# Physics 3

Forces and Motion

Magnetism and Stars

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## Turning Force and Centre of Mass

Moment (Nm) = Force (N) X Perpendicular Distance (m) between line of action and pivot.

• The force on the spanner causes the moment/turning effect on the nut. A larger force = a large moment.
• Using a longer spanner the same force can exert a larger moment because the distance form the pivot is greater.
• To get the maximum moment (or turning effect) you need to push at right angles perpendicular to the spanner. Any other angle = a smaller moment because the perpendicular distance between the line of action and the pivot is smaller.
• Finding the Centre of Mass:
• Suspend a shape and a plumb line from the same point and wait until they stop moving. Draw a line along the plumb line and so the same thing again but from a different point. The Centre of Mass is where the lines cross
• For simple shapes you can draw all the line of symmetry and where the lines cross that is the Centre of Mass.
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## Balanced Moments and Stability

Total Anticlockwise Moment = Total Clockwise Moment

Force x Distance = Force x Distance

If the Total Anticlockwise Moments do not equal the Total Clockwise Moments, there will be a Resultant Moment

• The most stable objects have a wide base and a low centre of mass.
• An object will begin to tip over if its centre of mass moves beyond the edge of its base.
• This is because of moments: If the weight doesn't act in line with the pivot it will cause the a resultant moment which will either right the object or tip it over.
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## Circular Motion

• Velocity is the speed and direction of an object
• If an object is travelling in a circle then it is constantly changing direction and therefore is constantly accelerating.
• The force that acts towards the centre of a circle is and keeps an object moving in a circle is called the: Centripetal Force
• Centripetal Force depends on:
• Mass
• The heavier the object the bigger the centripetal force has to be that keeps it moving in a circle
• Speed
• The faster the object the bigger the centripetal force has to be that keeps it moving in a circle.
• The smaller the circle the object is moving in the larger the force needed to keep it going in a circular movement. - It has more turning to do.
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## Gravity and Planetary Orbits (1)

• Gravity is the Centripetal Force that keeps Planets in Orbit
• Gravity is the force of attraction between two masses.
• The larger the masses the greater the force between them
• The Gravitational force can act as a centripetal force.
• An orbit is possible when there's a balance between the forward motion of the object and the gravitational force pulling it upwards.
• Planets always orbit around stars - slightly elliptical
• The further away from the sun the longer the orbit takes.
• Gravity Decreases quickly as you get further away
• With stars and planets gravity is very bis and felt a long way out
• The closer you get to one the stronger the force of attraction
• Comets, moons, satellites, space stations are held in place by gravity
• The size of the force of gravity decreases very quickly with distance
• In Practice
• URANUS = long way away from the sun = weaker gravitation effect = bigger orbit so traces slower and takes longer
• MERCURY = Close to the sun so stronger gravitational effect = smaller orbit and takes less time to complete its orbit.
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## Satellites

• Artificial Satellites made by humans were sent up for:
• Monitoring the earths weather and climate
• Communications
• Space Research
• Spying
• Geostationary satellites
• Have high orbits above the equator which take 24 hours to complete
• They stay above the same point on the Earths surface
• Used for Communications as are always in the same place
• Low Polar Orbit Satellites
• Sweeps over both poles as the Earth rotates beneath it
• The orbit takes a few hours to complete
• Each time the satellite goes around the Earth it scans it which allows the whole surface of the planet to be monitored each day.
• Used for The Weather and Spying  as their low orbits give them a better view of Earth.
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## Images

A Real Image is an image formed by a lens or Concave Mirror that can be projected on to a screen - the eye

A Virtual Image, seen in a mirror or lens, from which light rays appear after being reflected from a mirror or refracted from the lens.

