Physics unit 3

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  • Created by: Megan
  • Created on: 10-04-13 15:50


1. X-rays are high frequency, short wavelength electromagnetic waves. Their wavelength is roughly the same size as the diameter of an atom

2. They are transmitted by healthy tissue, but absorbed by denser materials like bones and metal

3. They affect photographic film in the same way as light, which means they can be used to take photographs. 

4. X-ray photographs can be used to diagnose many medical conditions such as bone fractures or dental problems

5. X-ray images can be formed electronically using charge-couple devices. CCDS are silicon chips about the size of a postage stamp, divided up into a grid of millions of identical pixels. CCDs detect X-rays and produce electronic signals which are used to form high resolution images. The same technology is used to take photographs in digital cameras

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CT scans

Computerised axial tomography (CT) scans use X-rays to produce high resolution images of soft and hard tissue. The patient is put inside the cylindrical scanner, and an X-ray beam is fired through the body from an X-ray tube and picked up by detectors on the opposite side. The X-ray tube and detectors are rotated during the scan. A computer interprets the signals from the detectors to form an image of a two-dimensional slice through the body. Multiple two-dimensional CT scans can be put together to make a three-dimensional image of the inside of the body. 

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Treating cancer

X-rays can cause ionisation - high doses of X-rays will kill living cells. They can therefore be used to treat cancers, just like gamma radiation. The X-rays have to be carefully focused and at just the right dosage to kill the cancer cells without damaging too many normal cells


1. The X-rays are focused on the tumour using a wide beam

2. This beam is rotated round the patient with the tumour at the centre. 

3. This minimisesb the exposure of normal cells to radiation, and so reduces the chances of damaging the rest of the body. 

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Radiographers take precautions

Prolonged exposure to ionising radiation can be very dangerous to your health. 

1. Radiographers who work with X-ray machines or CT scanners need to take precautions to minimise their X-ray dose. 

2. They wear lead aprons, stand behind a lead screen, or leave the room while scans are being done. 

3. Lead is used to shield areas of the patient's body that aren't being scanned, and the exposure time to the X-rays is always kept to an absolute minimum. 

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Electrical systems can be made which produce electrical oscillations of any frequency. These can easily be converted into mechanical vibrations to produce sound waves of a higher frequency than the upper limit of human hearing (the range of human hearing is 20 to 20000 Hz). This is called ultrasound


  • When a wave passes from one medium into another, some of the wave is reflected off the boundary between the two media, and some is transmitted (and refracted). This is partial reflection
  • What this means is that 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 it takes for the reflections to reach a detector can be used to measure how far away the boundary is. 
  • This is how ultrasound imaging works. 
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Find boundaries

1. The oscilloscope traces on the right shows an ultrasound pulse reflecting off two separate boundaries. 

2. Given the 'seconds per division' setting of the oscilloscope, you can work the time between the pulses by measuring on the screen. 

3. If you know the speed of sound in the medium, you can work out the distance between the boundaries, using this formula: 

s = v x t 

s is distance in metres, m 

v  is speed in metres per second, m/s 

t is time in seconds, s 

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Ultrasound waves used in medicine

Ultrasound has a variety of uses in medicine, from investigating blood flow in organs, to diagnosing heart problems, to checking on fetal development. The examples below are two of the most common... 

BREAKING DOWN KIDNEY STONES - Kidney stones are hard masses that can block  the urinary tract. An ultrasound beam concentrates high-energy waves at the kidney stone and turns it into sand- like particles. These particles then pass out the body in the urine. It's a good method because the patient doesn't need surgery and it's relatively painless

PRE-NATAL SCANNING OF FETUS - Ultrasound waves can pass through the body, but whenever they reach a boundary between two different media (like fluid in the womb and the skin of the fetus) some of the wave is reflected back and detected. The exact timing and distribution of these echoes are processed by a computer to produce a video image of the fetus. 

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Medical images

Doctors have to make compromises between getting a good enough image to be able to diagnose problems, whilst putting the patient at as low a risk as possible. X-ray and ultrasound imaging both have their advantages and disadvantages... 


