Further Physics (P3)



* X-rays are part of the elctromagnetic spectrum. They sit between Gamma rays and Ultraviolet waves. Properties of X-rays include:

  - They have a very high frequency and a very short wavelength (about the size of an atom)

  - They affect photographic film in the same way as light

  - They are absorbed by metal and bone and are transmitted by soft tissue

* They cause ionisation as they can knock off electrons from an atom.

* X-rays can be used to diagnose and treat some medical conditions. Examples include CT scans, bone fractures, dental problems and killing cancer cells. However, they can also cause cancer.

* A CT scanner uses X-rays to produce digital images of a cross-section through the body. They produce a higher dose of X-rays and can produce a 3D image, however they are more expensive.

* Charged-Coupled Devices (CCDs) can be used to form electronic images of X-rays.

* Precautions need to be taken when X-ray machines and CT scanners are in use: Lead Shielding (absorbs X-rays), Film Badges (contain photographic fim to show how many X-rays a person has been exposed to).

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* The range of human hearing is about 20Hz - 20,000Hz.

* Ultrasound waves have frequencies higher than the upper limit of human hearing.

* Ultrasound waves are partially reflected when they meet a boundary between two different media. The time taken for the reflections to reach a detector can be used to determine how far away such a boundary is.

* To take an ultasound scan, we need a transducer to send the waves and a screen to display the image.

* Ultrasound waves are partially reflected when they meet different media. The time taken for the reflections to reach the detector are measured. Each boundary distance is measured, and an image is formed.

* To calculate the distance between interfaces in various media, we use the following formula; s (m) = v (m/s) x t (s)

* Ultrasound waves can be used in medicine:

   - Pre-natal Scanning

   - Removal of Kidney Stones

* Ultrasound is non-ionising and it produces accurate images of soft tissue, unlike X-rays.

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* Refraction is the change of direction of light as it passes from one medium to another.

* A lens form an image by refracting light.

* Refractive Index (n) = The measure of how much a substance can refract a light ray.

* When light travels through something more dense than air, it changes angles (near to 90*)

* To calculate Refractive Index, we use this formula: Sin i / Sin r (i= a.o.Incidence, r= a.o.Refraction)

* In a Convex (Converging) lens, parallel rays of light are brought to a focus at the principal focus. The distance from the lens to the principal focus is called the focal length.

* Convex lenses are used in magnifying glasses (to produce a virtual image) and in camera lenses (to produce a real image)

* Concave (Diverging) lenses always produce virtual images.

* The magnification produced by a lens is calculated using the equation:                                     Image Height / Object Height

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The Eye

* The structure of the eye is limited to:

  -> Retina ---> Lens ---> Cornea ---> Pupil ---> Iris ---> Ciliary Muscle ---> Suspensory Ligaments

* Lenses can be used to correct vision using convex and concave lenses to produce an image on the retina:

   - Long sight, cause by the eyeball being too short or the eye lens being unable to focus, can be helped with Converging lenses.

   - Short sight, caused by the eyeball being too long or the eye lens being unable to focus, can be helped with diverging lenses.

* The human eye has a range of vision from aproximately 25cm (near point) to infinity (far point)

The Camera - Coverging lens fixed focus, focus acheived by varying lens position, image focused on film or CCD, brightness controlled by aperture and diaphragm.

The Eye - Converging lens with variable focus, focus achieved by contraction of ciliary muscle, Image focused on retina, brightness controlled by iris and pupil.

* The the camera and the eye produce an image that is REAL, INVERTED and DIMINISHED.

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Focal Length and Lens Power

* To calculate the power of a lens, we use the equation; P (D) = 1 / f (m)

* The power of a lens is measured in Dioptres (D).

* The power of a converging lens is positive and the power of a diverging lens is negative.

* The focal length of a lens is determined by:

   - The curvature of the two surfaces of the lens (thickness) - The focal length becomes shorter

   - The refractive index of the material from which the lens is made - The higher the refractive index, the shorter the focal length.

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Other Applications using Light

* The critical angle is the angle of incidence of a light ray in a transparent substance which produces feraction along the bondaryb.

* Total internal reflection occurs when the angle of incidence of a light ray in a transparent substance is greater than the critical angle.

* Visible light can be sent along optical fibres.

* An endescope is a device used to look inside a patients body without cutting it open.

    l---> It contains bundles of optical fibres which aremade from glass, very thin and flexible.

* A laser produces an intense narrow beam of light. It may be used as an energy source for cutting through materials, burning and cauterising (destroy dammaged tissue/stop bleeding)

* During laser eye surgery, a laser light passes straight through the cornea at the front of the eye but is absorbed by the retina at the back

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Centre of Mass

* The centre of mass of an object is that point at which the mass of the object may be thought to be concentrated.