• A virtual image looks bigger + further than the object actually is.
• To describe an image properly you need to say:
• How big it is compared to the object
• Whether its upright or inverted
• Whether its real or virtual

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## Reflection and Refraction

• Reflection is what allows us to see objects as the light bounces of them and into our eyes.
• When light reflects of an uneven surface, light reflects off at all different angles and you get a diffuse reflection
• When light reflects from an even surface then the light it all reflected at the same angle and you get a clear reflection.
• The Law of reflection applies to every reflected ray:

Angle of Incidence = Angle of Reflection

• Refraction
• Refraction of light is when the light waves change direction as they enter a different medium
• This is caused entirely by a change in speed of the waves
• E.G Makes a pond look shallower than it is as light reflects of the bottom and speeds up when it reaches the air and leaves the water.
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## Plane Mirrors

• The Image is the same size and the object
• The Image is as far behind the mirror as the object is in front of it
• The image is formed from diverging rays = a virtual image.
• Drawing a Ray Diagram
• The object the mirror and the eye are already draw for you.
• 1) Draw the virtual image the same size and distance from the mirror line as the object.
• 2) Draw the reflected ray from the top of the virtual image to the top of the eye. Use a dotted line from the virtual image to the mirror and a solid line form the mirror to the eye.
• 3) Draw the incident ray from the top of the object to the mirror where it meets the reflected ray. Use a solid line
• 4) Repeat steps 2 and 3 again for the bottom of the eye - the reflected ray going from the image to the bottom of the eye and the incident ray going from the object to the mirror.
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## Curved Mirrors

Concave Mirrors are a curved mirror with a surface that bends in and creates a Real Image

• Shiny on the inside of the curve.
• Light shining on a concave mirror converges.
• Centre of Curvature and Focal Point are in front of a concave mirror.

Convex Mirrors are a curved mirror with a surface that bends out and creates a Virtual Image

• Shiny on the outside of the curve.
• Light Shining on a convex mirror diverges.
• Centre of Curvature and Focal Point are behind a convex mirror
• The centre of the sphere is the centre of curvature and is marked with a C.
• The centre of the mirrors surface is called the Vertex.
• Halfway between the centre of curvature and the vertex is called the Focal Point - F.
• The centre of curvature, vertex and focal point lie on al line down the middle of the mirror called the Principle Axis
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## Concave Mirror Ray Diagrams

An Incident Ray parallel to the Principle Axis will pass through the focal point when it is reflected.

An Incident Ray passing through the focal point will be parallel to the Principle Axis when it is reflected.

• The Mirror, object, principle axis and focal point have been drawn for you
• 1) Draw a ray from the top of the object to the mirror parallel to the Principle axis which is then reflected back through the Focal Point
• 2) Draw a ray from the top of the object to the mirror going through the focal point and then is reflected back parallel to the Principle axis
• 3) Mark where the two rays meet - That is the top of the image.
• 4) Repeat the process for the bottom of the object. If the bottom of the object is on the bottom of the axis so will the images bottom be.
• If an object is at the Centre of Curvature you get a real upside down image the same size as the object.
• If an object is between C and F the image is real, upside down and bigger.
• If an object is closer to the mirror than the Focal Point you get a virtual image that is the right way up.
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## Convex Mirror Ray Diagrams

An Incident Ray parallel to the Principle Axis will reflect so that the reflected ray seems to come from the Focal Point.

An Incident Ray that can be extended to pass through the Focal Point will be parallel to the Principle Axis when it is reflected.

• The Mirror, object, principle axis and focal point have been drawn for you
• 1) Draw a ray going from the top of the object to the mirror parallel to the Principle Axis and is reflected down through the Focal Point
• 2) Draw another ray form the top of the object to the mirror and passing directly through to the focal point on the other side.
• 3)The Incident ray that passes through the Focal Point is reflected parallel to the Principle Axis
• 4) Mark where the two reflected rays meet behind the mirror. That is the top of the image - Repeat the process to find the bottom of the image.
• The image in a convex mirror will always be virtual, upright, smaller than the object and behind the mirror but closer than the focal point.
• The further away the object the smaller the image.
• You can see a wide area with a convex mirror which is why they put them on dodgy road corners.
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## Lenses (1)

