Yr 11 Physics iGCSE: Revision Notes 3

Formulae you will need to know are in bold type.

CONTENTS

1. PRESSURE AND GASES
2. TURNING EFFECTS OF FORCES
3. CIRCULAR MOTION
4. ASTRONOMY
5. LIGHT AND SOUND
6. MAGNETISM AND ELECTROMAGNETISM
7. ELECTROMAGNETIC INDUCTION

1. PRESSURE AND GASES

• 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 = 8cm of water (= 0.08m X 1,000 kg/m3 X 9.8 N/kg  =  780 Pa), this is the

pressure above external air pressure, so total gas pressure = A.P. + 780 Pa = 101,000 Pa + 780 Pa = 101780 Pa.

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

the container - 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.

• pressure law:               p1  /  T1   =   p2  /  T2,     

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

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2. Turning effect 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.  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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4.  Astronomy

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

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

• The bigger the masses of the bodies the bigger the force of gravity between them.

• As the distance between two bodies increases, the force of gravity between them decreases.

• The orbit of any planet or comet is an ellipse (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.

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

   must move at a particular speed around larger bodies.

•  orbital speed  =  orbit circumference / time of orbit ,         v  =   2 π r  / T

• Communications satellites are usually put into a geostationary orbit above the equator.

• Data collection satellites are usually put into a low polar orbit.

• our solar system is in the Milky Way galaxy

• a galaxy contains billions of stars

• the universe is a collection of billions of galaxies

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5. 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.

• 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

•  Snell’s Law: you need to be able to describe the method we used to get  n   =  sin i  / sin r ,

  (apparatus: ray box, rectangular glass block, protractor, pencil, ruler) 

•  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˚)

   Use: sin c  =  1 / n  , to work out critical angle c

•  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

• Communication signals may be analogue (continuously varying) or digital (on or off only).

   Digital signals are less prone to interference than analogue, carry more information,

   and can be easily processed by computers.

                    ANALOGUE:                                                                 DIGITAL:

   

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

•  Pitch means the same as frequency, but has no scale

•  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 an experiment to measure the speed of sound in air.

• Diffraction occurs with sound and light waves, it is the bending of waves around an obstacle

  OR the spreading out of waves after passing through an aperture:

Examples of wave diffraction: 1st diagram is of waves travelling through a gap. The waves bend round the gap when they pass through the gap, forming semi-circular waves. The 2nd diagram shows waves bending round the edge of a barrier. The 3rd diagram shows waves passing a barrier - the waves bend round the edges of the barrier.

  Diffraction is more noticeable when the gap width is similar to the wavelength of the waves.

  Diffraction of light is harder to observe as the wavelength of light is so small.

 Radio waves on the other hand have a much bigger wavelength and easily diffract around hills.

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6. 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:

•  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

•  3 electromagnetic fields you need to know:

     STRAIGHT WIRE                SHORT COIL                         SOLENOID

• the fields above only exist when the current flows

• 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:

    

 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.

•  This effect causes 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.

•  The force also acts more fundamentally on a charged particle moving through a magnetic

   field (force is most when the particle moves at right angles to the field, zero when it moves

   parallel to the field).

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7. 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 close to a coil, the magnet's field lines

  then cut through the coil as it spins and an ac is produced - X is a magnet:

•   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  =  ε Ip Vp 

where ε = efficiency of the transformer, for an ideal transformer ε = 1.

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