zero  0 K  =  - 273 0C

 at absolute zero molecules stop moving so average KE of molecules is zero.

 Temperature in K = temperature in 0C + 273,  Temperature in 0C  = temperature in K – 273

  •  pressure is proportional to absolute temperature (Kelvin Temperature), so doubling the

   Kelvin temperature would double the pressure.

    

  • the Kelvin temperature of a gas is proportional to the average KE of its molecules:

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 2. TURNING EFFECTS OF FORCES

  • The turning effect of a force is called the moment.

  • The turning moment is given by the equation:

  moment = force Χ perpendicular distance from the line of action of the force to the pivot

 (newton metre, Nm) (newton, N)  (metre, m)

  • The centre of mass of a body is the point through which its weight acts.

  • If suspended, a body will come to rest with its centre of mass directly below the point of suspension.

 • If a body is balanced, the total clockwise moment = the total anticlockwise moment.

 

 eg. W1 d1  =  W2 d2  (P is the pivot)

 • Stability of a body is improved with a lower centre of mass or a bigger base area.

 • If the line of action of the weight of a body lies outside the base of the body there will be a

  resultant moment making it topple:

           line of action of weight

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3. GEARS

Gears can be used to:

   • reverse rotation direction eg. if A is turned clockwise, B turns anticlockwise, and C turns clockwise

   •  increase the turning moment eg. in the example above R = 2r, so if D is turned the turning moment acting on wheel E is doubled

   • change speed of rotation or number of rotations per second. D has 12 teeth but E has 24 teeth giving a ratio of 1: 2.

eg. if D is rotated at 20 rpm then E will turn at 10 rpm – a speed reducer.

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4.  CIRCULAR MOTION

• When a body moves in a circle it continuously accelerates towards the centre of the circle.

    This acceleration changes the direction of motion of the body, so its velocity changes,

    but not its speed.

• The resultant force causing this acceleration is called the centripetal force (F).

• The direction of the centripetal force is always towards the centre of the circle.

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5.  ASTRONOMY

• The Earth, Sun, Moon and all other bodies attract each other with a force called gravity.

• Your weight, or the force of gravity on you, would be different on different planets because of the variation in gravitational field strength g

• Gravity causes: moons to orbit planets, planets and comets to orbit the sun, satellites to orbit the earth

• You should know basic structure of the solar system: 8 planets with their natural satellites (moons), asteroids, comets, dwarf planets like Pluto.

 • You should know that until mid sixteenth century it was thought that the Earth (geocentric theory) was at the centre of solar system not the Sun (heliocentric theory),

 • the geocentric theory (central Earth) was believed because: the Sun, Moon, planets and stars all appeared to revolve around the Earth, AND to an

   observer on Earth, Earth appears to be stationary. Also, the church was a strong supporter of a central Earth.

   • Detailed observations of planets show a complex to and fro motion not explained by the geocentric model. Also, if the Earth is rotating then much

     of the geocentric evidence is unsound.  

• Some artificial satellites / moons / planets can have near circular orbits OR they may be elliptical orbits

• Comets tend to be elliptical (slightly squashed circle), with the Sun at one focus.

• The further away an orbiting body is the longer it takes to make a complete orbit.

•  For circular orbits the centripetal force is provided by gravity

• To stay in stable orbit at a particular distance, smaller bodies, including planets and satellites,

   must move at a particular speed around larger bodies. This speed decreases as r increases.

Cosmology, origins and evolution of the Universe: Steady State vs. Big Bang Theory

• If a wave source is moving relative to an observer there will be a

  change in the observed wavelength and frequency: called Doppler Shift:


 

• There is a red-shift in light observed from most distant galaxies. The further away galaxies are the bigger is the red-shift.

• Observations of the Sun show red shifted light from one edge, but blue shifted light from the opposite edge - why is this?

• You should do the example on the specimen paper 1 (P1, Qu.3)

• The observed red-shift provides evidence that the universe is expanding and supports both the ‘Big Bang’ theory (that the universe

   began from a very small initial point), but also the Steady State Theory (since the density of the universe would be constant in an

   expanding universe, due to the continuous creation of matter).

• Further evidence came with the discovery of microwave radiation coming from all directions of the sky. This cosmic background

   radiation (CMB) could be the radiation left over from a 'Big Bang' (explosion) type of event, its wavelength, very short at first, would

  have increased over time to match the value we see today. The Steady State theory fails to give a convincing explanation for this, making

  the Big Bang Theory currently the widely accepted theory for the evolution of the universe.