  • Ultrasound waves are non-ionising and, as far as anyone can tell, safe
  • X-rays are ionising. They can cause cancer if you're exposed to too high a dose, and are definetely NOT safe to use on developing babies. 
  • CT scans use a lot more X-ray radiation than standard X-ray photographs, so the patient is exposed to even more ionising radiation. Generally CT scans aren't taken unless they are really needed because of the increased radiation dose


  • Ultrasound images are typically fuzzy - makes it harder to diagnose conditions. 
  • X-ray photographs produce clear images of bone and metal
  • CT scans produce detailed images and ca be used to diagnose complicated illnesses. 
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Refraction is when waves change direction as they enter a different medium. This is caused by the change in density from one medium to the other - which changes the speed of the waves. 

1. When waves slow down they bend towards the normal. 

2. When light enters glass or plastic it slows down - to about 2/3 of its speed in air

3. If a wave hits a boundary at 90 degrees (i.e. along the normal) it will not change direction - but it'll still slow down. 

4. When light hits a different medium (e.g. plastic or glass) some of the light will pass through the new medium but some will be reflected - it all depends on the angle of incidence (the angles it hits the medium). 

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Refractive index

1. Refractive index of a medium is the ratio of speed of light in a vacuum to speed of light in that medium. 

2. The angle of incidence, i, angles of refraction, r, and refractive index, n, are all linked

3. When an incident ray passes from air into another material, the angle of refraction of the ray depends on the refractive index of the material: 


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Lenses form images by refracting light and changing its direction. There are two main types of lens - converging and diverging. They have different shapes and have opposite effects on light rays. 

  •  A converging lens is convex - it bulges outwards. It causes parallel rays of light to converge (move together) at the principal focus
  • A diverging lens is concave - it caves inwards. It causes parallel rays of light to diverge (spread out). 
  • The axis of a lens is a line passing through the middle of the lens. 
  • The principal focus of a converging lens is where rays hitting the lens is parallel to the axis all meet
  • The principal focus of a diverging lens is the point where rays hitting the lens parallel to the axis appear to all come from - you can trace them back until they all appear to meet up at a point behind the lens. 
  • There is a principal focus on each side of the lens. The distance from the centre of the lens to the principal focus is called the focal length
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Converging and diverging


  • An incident ray parallel to the axis refracts through the lens and passes through the principal focus on the other side. 
  • An incident ray passing through the principal focus refracts through the lens and travels parallel to the axis.
  • An incident ray passing through the centre of the lens carrie on in the same direction.


  • An incident ray parallel to the axis refracts through the lens, and travels in line with the principal focus (so it appears to have come from the principal focus). 
  • An incident ray passing through the lens towards the principal focus 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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Real and virtual images

1. A real image is where the light from an object comes together to form an image on a screen - like the image formed on an eye's retina (the screen at the back of an eye). 

2. A virtual image is when the rays are diverging, so the light from the object appears to be coming from a completely different place

3. When you look in a mirror you see a virtual image of your face - because the object appears to be behind the mirrror. 

4. You can get a virtual image when looking at an object through a magnifying lens - the virtual image looks bigger than the object actually is. 

To describe an image properly, you need to say 3 things: 1. How big it is compared to the object 2. Whether it's upright or inverted (upside down) relative to the object 3. Whether it's real or virtual. 

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Ray diagram (converging)

1. Pick a point on the top of the object. Draw a ray going from the object to the lens parallel to the axis of the lens. 

2. Draw another ray from the top of the object going right through the middle of the lens. 

3. The incident ray that's parallel to the axis is refracted through the principal focus (F). Draw a refracted ray passing through the principal focus

4. The ray passing through the middle of the lens doesn't bend. 

5. Mark where the rays meet. That's the top of the image

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Distance from the lens

1. An object at 2F will produce a real, inverted (upside down) image the same size as the object, and at 2F

2. Between F and 2F it'll make a real, inverted image bigger than the object, and beyond 2F

3. An object nearer that F will make a virtual image the right way up, bigger than the object, on the same side of the lens. 

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Ray diagram (diverging)

1. Pick a point on the top of the object. Draw a ray going from the object to the lens parallel to the axis of the lens.

2. Draw another ray from the top of the object going right through the middle of the lens. 

3. The incident ray that's parallel to the axis is refracted so it appears to have from the principal focus. Draw a ray form the principal focus. Make it dotted before it reaches the lens. 

