1. PHYSICS 3 FORCES AND WAVES & MAGNETISM AND STARS

2. FORCES & WAVES

3. Turning Forces A Moment is the Turning Effect of a Force the force causes a turning effect or moment, a larger force would cause a larger moment making the distance longer, the same force can make a larger moment because the distance from the pivot is greater moment (Nm) = force (N) x perpendicular distance (m) between line of action and pivot

4. Centre of Mass Centre of Mass Hangs Directly Below Point of Suspension the centre of mass of an object is the point at which the whole mass seems to be concentrated a freely suspended object will swing until its centre of mass is vertically below the point of suspension the objects weight acts at a distance from the pivot which creates a moment about the pivot, making it swing until the centre of mass is at its lowest (directly under the pivot) it rests in this position as there’s no moment, the pivot is in line with the line of action of the force Finding the centre of mass of a flat shape: 1) hang the shape and a plum line from the same point 2) draw along the line on the shape 3) do the same thing again but hang the shape from a different pivot point 4) the centre of mass is where the two lines meet freely suspended from this point centre of mass weight pivot weight pivot centre of mass

5. For The Seesaw two balance: total anticlockwise moments = total clockwise moments Balanced Moments & Stability If They’re Not Equal if the anticlockwise moments do not equal the clockwise moments, there will be a resultant moment so the object will turn an object will begin to tip over if its centre of the mass moves beyond the edge of its base if the weight doesn’t act in line with the pivot it’ll cause a resultant force, this will: put the object up right if the line of gravity is inside the base tip it over if the line of gravity is outside the base the most stable objects have a wide base and a low centre of mass Are The Moments Equal? if the anticlockwise moments are equal to the clockwise moments, the object won’t turn

6. Centripetal Force in a circular motion the velocity is always changing because the direction of motion is changing so it has acceleration even though the speed can be constant in the circular path there is a force acting toward the centre preventing the object from continuing outward and flying off, so the acceleration if towards the centre of the circle the force acting towards the centre of the circle is the centripetal force the centripetal force required will increase when: the mass is increased the radius is decreased the velocity is increased a car making a turn: the force of friction on the car’s wheels provide the centripetal force a bucket of water spun in a circle: the tension acting on the string provides the centripetal force the moon orbiting the Earth: gravity acting on the moon provides the centripetal force

7. Gravity & Planetary Orbits Gravity – Centripetal Force Keeping Planets In Orbit gravity is the force of attraction between masses, the larger the masses the greater the force of gravity between them gravitational force can act as the centripetal force that keeps one object moving in a circular path (orbit) round another an orbit is possible when there’s balance between the forward motion of the object and the gravitational force pulling it inwards not balanced: weaker centripetal force – planets will fly off stronger centripetal force – planets will be pulled inwards to the sun planets always orbit around stars these orbits are all slightly elliptical (elongated circles) with the Sun at one focus of the ellipse the further the planet is from the sun the longer its orbit takes Gravity Decreases with very large masses like stars and planets, gravity is very big and is felt a long way out the closer you get to a star or planet the stronger the force of attraction to counteract the stronger gravity, planets nearer the Sun move faster covering their orbit quicker comets, moons, satellites and space stations are all held in orbit by gravity the size of the force of gravity decreases very quickly with increasing distance weaker gravitational effect means bigger orbit, travels slower and takes longer to complete its orbit

8. Artificial Satellites Artificial Satellites were sent up by humans for: monitoring the Earth for things like weather and climate communications like phone and TV space research like the Hubble Space Telescope spying on baddies The two main kinds of orbits useful for satellites are: Geostationary Satellites they are high orbits over the equator which take exactly 24 hours to complete they stay above the same point on the Earth’s surface as the Earth rotates with them makes them ideal for communications as they’re always in the same place and they can transfer signals from one side to another really quickly Low Polar Satellites the satellites sweeps over both poles whilst the Earth rotates beneath each full orbit takes just a few hours each time it comes around it can scan the next bit of the globe, this allows the whole surface of the planet to be monitored each day as they are low they are useful for weather and spying