• stars are able to maintain their energy output for billions of years (our Sun at 5 billion years old is about half way through it's life)

• Stars form when enough dust and gas from space is pulled together into a lump, by gravity (see diagram below)

• For most of it's life, gravitational forces balance radiation pressure to make a star stable.

• Be familiar with the life cycle of a star:

• Fusion processes in stars create elements, eg helium is created from hydrogen

• These elements may be distributed throughout the Universe by the explosion of a star (supernova) at the end of its life.

• The lifetime of a giant star is much shorter than our Sun. Despite their greater mass (amount of fuel),

    the conversion of mass to energy (fusion) occurs at a collassal rate

• optical telescopes: Earth based vs Space telescope - advantages and disadvantages of each type

   eg. space telescopes don't suffer from light pollution or atmospheric absorption but are much more expensive to

         set up and to maintain.

 

               http://www.skinnerscience.com/physics%20title%20page/j0083211.gif      http://www.jb.man.ac.uk/vlbi/images/tel72/thwett.gif

• Telescopes may detect visible light or other EM radiations such as radio waves or X-rays.

• radio telescopes: uses, eg. SETI

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6. LIGHT AND SOUND

• light waves are transverse, and sound waves are longitudinal

• Reflection: angle of incidence = angle of reflection

 we get a 'virtual' image in a plane mirror, the same distance behind the mirror as the object is in front.

• we get specular (perfect) reflection from a mirror, but diffuse (irregular) reflection from a rough surface

• we can use reflection to explain the colour an object appears to be.

    eg. if it appears red, then we say it reflects red light, but absorbs the other colours of light shining on it. 

   eg. if we shine red light on an object which then appears to be black, then the object has no red in it.

• Colour filters only allow their own colour of light to pass through.

      eg. a blue filter will absorb all light falling on it except blue which it transmits.

• Refraction: coming from a less dense to a more dense medium the ray bends towards the normal

   coming from a more dense to a less dense medium the ray bends away from the normal

•  The reason for refraction is that the light travels at different speeds in different materials,

    it is fastest in a vacuum, almost as fast in air, slower in water, slower still in glass.

   The denser the material, the slower the speed, and the greater the refraction (light bending) effect.

• Converging and diverging lenses work by refracting light

•  The shorter the focal length the more powerful the lens (and the thicker the lens also)

•  Critical angle is the maximum angle of incidence for which light inside the denser medium

 can still enter the less dense medium. (this happens when the angle of refraction = 90˚)

•  Total internal reflection occurs when the angle of incidence is greater than the

    critical angle for the material, then all the light is reflected inside the denser material.

•  You should know that fibre optics are used to transmit digital data via total internal reflection

                    ANALOGUE:                                                                 DIGITAL:

   

•  Like all waves, light, sound, and water waves carry energy away from a source (not matter)

•  Sounds are caused by vibrating objects : human audible frequency range = 20 Hz - 20,000 Hz.

•  A sound wave is carried through a material via vibrations of the particles of the material

•  Sound vibrations in air can be absorbed / transferred to surrounding materials

   eg. in the ear the eardrum is set vibrating at the same frequency as the incoming sound

•   If a sound wave passes into a more dense material it will speed up, and refraction (direction change) will occur

•  Sound of frequency > 20kHz is called ultrasound, sound of frequency < 20 Hz is called infrasound

•  Uses of ultrasound include sonar, foetal scans. Uses of infrasound include siesmic wave analysis.

•  A sound wave can be displayed by connecting a microphone to an oscilloscope, the frequency

   can be measured by first measuring the time period for 1 cycle, then using,    f  =  1 / T

• The greater the amplitude, the louder the sound (see above)

• You need to be able to describe experiments to measure both the speed of sound in air, and the speed of water waves.

   For water waves you could simply measure the time taken for the wavefront to travel a measured distance, the use v = d / t.

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7. INFRA RED RADIATION

• infra red radiation is the transfer of energy by electromagnetic waves

• If an object is hotter than its surroundings then it will emit infra red radiation

• If an object is cooler than its surroundings then it will absorb infra red radiation

• If an object is at the same temperature as its surroundings then it emits at the same rate it absorbs infra red radiation - it is in EQUILIBRIUM

   eg. if the average power it radiates is less then the average power it absorbs it will heat up

   eg. if the average power it radiates is more then the average power it absorbs it will cool down

• the temperature of the Earth depends on the balance between incoming radiation from the Sun and outgoing radiation

    This is how we explain the greenhouse effect 

• ALL wavelengths of electromagnetic radiation may be emitted by very hot objects (stars).