• Light is refracted when it enters or leaves a glass prism
• Ray bends towards the normal as it enters the denser material
• Ray bends away from the normal as it emerges into a less dense
• Different wave lengths refract different amounts.
• Different Lenses produce different kinds of image
• Converging = Convex + Diverging = Concave
• Rules for Refraction of a Converging Lens
• An Incident ray parallel to the axis refracts through the lens and passes through the focal point on the other side.
• An Incident ray passing through the focal point refracts through the lens and travels parallel to the axis.
• An incident ray passing through the centre of the lens carries on in the same direction
• Rules for Refraction of a Diverging Lens
• An Incident ray parallel to the axis refracts through the lens and travels in line with with the focal point (appears to have come from it)
• An incident ray passing towards the focal point refracts through the lens and travels parallel to the axis.
• An incident ray passing through the centre of the lens carries on in the same direction.
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## A Ray Diagram on a Converging Lens

• The object, lens, axis and focal point are drawn for you
• 1) Draw a ray going from the top of an object parallel to the Principle axis to the vertical axis in the centre of the lens and then refracted through the focal point
• 2) Draw a ray going from the top of the object straight through the middles of the lens - It doesn't bend.
• 3) Mark where the rays meet - this is the top of the image.
• 4) Repeat the steps again to find the bottom of the image.
• The Distance form the lens effects the image
• An object nearer to the mirror an the Focal Point will make a virtual image that is the right way up and bigger than the object on the same side go the lens.
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## A Ray Diagram on a Diverging Lens

• The object, lens, axis and focal point are drawn for you
• 1)Draw a ray from the top of the object to parallel to the Principle Axis to the vertical Axis in the centre on the lens it is refracted up and outwards and appears to have come from the Focal Point.
• 2)Draw a ray passing directly through the middle of the lens so it doesn't bend.
• 3)Track back the rays to the original side of the mirror where the object is using a dotted line. Where the lines meet make a mark - that is the top of the image.
• 4)Repeat the process to find the bottom of the image
• The image is always virtual, the right way up and smaller and on the same side of the lens as the object no matter where the object is.
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## Uses of Lenses - Magnification and Cameras

• Magnifying Glasses
• Use Convex Lenses
• Creating a virtual image
• The object being magnified must be closer than the focal length.
• The Magnification Formula:

Magnification = Image Height / Object Height

• Taking a Photo
• Forms a Real Image on the film.
• The image is smaller that the object because it is a lot further away than the focal point of the lens.
• The image is inverted - upside down
• A real inverted image forms on your retina
• Your brain flips the image so that we see it the right way up
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## Sound Waves (1)

• Sound travels in waves
• Caused by vibrating particles which are passed through the surrounding medium in a series of compressions known as Longitudinal Waves.
• You hear when the sound reaches ear drums making them vibrate.
• The denser the medium, the faster the sound travels through it = faster in solids than in liquids and faster in liquids than in gasses.
• Sound Waves can Reflect and Refract
• Sound waves will by hard flat surfaces
• Things like Carpets will absorb sounds rather than reflecting them
• Sound waves will refract when they enter different mediums - as they enter denser material they speed up.
• Range of sound
• Frequency of wave is the number of waves in 1 second
• Humans can hear between 20 - 20000 Hz
• Sound does not travel in a Vacuum
• There are no particles to vibrate in a vacuum so there can be no sound.
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## Sound Waves (2)

• Sound Increases with Amplitude
• The greater the amplitude the more energy it carriers so the sound will be louder.
• The bigger the amplitude the louder the sound
• The Higher the Frequency the Higher the Pitch
• Higher frequency sound wave sound higher pitched - mouse
• Lower frequency sound wave sound lower pitch - cow
• High frequency = shorter wavelength
• Frequency is number of complete vibrations each second
• Common Units are kHz (1000 Hz) and MHz (1000000 Hz)
• The Quality of a note depends on the waveform
• Sine wave - Clear, pure sound
• Sawtooth wave - Buzzy, brassy sound
• Pulse wave - Thin, reedy sound
• Square wave - Hollow sound
• Triangle wave = Weak and Mellow
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## Ultrasound (1)

• Has a higher frequency than we can hear - above 20 kHz
• You can use CRO traces to compare amplitudes and frequencies
• Ultrasound get partially reflected at a boundary between media
• When a wave passes from one medium to another, some wave is reflected off the boundary of the two media and some is transmitted. This is called Partial Reflection.
• This means that you can point a pulse of ultrasound at an object and wherever the boundaries between one substance and another some ultrasounds can be reflected back.
• The time taken for the reflections to reach the detector can be used to measure how far away the boundary is this is called Ultrasound imaging.
• You can use Oscilloscope Traces to find Boundaries
• Using the formula Distance = Velocity X Time
• After sending an ultrasound wave into a block of metal and using the Oscilloscope to show when the pulses returned.
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## Ultrasound (2)