4. The ray passing through the middle of the lens doesn't bend. 

5. Mark where the refracted rays meet. That's the top of the image.


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Magnifying glass (converging)

Magnifying glass work by creating a magnified virtual image. 

1. The object being magnified must be closer to the lens than the focal length

2. Since the image produced is a virtual image, the light rays don't actually come from the place where the image appears to be. 

3. Remember 'you can't project a virtual image onto a screen'. 

4. Magnification formula:


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Powerful lens

 1. Focal length is related to the power of the lens. The more powerful the lens, the more strongly it converges rays of light, so the shorter the focal length.        2. The power of a lens is given by the formula: 

POWER (D) = 1 / FOCAL LENGTH (m)  P = 1 / f 

 3. For a coverging lens, the power is positive. For a diverging lens, the power is negative.                                       4. The focal length of a lens is determined by two factors: a. the refractive index of the lens material, b. the curvature of the two surfaces of the lens.                                        5. To make a more powerful lends from a certain material like glass, you just have to make it with more strongly curved surfaces.                                                                 6. For a given focal length, the greater the refractive index of the material used to make the lens, the flatter the lens will be.        7. This means powerful lenses can be made thinner by using materials with high refractive indexes

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Structure of the eye

1. The cornea is a transparent 'window' with a convex shape, and a high refractive index. The cornea does most of the eye's focusing

2. This iris is the coloured part of the eye. It's made up of muscles that control the size of the pupil - the hole in the middle of the iris. This controls the intensity of light entering the eye. 

3. The lens changes shape to focus light from objects at  varying distances. It's connected to the ciliary muscles by the suspensory ligaments pull the lens into a thinner, flatter shape

4. Images are formed on the retina, which is covered in light-sensitive cells. These cells detect light and send signals to the brain to be interpreted. 

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Near and far points

1. The far point is the furthest distance that the eye can focus comfotably. For normally-sighted people, that's infinity

2. The near point is the closest distance that the eye can focus on. For adults, the near point is approximately 25 cm

3. As the eye focuses on close objects, its power increases - the lens changes shape and the focal length decreases. But the distance between the lens and the image stays the same

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A camera (similar to eye)

When you take a photograph of a flower, light from the object (flower) travels to the camera and is refracted by the lens, forming an image on the film. 

1. The image on the film is a real image because light rays actually meet there. 

2. The image is smaller than the object, becuse the object's a lot further away than the focal length of the lens. 

3. The image is inverted

4. The same thing happens in our eye - a real, inverted image forms on the retina. Our very clever brains flip the image so that we see it right way up. 

5. The film in a camera, or the CCD in a digital camera, are the equivalent of the retina in the eye - they all detect the light focused on them and record it. 

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Short sight (diverging lenses)

1. Short-sighted people can't focus on distant objects - their far point is close than infinity

2. Short site is caused by the eyeball being too long, or by the cornea and lens system being too powerful - this means the eye lens can't produce a focused image on the retina where it is supposed to. 

3. Images of distant objects are brought into focus in front of the retina instead. 

4. To correct short sight you need to put a diverging lens (with a negative power) in front of the eye. This diverges light before it enters the eye, which means the lens can focus it on the retina. 

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Long sight (converging lenses)

1. Long-sighted people can't focus clearly on near objects - their near point is further away than normal.

2. Long sight happens when the cornea and lens are too weak or the eyeball is to short.

3. This means that images of near objects are brought into focus behind the retina

4. To correct long sight a converging lens (with a positive power) can be put in front of the eye. The light is refracted and starts to converge before it enters the eye, and the image can be focused on the retina where it belongs. 

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Laser surgery

A laser is an narrow, intense beam of light. The light waves that come from a laser all have the same wavelength

1. Lasers can be used in surgery to cut through body tissue, instead of using a scalpel. 

2. Lasers cauterise (burn and seal shut) small blood vessels as they cut through the tissue. This reduces the amount of blood the patient loses and helps to protect against infection

3. Lasers are used to treat skin conditions such as acne scars. Lasers can be used to burn off the top layers of scarred skin revealing the less-scarred lower layers. 