9. Light & Images Light light is a transverse wave which travels in a straight line Reflection allows us to see all objects smooth surface: rays of light are reflected off at the same angle, speed, frequency and wavelength as they approached it giving a clear reflection uneven surface: rays are reflected off at different angles so the reflection is diffused black surface: no reflection is produced as all light is absorbed Refraction when waves change direction as they enter a different medium caused by the change in speed of the waves incident ray: incoming light wave which hits the surface reflected ray: outgoing light wave which bounces of the surface normal: line perpendicular to the point the ray hits the surface angle of incidence: angle the incoming ray approaches the surface angle of reflection: angle the outgoing ray bounces off the surface angle of incidence = angle of reflection Real Images real light rays are converging and coming together they are brought to a focus and the image can be projected onto a screen Virtual Images formed when imaginary lines cross rays do not come from where the image seems to be image cannot be projected onto a screen real rays are diverging going different directions and moving apart

10. Plane Mirrors An image is formed as rays of light from an object hit the surface of the mirror, these rays of light are reflected back into the eyes and the brain interprets the image in picture form. 1) draw the virtual image 2) draw a reflected ray from the top of the virtual image to the top of the eye bold line: real ray between mirror and eye dotted line: virtual ray between mirror and virtual image 3) draw an incident ray from the top of the object to the point where the reflected ray meets the mirror 4) repeat steps two and three again for the bottom of the eye the image is formed upright but laterally inverted (left and right sides seem to be interchanged/swapped) image appeared to be exactly the same size and distance away from the mirror as the actual object virtual image: formed by diverging rays that do not cross, imaginary rays cross upright, virtual, same size, laterally inverted

11. Curved Mirrors C= centre of curvature: middle of the circle F= focal point: middle of C and V where the rays cross V= vertex: middle point of the mirror focal length: distance between focal point and mirror Convex Mirror: curve outwards shiny on the outside of the curve causes diverging rays C and F are behind the mirror Concave Mirror: curve inwards shiny on the inside of the curve causes converging rays C and F are in front of mirror

12. Convex Mirrors incident ray parallel to axis will reflect so that the reflected ray seems to come from the focal point incident ray that can be extended to pass through the focal point will be parallel to the axis when it’s reflected incident ray that aims at the virtual centre of curvature of the mirror is reflected back on its own path rays are diverging so the image must be virtual, it is also upright and diminished further away the object is the smaller the image a wide area can be seen using convex mirrors so they are used in security mirror and to see around corners 1) Mark C and F 2) Draw a parallel ray to the axis from the top of the object to the mirror and it will reflect as if it has come from the focal point 3) Draw a ray from the top of the object at an angle as if it were going to extend through the focal point and it will reflect parallel to the axis 4) Where the imaginary rays appear to cross, the top of the image is formed

13. Concave Mirrors At Centre Of Curvature inverted same size real image Between C and F inverted magnified real image Outside Centre Of Curvature inverted diminished real image At Focal Point image formed at infinity rays are parallel rays never meet Inside Focal Distance upright magnified virtual image parallel rays reflect through focal point angled rays through focal point reflected parallel

14. Lenses Refraction is when light passes from one medium to another and it bends as the speed of light changes because it passes through medium with different densities. when light passes from air to glass it bends towards the normal when light passes from glass to air it bends away from the normal Dispersion in a prism is caused by each of the light colours having a different wavelength and so different speeds in the medium. All colours of light have the same speed in a vacuum. When different coloured lights enter the glass they slow down and so are refracted. As each colour light slows down at specific amounts the colours will be refracted through different angles. red light has the longest wavelength in the visible spectrum and is slowed down the least, meaning red light is refracted through the smallest angle violet light has the shortest wavelength, as it enters it is slowed down the most and refracted through the biggest angle

15. Lenses critical angle is where the light is refracted perpendicular to the normal above this angle the whole light way is reflected internally which is total internal reflection total internal reflection can happen within a glass prism light in the glass hits the surface at 45° which is greater than the critical periscopes use, mirrors produce a poorer quality image and reflects less light than a prism if the prism is orientated differently, two total internal reflections can be achieved which reflects the light back towards its source this is used in cats eyes and reflective strips