• Industrial Cleaning
• Clean delicate mechanisms without them having to be taken apart
• Can be directed into very precise areas and so are very effective
• High frequency vibrations of ultrasound make the equipment vibrate too and therefore the dirt on the equipment which is broken down by the vibrations and fall of the equipment.
• Same thing happens when dentists clean teeth
• Quality Control
• Ultrasound passing through a metal casing will be reflected and detected where it reached the boundary between two different media.
• Tee exact timing and distribution of these echoes can gives detailed into about its internal structure and can be processed by a computer
• Pre-Natal Scanning
• In the uterus there are boundaries between the amniotic fluid that the fetes floats in and the body tissues of the foetus
• As the ultrasound hit the boundary it is reflected back and the reflected waves are processed by the computer to produce a video image.
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## Magnetic Fields

A Magnetic Field is a region where magnetic materials like iron and steel and also wires carrying currents experience a force acting on them.

• Magnetic fields are represented by field diagrams - the field lines always point from the north pole of the magnet to the sought pole.
• The Magnetic field around a straight current carrying wire is made up of concentric circles with the wire on the centre.
• The Right Hand Thumb Rule shows with way the magnetic field goes - Your thumb points in the direction of the current.
• The magnetic field inside a solenoid - coil of wire - is strong and uniform
• The ends of the solenoid act like the poles of a bar magnet and so does the magnetic field outside the solenoid.
• You can increase the strength of the magnetic field around a solenoid by adding a magnetically soft iron core through the middle of the coil = This makes an Electromagnetic,
• Magnetically soft material magnetises and demagnetises very easily so as soon as you turn of the current in the solenoid the magnetic field disappears.
• Irons, Steel and Nickel are Magnetic
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## The Motor Effect

When a current is passed along a wire in a magnetic filed and the wire is not parallel to the lines of the magnetic field, a force is exerted on the wire by the magnetic field.

• A Current in a Magnetic Field feels a Force
• At all angles the wire will experience some force BUT:
• To experience the full force the wire has to be at a 90 Degree angle to the Magnetic Field
• If the wire runs along the magnetic field then it won't experience any force at all.
• The Force gets bigger if either the current or the magnetic field is made bigger.
• Flemming's Left Hand Rule tells you which way the force is acting:
• Point your First Finger in the direction of the Magnetic Field
• Point your SeCond Finger in the direction of the Current
• Your ThuMb is pointing in the direction of the Force (Motion)
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## The Simple Electric Motor

• 4 Factors which speed it up
• More Current
• More Turns on the coil
• A Stronger Magnetic Field
• A Soft Iron Core in the coil
• How does the Simple Electric Motor work
• The forces are the usual forces which act on any current in a magnetic field.
• The coil is on a spindle and the forces act as one up and one down, it rotates
• The split-ring commutator is a way of swapping the contacts every half turn to keep the motor rotating in the same direction.
• The direction of the motor can be reversed weather by swapping the polarity of the DC supply or swapping the magnetic poles over
• Electric Motors in every day life
• CD Players, Food Mixers, Fans, Printers, Drills, Hairdryers,
• Link the coil to an axel and the axel spins around.
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## Electromagnetic Induction

Electromagnetic Induction is the creation of a VOLTAGE (and maybe a current) in  wire which is experiencing a CHANGE IN MAGNETIC FIELD

• You can do this by moving a magnet inside a coil of wire or moving a conductor in a magnetic field. Shifting it from side to side creates a current.
• If you move the magnet in the opposite direction the voltage/current will be reversed to.
• If you reverse the polarity of the magnets then the voltage/current will be reversed to.
• If you keep moving a magnet backwards and forwards you produce a voltage that keeps swapping direction - an AC Current
• If you turn the magnet in the coil the magnetic field in the coil changes which induces a voltage which can make a current flow through the wire.
• 4 Factors that affect the size of the Induced Voltage
• The Strength of the Magnet
• The Number of Turns on the Coil
• The Area of the Coil
• The Speed of the Movement