    Higher temperature objects (eg blue stars) are strong emitters of all wavelengths, but particularly the SHORT wavelengths

   Lower temperature objects (eg red stars) are strong emitters of LONG wavelengths

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8.  MAGNETISM AND ELECTROMAGNETISM

•  Magnets attract magnetic materials: iron, nickel, and cobalt.

  the nails and paperclips become temporary magnets 'induced' by the presence of the bar magnet.

•  NB. so called 'soft' magnetic materials make good temporary magnets, they do not retain their magnetism.

   To make permanent magnets 'hard' materials like steel are needed.

•  Magnets law: like poles repel, unlike poles attract

•  Magnetic field direction, lines go from North to South

•  The Earth magnetism can be accounted for by assuming a huge bar magnet, with S pole uppermost, at its centre:

•  a magnetic field is the region of the force a magnet can exert, it can be mapped either by

   using iron filings OR by using a plotting compass

•  Between 2 like poles is a point of zero resultant field: a neutral point

•  Between 2 unlike poles it is possible to have a large uniform field region eg.

    between the poles of magnadur magnets

•  2 electromagnetic fields you need to know:

     STRAIGHT WIRE                            SOLENOID

• The field around a straight wire is non uniform, the field inside the solenoid is uniform

• the direction of the field and current are related using the RIGHT HAND GRIP RULE,

   where for your right hand, Thumb = Current, Fingers = magnetic field

• the fields above only exist when the current flows, then the greater the current the stronger the field

• to create a permanent magnet a 'hard' material, like steel, is inserted into the solenoid above

   and the current is switched on briefly

• When a current carrying wire lies inside a magnetic field it experiences a force

    (an equal but opposite force acts on the magnet)

•  it can be explained by the interaction of the wire's magnetic field with that of the permanent magnet:

    

•  The direction of the force is given by Fleming’s left hand rule : thumb = force direction,

        1st finger = field direction,  2nd finger = current direction.

•  the size of the force is increased by increasing the current, increasing the field strength,

   or using a longer wire.

•  when the wire is at right angles to the magnetic field, the force can be calculated from  F  =  B I L 

     where B = magnetic field strength,  I  = current,    L =  length of wire

     magnetic field strength B has the units Tesla T. 

•  This effect can cause a turning force (turning moment) to make a motor coil spin, and when

    ac is used in a speaker coil it causes the coil to vibrate so producing sound waves.

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9. ELECTROMAGNETIC INDUCTION

•  when a wire is moved so as to cut through field lines a voltage is induced in the wire. If

    a complete circuit exists then an induced current will flow.

•  to increase the size of the induced voltage:  move the wire faster, use a stronger field,

    use a longer wire.

•  to reverse the direction of the induced voltage then either reverse the direction of motion

  of the wire or reverse the polarity of the field.

•  Alternatively a magnet can be moved so that its field lines cut through a coil:

   To increase the size of the induced voltage: move the magnet faster, use a stronger field,

    use a coil with more turns.

•  A practical generator usually involves spinning a magnet, X, close to a coil, the magnet's field lines

  then cut through the coil as it spins and an ac is produced (so called alternator)

•   a similar set up can be used in a dynamo to produce dc electricity

•  the direction of the induced current is always so as to oppose the effect that caused it

•   In a transformer (below), field lines coming out of the primary coil reach across and cut through

    the secondary coil. This causes an induced voltage, Vs in the secondary coil, since Vp is ac

    the primary field lines are continually moving causing Vs to be an ac as well.

  NB. the CORE is made of laminated IRON.

 

•   If  Ns  >  Np  ,  then  V >  Vp   output voltage is more than input voltage, we have a step up

•   If  Ns  <  Np  ,  then  V <  Vp   output voltage is less than input voltage, we have a step down

    you will need the turns ratio equation ,     V /  Vp   =   Ns / Np  ,

and the power equation,  Power out = Power in,     Is Vs  =   Ip Vp 

•  on the national grid, transformers are used to produce high transmission voltages. This allows lower

currents to be used, so less heating occurs and greater efficiency is achieved.

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