16. Curved Lenses A lens can be made of glass or plastic, as they reflect light in the same way they have been incorporated into optical appliances like cameras, glasses and binoculars. Concave Lenses: DivergingConvex Lenses: Converging

17. Concave Lenses Concave lenses refract light causing them to diverge. They bend light outwards and become further apart or more spread out. incident ray parallel to axis refracts through the lens and travels so it appears to have come from the focal point incident ray passing towards the focal point refracts through the lens and travels parallel to the axis incident ray passing through the centre of the lens carries on in the same direction 1) draw a ray from the top of the object parallel to the lens 2) this is refracted so it appears to have come from the focal point, so draw a dotted ray from the focal point to the point on the lens and continue afterwards as a real ray 3) draw a ray from the top of the object going right through the middle of the lens and this doesn’t bend 4) where the rays cross is the top of the image Image Formed rays are diverging so a virtual image is formed upright diminished same side of the lens as the object

18. Convex Lenses Convex lenses refract light causing them to converge. They bend light inwards and become closer together. incident ray parallel to axis refracts through the lens and passes through the focal point on the other side incident ray passing towards the focal point refracts through the lens and travels parallel to the axis incident ray passing through the centre of the lens carries on in the same direction 1) draw a ray from the top of the object parallel to the lens 2) this is refracted so it passes through the focal point 3) draw a ray from the top of the object going right through the middle of the lens and this doesn’t bend 4) where the rays cross is the top of the image thicker lens, shorter focus length as it converges rays strongly Image Formed rays are converging so a real image is formed inverted magnified/diminished

19. Convex Lenses Between C and F inverted, magnified, real image opposite, greater distance At Focal Point rays are parallel and never meet Outside Centre Of Curvature inverted, diminished, real image opposite, between F and C Inside Focal Distance upright, magnified, virtual image same side, greater distance At Centre Of Curvature inverted, same size, real image opposite, same distance at C

20. Lens Uses Glasses Opticians use a series of complex tests to judge if a person needs glasses and the strength of glasses if they are needed. Camera film camera: film digital camera: charged couple device (CCD) unit a shutter controls the length of time light passes through the camera to the film by changing the distance between the lens and the film or CCD, the camera can focus on objects at different distances and a permanent image captured

21. Sound Waves Sound Travels As Waves caused by vibrating objects mechanical vibrations are passed through the surrounding medium as a series of compressions, they’re known as longitudinal waves the sound will eventually reach someone’s eardrum at which point the person might hear it if it’s loud enough and in the right frequency range denser the medium the faster sound travels through it, sound generally travels faster in solids than in liquids and faster in liquids than in gases Reflect & Refract they will be reflected by hard flat surface, things like carpets and curtains are absorbing surfaces which will absorb them they will refract (change direction) as they enter different media, as they enter denser material they speed up Sound Waves frequency of a wave (in Hz) is the number of waves in 1 second human ears are capable of hearing sounds with frequencies between 20Hz - 20000Hz they are transmitted by vibrating particles, so they can’t travel though a vacuum

22. Sound Waves Loudness Increases with Amplitude greater the amplitude of a wave the more energy it carries in sound this means it’ll be louder bigger amplitude mean a louder sound Higher Frequency Higher Pitch high frequency sound waves are high pitched low frequency sound waves are low pitched frequency is the number of complete vibrations each second common units are kHz (1000Hz) and MHz (1000000Hz) high frequency means shorter wavelengths Quality of a Note on a cathode ray oscilloscope, a clear pure sound produces a smooth rounded waveform called a sine wave other kinds of sounds produce different CRO traces Ramp Up: buzzy brassy sound Pulse: thin reedy sound Square: hollow sound Triangle: weak mellow wound

23. Ultrasound Ultrasound is a sound with a higher frequency than we can hear. The vibrations produced are higher than the range of human hearing (frequency above 20kHz) Partially Reflected at a Boundary Between Media when a wave passes from one medium into another, some of the wave is reflected off the boundary between the media and some is transmitted and refracted, this is partial reflection you can point a pulse of ultrasound at an object and wherever there are boundaries between one substance and another, some of the ultrasound gets reflected back the time taken for the reflections to reach a detector can be used to measure how far away the boundary is