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## Generators

• Rotate a coil in a magnetic field
• Have a construction like a motor
• As the coil spins the current is induced in the coil which changes direction every half turn.
• They have split rings and brushes so the contacts don't have swap every half turn.
• They produce an AC Voltage
• Dynamos
• You turn the magnet instead of the coil
• This still causes the field through the coil to swap every half turn so the output is the same a generator
• Dynamos are used on bikes to power lights. The cog wheel at the top is positioned so that it touches on of the wheels SO as the wheel moves round it turns the cog attached to the magnet = AC Current
• DISADVANTAGE is that when the wheels stop moving the light turns off for example if you stop at traffic lights someone might not see you = Dead
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## The Early Universe (2)

• When the Helium core runs out:
• Carbon, Oxygen and Neon combine to make silicon.
• In the biggest stars nuclei keeps on combining by fusion until they've formed an Iron Core.
• At the end of their lives massive stars fire gas out into space in explosions called SUPERNOVAS
• In the explosions heavy nuclei combine with each other and neutrons to make pretty much all the elements in the Universe.
• The dust and gas from the supernovas can form new stars and planets called SECOND GENERATION STARS which contain heavy elements as well as hydrogen.
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## Transformers (2)

• Transformers are used on the National Grid because:
• To transmit lots of power you either need high voltage or high current
• Hight current results in a loss of power due to resistance in cables
• Power Loss = Current Squared X Resistance
• It is much cheaper to have a high voltage (400000V) + a low current
• The transformers are used to step up the voltage before going onto the pylons for effective transmission and to step down the voltage before it enters your homes so it is safe and useable.
• The Transformer Equation:
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## Stars and Galaxies

• Stars form from cloud of gas and dust which spiral together due to gravitational attraction which compresses the matter so much that intense heat develops and sets of nuclear fusion reactions.
• At the same time other lumps may develop from the cloud which gather together to eventually from planet which orbit around the stars.
• Our SUN is one on millions of stars which form the Milky Way Galaxy
• Gravity is the force that keeps the stars together in a galaxy which rotates.
• Galaxies are often millions of times further apart than the stars within a galaxy so most of the universe is empty space.
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## The Early Universe (1)

• Right after The Big Bang only hydrogen was present - as the universe expanded they clumped together to form stars.
• In Stars cores hydrogen nuclei smash together to from helium nuclei
• When stars get older all the hydrogen in the sore turns to helium
• Helium then fuses to form other heavier elements:
• 3 helium nuclei combined together can make 1 Carbon Nucleus
• More helium combines with carbon to make oxygen and neon
• This all happens in GIANT RED STARS
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## The Life Cycle of Small Stars

• Clouds of Dust and Gas ---> From which stars are initially formed
• Protostar ---> The concentration of dust clouds and gas in a space that forms a star
• Main Sequence Stars ---> When the stars are in a stable period due to the large amount of hydrogen that they have in their core
• Red Giant ---> A star that has expanded and cooled resulting in it becoming red and much larger and cooler than it was before it expanded
• White Dwarf ---> A star that has collapsed from the red giant stage to become much hotter and denser than it was before
• Black Dwarf ---> When the light has faded completely from a white dwarf
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## The Life Cycle of Big Stars

• Clouds of Dust and Gas ---> From which stars are initially formed
• Protostar ---> The concentration of dust clouds and gas in a space that forms a star
• Main Sequence Star ---> When the stars are in a stable period due to the large amount of hydrogen that they have in their core
• Red Giant ---> A star that has expanded and cooled resulting in it becoming red and much larger and cooler than it was before it expanded
• Supernova ---> The explosion of a massive star after fusion in its core ceases and the matter surrounding the core collapse onto the core and rebounds.
• EITHER
• New Planetary Nebula
• New Solar System
• OR
• Neutron Star ---> The highly compressed core of a massive star that remains after a supernova explosion
• Black Hole ---> An object in space that has so much mass that nothing, not even light, can escape its gravitational field.
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