4. One of the most common types of laser surgery is eye surgery. A laser can be used to vaporise some of the cornea to change its shape - which changes its focusing ability. This can increase or decrease the power of the cornea so that the eye can focus images properly on the retina

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Total internal reflection

 1. Optical fibres can carry visible light over long distance.                     2. They work by bouncing waves off the sides of a thin inner core of glass or plastic . The wave enters one end of the fibre and is reflected repeatedly until it emerges at the other end.                                     3. Optical fibres work because of total internal inflection.                                             4. Total internal reflection can only happen when a wave travels through a dense substance like glass or water towards a less dense substance like air.     5. It all depends on whether the angle of incidence is bigger than the critical angle...


 ...LESS than the critical angle: most of the light passes out but a little bit's internally reflected.              ... EQUAL to critical angle: the emerging ray comes out along the surface. There's quite a bit of internal reflection.                             ... GREATER than critical angle: no light comes out. It's all internally reflected, i.e. total internal reflection

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Refractive index

1. A dense material with a high refractive index has a low critical angle

2. If a material has a high refractive index, it will totally internally reflect more light - more light will be incident at an angle bigger than the critical angle. 

3. For example, the critical angle of glass is around 42 degrees, but for diamond the critical angles is just 24 degrees, so more light is totally internally reflected - which is why diamonds are so sparkly

4. Refractive index and critical angle (c) are related by this formula: 


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1. An endoscope is a thin tube containing optical fibres that lets surgeons examine inside the body

2. Endoscopes consist of two bundles of optical fibres - one to carry light to the area of interest and one to carry an image back so that it can be viewed. 

3. The image can be seen through an eyepiece or displayed as a full-colour moving image on a TV screen.

4. The big advantage of using ensocopes is that surgeons can now perform many operaitons by only cutting teeny holes in people - this is called keyhole surgery, and it wasn't possible before optical fibres. 

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The size of the moment of the force is given by: 

MOMENT = FORCE x perpindicular DISTANCE for the line of action of the force to the pivot

M =F x d 

M - Moment of the force in newton - metres (Nm)       F - Force in newtons (N)       d - Distance in metres (m)

1. The force on the spanner causes a turning effect or moment on the nut (which acts as a pivot). A larger force would mean a larger moment.       2. Using a loner spanner, the same force can exert a larger moment because the distance from the pivot is greater.                3. To get the maximum moment (or turning effect) you need to push at right angles (perpendicular) to the spanner.                4. Pushing at any other angle means a smaller moment because the perpendicular distance between the line of action and the pivot is smaller

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Centre of mass

1. You can think of the centre of mass of an object as the point at which the whole mass is concentrated. 

2. A freely suspended object will swing until its centre of mass is vertically below the point of suspension

3. This means you can find the centre of mass of any flat shape like this:         a. Suspend the shape and a plumb line from the same point, and wait until they stop moving.         b. Draw a line along the plumb line        c. Do the same thing again, but suspend the shape from a different pivot point.         d. The centre of mass is where your two lines cross

4. But you don't need to go to all that trouble for symmetrical shapes. You can quickly guess where the centre of mas is by looking for lines of symmetry

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Simple levers

Levers use the idea of balanced moments to make it easier for us to do work (e.g. lift an object): 

1. The moment needed to do work = force x distance from the pivot. So the amount of force needed to do work depends on the distance the force is applied from the pivot. 

2. Levers increase the distance from the pivot at which the force is applied - so this means less force is needed to get the same moment

3. That's why leavers are known as force multipliers - they reduce the amount of force that's needed to get the same moment by increasing the distance. 

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Unstable objects tip over easily - stable ones don't. The position of the centre of mass is all-important

1. The most stable objects have a wide bass and a low centre of mass

2. An object will begin to tip over if its centre of mass moves beyond the edge of the bass. 

3. Again, it's because of moments - if the line of action of the weight of the object lies outside of the base of the object, it'll cause a resultant moment. This will tip the object over. 