24. Ultrasound Uses Ultrasound Vibrations Used in Industrial Cleaning they can be used to clean delicate mechanisms without them having to be dismantled waves can be directed onto very precise areas and are extremely effective at removing dirt and other stuff high frequency vibrations of ultrasound make the object vibrate at a high frequency causing the dirt on it to vibrate as well vibrations breaks up dirt and crud into very small particles which fall off the object this is sometimes used by dentist to clean teeth Ultrasound is Used in Industrial Quality Control ultrasound waves can pass through things like metal casting and when they reach a boundary between two different media some of the wave is reflected back and detected exact timing and distribution of these echoes give detailed information about the internal source echoes re processed by computers to produce a visual image of what the object must be like inside, if there are cracks they’ll show up Used for Pre Natal Scanning of a Fetus as the ultrasound hits boundaries between different media some of the wave is reflected back in the uterus there are boundaries between the amniotic fluid and the body tissues of the fetus reflected waves are processed by computers to produce a video image of the fetus image can be used to see if it is developing properly and what sex it is

25. MAGNETISM & STARS

26. 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. They can be represented by field diagrams where the arrows on the field lines always point form the north pole of the magnet to the south pole. Around A Wire Carrying Current there is a magnetic field around a straight current carrying wire the field is made up of concentric circles with the wire in the centre Around A Solenoid magnetic field inside a solenoid (coil of wire) is strong and uniform outside the coil, the magnetic field is just like one round a bar magnet the ends of a solenoid act like the north and south pole if the direction of the current is reversed the N and S poles will swap ends the direction of current flow at either end of the solenoid tells you whether it’s the N or S pole you can increase the strength of the magnetic field by adding a magnetically soft iron core or increasing the number of coils magnetically soft material magnetises and demagnetises very easily Isaac Newton Stole Cars Iron Nickel Steel Cobalt

27. Motor Effect Current in a Magnetic Field Experiences a Force passing an electric current through a wire produces a magnetic field around the wire if you put that wire into a magnetic field, you have two magnetic fields combining which puts a force on the wire to experience the full force, the wire has to be at right angles to the magnetic field if the wire runs along the magnetic field it won’t experience any force at all at angles in between it’ll feel some force Fleming's Left Hand Rule thuMb Motion: direction of the force (motion) First Finger Field: direction of the field seCond finger Current: the direction of the current

28. Simple Electric Motor the rotating coil has two sides with two forces acting each one, one up and one down because on each side the electric current is going in opposite direction the split ring communicator 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 either by swapping the polarity of the DC supply or swapping the magnetic poles 4 Factors Which Speed It Up more current more turns in the coil stronger magnetic field a soft iron core in the coil Uses Link the coil to an axle and the axle spins round. Electric motors are used in: cd players food mixers fan heaters fans printers drills hair dryers cement mixers

29. Electromagnetic Induction Electromagnetic Induction: the creation of a voltage (and maybe current) and in a wire which is experiencing a change in magnetic field Moving Magnet In A Coil Of Wire Induces A Voltage electromagnetic induction means creating a voltage (and maybe current) in a conductor, this can be done by moving a magnet in a conductor or moving a conductor in a magnetic field the wire cuts through a magnetic field line which makes the electrons in the wire move inducing (creating) a voltage by moving the magnet in the opposite direction or reversing the polarity of the magnet, the voltage or current will be reversed too if the magnet or coil moves backward and forwards, a voltage that keeps swapping is produces, alternating current 4 Factors to increase the voltage, increase: strength of the magnet area of the coil number of turns on the coil speed of movement

30. Generators AC Generators generators generate electricity by rotating a coil in a magnetic field as the coil spins it is cutting through magnetic field lines this induces a voltage which can make current flow in the wire when the coil turns half way the direction of the magnetic field on each side of the coil reverses, so the voltage reverses and the current flows in the opposite direction around the coil of wire creating an alternating current they have slip rings and brushes so the contacts don’t swap every half turn, the coil is free to move Dynamos dynamos are a type of generator that rotate the magnet instead of the coil this still causes the field through the coil to swap every half turn so it will also generate alternating current they are often used on bikes to power lights the cog wheel at the top is positioned to that it touches one of the wheels as the wheel moves around it turns the cog which is attached to the magnet