4. Lots of objects are specially designed to give them as much stability as possible. 

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1. A simple pendulum is made by suspending a weight from a piece of string. When you pull back a pendulum and let it go, it will swing back and fourth. 

2. The time taken for the pendulum to swing from one side to the other and back again is called the time period

3. The time period for each swing of a given pendulum is always the same

4. The time period can be calculated using this formula:


5. The time period of a pendulum depends on its length. The longer the pendulum, the greater the time period. So the shorter the length, the shorter the time period. 

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Liquids (incompressible)

1. Liquids are virtually incompressible - you can't squash them, their volume and density stay the same

2. Because liquids are incompressible and can flow, a force applied to one point in the liquid will be transmitted to other points in the liquid. 

3. Imagine a balloon full of water with a few holes in it. If you squeeze the top of balloon, the water will squirt out of the holes. This shows that force applied to the water at the top of the balloon is transmitted to the water in other parts of the balloon . This also shows that pressure can be transmitted throughout a liquid. 


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Pressure - pascals (Pa) 

Force - newtons (N) 

Cross - sectional area - metres^2 (m^2) 

F / P x A 

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Hydraulic systems

1. Hydraulic systems are used as force multipliers - they use a small force to produce a bigger force. They do this using liquid and a sneaky trick with cross - sectional areas

2. A simply hydraulic system has two pistons, one with a smaller cross-sectional area than the other. Pressure is transmitted equally through a liquid - so the pressure at both pistons is the same

3. Pressure = force / area, so at the 1st piston, a pressure is exerted on the liquid using a small force over a small area. The pressure is transmitted to the 2nd piston. 

4. The 2nd piston has a larger area, and so as force = pressure x area, there will be a larger force.  

5. Hydraulic systems are used in all sorts of things e.g. car braking systems, hydraulic car jacks, manufacturing and deployment of landing gear on some aircraft. 

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Circular motion

1. Velocity is both the speed and direction of an object. 

2. If an object is travelling in a circle it is constantly changing direction. This means its velocity is constantly changing (but not its speed) - so the object is accelerating.This acceleration is towards the centre of the circle. 

3. There must be a resultant force acting on the object causing this acceleration. This force acts towards the centre of the circle. 

4. This force that keeps something moving in a circle is called a centripetal force.

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Centripetal force

1. The faster an object's moving, the bigger the centripetal force has to be to keep it moving in a circle

2. The larger the mass of the object, the bigger the centripetal force has to be to keep it moving in a circle

3. And you need a larger force to keep something moving in a smaller circle - it has more turning to do. 

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Magnetic field


Magnetic fields can be represented by field diagrams. The arrows on the field lines always point from the North pole of the magnet to the South pole. 

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Magnetic field around current-carrying wire

1. When a current flows through a wire, a magnetic field is created around  the wire. 

2. The field is made up of concentric circles with the wire in the centre. 

Magnetic field round a coil of wire 

1. The magnetic field inside a coil of wire is strong and uniform

2. Outside the coil, the magnetic field is just like the one round a bar magnet

3. 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. It's then called an ELECTROMAGNET

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An electromagnet must be constantly supplied with current - as that's what produces the magnetic field. So if the current stops, then it stops being magnetic. Magnets you can switch off at your whim can be really useful…. 


Cranes used for lifting iron and steel - using an electromagnetic means the magnet can be switched on when you want to and attract and pick stuff up, then switched off when you want to drop it. Which is far more useful

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Motor effect

The force experienced by a current-carrying wire in a magnetic field is known as the motor effect

1. The force on a current-carrying wire placed in a magnetic field will get bigger if either the current or the magnetic field is made bigger. 

2.  Note the force on the wire is at 90 degrees to both the wire and to the magnetic field

3. If the direction of the current or magnetic field is reversed, then the direction of the force is reversed too. You can always predict which way the force will act using Fleming's left hand rule

4. To experience the full force, the wire has to be at 90 degrees to the magnetic field

5. If the wire runs along parallel to the magnetic field it won't experience any force

6. At angle in between it'll feel some force. 

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Fleming's left hand rule

1. Using your left hand, point your first finger in the direction of the field and your second finger in the direction of the current

2. Your thumb will then point in the direction of the force (Motion). 

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Electric motor


a. More current

b. Stronger magnetic field

1. There are forces acting on the two side arms of the coil of wire

2. These forces are just the usual forces which act on any current in a magnetic field

3. Because the coil is on a spindle and the forces act one up and one down, it rotates

4. The split-ring commutator is a clever way of swapping the contacts every half turn  to keep the motor rotating in the same direction

5. The direction of the motor can be reversed either by swapping the polarity of the direct current (DC) supply or swapping the magnetic poles over. 

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Electric motor uses


1. Link the coil to an axle, and the axle spins round

2. In the diagram there's a fan attached to the axle, but you can stick almost anything on a motor axle and make it spin around. 

3. For example, in a food mixer, the axle's attached to a blade or whisks. In a CD player the axle's attached to the bit you sit the CD on. Fan heaters and hair dryers have an electric heater as well as a fan. 

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Electromagnetic induction


For some reason they use the word induction rather than creation, but it amounts to the same thing

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Electromagnetic induction

1. Electromagnetic induction means creating a potential difference across the ends of a conductor.                2. You can do this by moving a magnet in a coil of wire or moving an electrical conductor in a magnetic field (cutting magnetic field lines). Shifting the magnet from side to side creates a little blip of current.       3. If you move the magnet in the opposite direction, then the potential difference/ current will be reversed too. Likewise, if the polarity of the magnet is reversed, then the potential difference/ current will be reversed too.        4. If you keep the magnet (or the coil) moving backwards and forwards, you produce a potential difference that keeps swapping direction - and this is how you produce an alternating current. 


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There are a few different types of transformer. The two you need to know about are step - up transformers and step - down transformers. They both have two coils, the primary and the secondary, joined with an iron core

STEP UP TRANSFORMERS -  Step the voltage up. They have more turns on the secondary coil than the primary coil. 

STEP DOWN TRANSFORMERS- Step the voltage down. They have more turns on the primary coil than the secondary. 

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Transformers (electromagnetic induction)

1. The primary coil produces a magnetic field which stays within the iron core. This means nearly all of it passes through the secondary coil and hardly any is lost.        2. Because there is alternating current in the primary coil, the field in the iron core is constantly changing direction - i.e. it is a changing magnetic field.        3. This rapidly changing magnetic field is then felt by the secondary coil.       4. The changing field induces an alternating potential difference across the secondary coil (with the same frequency as the alternating current in the primary) - electromagnetic induction of a potential difference in fact.        5. The  relative number of turns on the two coils determines whether the potential difference induced in the secondary coil is greater or less than the potential difference in the primary.        6. In a step-up transformer, the p.d. across the secondary coil is greater than the p.d. across the primary coil.        7. In a step-down transformer, the p.d. across the secondary coil is less than the p.d. across the primary coil.        8. If you supplied DC to the primary, you'd get nothing out of the secondary at all. 

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The iron core

1. The iron core is purely for transferring the changing magnetic field from the primary coil to the secondary. 

2. No electricity flows round the iron core

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The transformer equation

You can calculate the output potential difference from a transformer if you know the input potential difference and the number of turns on each coil. 


You can write this either way up. 

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Transformers are nearly 100% efficient

The formula for power supplied is: Power = current x potential difference or: P = I x V.

So you can write electrical power input = electrical power output as: 

VpIp = VsIs 

Vp = p.d. across primary coil (V)  Vs = p.d. across secondary coil (V)     Ip = current in the primary coil (A)      Is = current in the secondary coil (A)

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Switch mode transformers

1. Switch mode transformers are a type of transformer that operate at higher frequencies than traditional transformers.        2. They usually operate at between 50 kHz and 200 kHz.        3. Because they work at higher frequencies, they can be made much lighter and smaller than traditional transformers that work from a 50 Hz mains supply.        4. This makes them more useful in things like mobile phone chargers and power supplies, e.g. for laptops.                5. Switch mode transformers are more efficient than other types of transformer. They use very little power when they're switched on but no load (the thing you're charging or powering) is applied, e.g. if you've left your phone charger plugged in but haven't attached your phone. 

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Dom Townrow


Good source, copied the CGP book but this is better as the colours and pictures make the CGP book hard to read

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