31. Transformers Electromagnetic Induction alternating current passes through the primary coil wrapped around a soft iron core the changing current produces a changing magnetic field in the primary coil this creates a changing magnetic field in the iron core which produces a changing magnetic field in the secondary coil this induces an alternating voltage in the secondary coil making an alternating current the number of turns in the two coils of wire determine whether the voltage induced in the secondary coil is greater or less than the voltage in the primary coil iron core is for transferring the changing magnetic field, no electricity flows through it Transformers Change AC Voltages a changing magnetic field is needed to induce a voltage so a alternating current must be used transformers have a primary and secondary coil which are joined with a soft iron core there are two types and they both use electromagnetic induction: step up transformers step the voltage up as they have more turns on the secondary coil than the primary coil step down transformers step the voltage down as they have less turns on the secondary coil than the primary coil

32. Transformers Transformers in the National Grid to get a lot of power you need either a high voltage or a high current the problem with high current is the loss of power (as heat) due to resistance of the cables, the higher the current the more heat is lost to reduce this the national gird transmits electricity at a lower current, this needs a high voltage the transformers have to step the voltage up at one end for efficient transmission then bring it back down to safe useable levels at the other end Primary Voltage Number of Turns on Primary Secondary Voltage Number of Turns on Secondary =

33. Galaxies Stars & Solar Systems stars form from clouds of gas and dust which spiral in together because of gravitational attraction gravity compresses the matter so much that intense heat develops and sets off nuclear fusion reactions (joining of small atomic nuclei) the star begins emitting light and other radiation as the the star is forming, other lumps can develop from the same spiralling cloud which eventually gather together and form planets which orbit around the star Milky Way the sun is one of billions of stars which form our galaxy distance between stars is usually millions of times further apart than the distance between planets in our solar system gravity is the force which keeps the stars together in a galaxy and they rotate like a wheel but much slower Whole Universe galaxies are billions of times further apart than the stars are within a galaxy the universe is mostly empty space and is really big Early Universe just after the Big Bang there was only hydrogen, as the universe expanded these atoms clumped together to form stars in the core of stars hydrogen nuclei smash together to form helium nuclei (nuclear fusion) and as it grows older all the hydrogen in the core turns into helium once the hydrogen runs out, helium nuclei fuse to form heavier elements, this happens in red giant stars eventually the helium runs out and the heavier elements combine to make even heavier elements, in the biggest stars nuclei keep on fusing until they’ve formed iron at the end massive stars explode flinging gases out into space, heavy nuclei combine with each other and neutrons to make all the elements in the universe dust and gas from these supernova explosions can form new stars and planets containing heavier elements as well as hydrogen

34. Beginning of Stars 1) stars form from a nebulae which is a collection of clouds of dust and gas 3) when the temperature is hot enough, nuclear fusion forms helium nuclei from hydrogen nuclei and gives out massive amounts of heat and light, creating a star 2) the force of gravity slowly pulls the particles together and as they move inwards their gravitational potential energy is converted into heat energy and the temperature rises and a protostar 4) it enters a long stable period where the heat created by the nuclear fusion provides an outward pressure to balance the force of gravity pulling everything inwards, this period is called a main sequence star as the balanced forces stop everything exploding outwards or collapsing inwards and because of the huge amount of hydrogen stars have this stable period can last millions of years 5) eventually the hydrogen runs out and the star swells into a red giant, it is red as the surface cools because nuclear fusion is happening less

35. Ending of Stars Big Stars 6) big stars begin to glow brightly again as they continue more nuclear fusion expanding and contacting several times, forming heavier elements 7) eventually they will explode in a supernova, throwing outer layers of dust and gas into space and these will eventually form second generation stars 8) this leaves a very dense core called a neutron star or if the star is big enough it could become a black hole Small Stars 6) small stars will begin to cool and contract into a dense white dwarf 7) then finally as the light fades completely it becomes a black dwarf Neutron stars, white dwarfs and black dwarfs are made from a matter which is millions of times denser than any matter on Earth because the gravity is so strong it even crushes the atoms.