* You can find the centre of mass of a thin irregular sheet of material:

    - Suspend the thin sheet from a pin held in a clamp stand so it is able to turn.

    - When it comes to rest, hang a plumbline from the same pin.

    - Mark the position of the plumbline against the sheet

    - Hang the sheet from another point and repeat

    - The centre of mass is where the lines that marked the position of the plumbline cross.

* If freely suspended, an object will come to rest with its centre of mass directly below the point of suspension.

* The centre of mass of a symmetrical object is along the axis of symmetry.

* For a simple pendulum, we can calculate the period (time taken for each cycle): T (s) = 1 / f (Hz)

* The time period depends on ther length of a pendulum. The longer the pendulum, the longer the time.

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* Moment = The turning affect on a force.

* The size of a moment depends on two factors:

   - The size of the force applied

   - The perpendicular distance from the pivot to the line of action of the force

* Moment can be calculated using; M (Nm) = F (N) x d (m)

* If an object is not turning, the total clockwise moment must be exactly balanced by the total anticlockwise moment about any pivot.  > Clockwise Moment = Anti-Clockwise Moment

* If an object is in equilibrium, it isn't turning.

* A lever is a simple machine that makes work easier to do.

* Leavers reduce the force needed to do these tasks.

* When someone uses a leaver, they exert a force around a pivot to move an object.

* Leavers act as force multipliers - They reduce the effort needed to move the load by increasing the distance over which it is acting.

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* Liquids are virtually incompressible, and the preassure in a liquid is transmitted equally in all directions.

        l---> A force exerted at one point on a liquid will be transmitted to other points in the liquid.

* The use of different cross-sectional areas on the effort and load side of a hydraulic system enables the system to be used as a force multiplier.

* The pressure in different parts of a hydraulic system is given by: P (Pa) = F (N) / A (m2)

Pressure is increased by:

    - A bigger force

    - A smaller area

Hydraulic system uses the pressure in a fluid to exert a force.

* Hydraulic Machines: Diggers, Car jack, Car breaking systems

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Circular Motion

* When an object moves in a circle it continuously accelerates towards the centre of the circle. This acceleration changes the direction of motion of the body, but not its speed.

* Objects travelling in a circular motion are prevented from moving off in a straight line by Centripetal Force.

* The resultant force causing this acceleration is called the centripetal force and is always directed towards the centre of the circle.

* The centripetal force needed to make an object perform circular motion increases as:

    - The mass of the object increases

    - The speed of the object increases

    - The radius of the circle decreases

* A satellite orbiting the Earth has Centripital Force acting on it, and the force that keeps it in place is gravity. For another given object, Centripetal Force can be caused by Friction.

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* When a current flows through a wire, a magnetic field is produced around the wire.

* When a magnetic field is produced around a wire, it is only present whilst current is flowing. This effect is used in electromagnets.

* An electromagnet consists of a coil of insulated wire wrapped around a soft iron core.

* The size of the force can be increased by:

    - Increasing the th number of coils

    - Increasing the size of the current

* Applications of electromagnets includes their use on cranes for lifting iron/steel.

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The Motor Effect

* A simple electric motor can be built using a coil of wire that is free to rotate between two opposite magnetic poles.

* When an electric current flows through the coil, the coil experiences a force and moves. This is called the Motor Effect.

* A magnetic field goes from North to South.

* We can identify the direction of the force using Fleming's Left Hand Rule:

    - Thumb = Direction of Movement

    - First Finger = Direction of Magnetic Field

    - Second Finger = Direction of Current

* The direction of the force is reversed if either the direction of the current or the direction of the magnetic field is reversed.

* When a current passes through a coil, it spins as:

   - A force acts on each side of the coil due to the motor effect

   - The force on one side of the coil is in the opposite direction to the force on the other side.

* Force can be increased by: Increasing current, Increasing magnetic field.

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* If an electrical conductor 'cuts' through a magnetic field a potential difference is induced across the ends of the conductor.

* If a magent is moved into a coil of wire a potential difference is induced across the ends of the coil.

* A transformer consists of a soft iron core with two coils of insulated wire wrapped separately around it. Each coil has a different number of turns.

* A transformer changes the potential difference of electricity.

* It only works with alternating current electricity.. The primary coil is connected to an a.c. supply - it acts like an electromagnet. The secondary coil is where an alternating potential difference is induced.

* A step-up transformer increases the potential difference, whereas a step-down transformer reduces the potential difference.

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How Transformers Work

* Insulating wires carry an alternating current in the primary coil.

* This produces a changing magnetic field in the iron core.

* This produces a changing magnetic field in the secondary coil.

* This induces or generates an alternating potential difference or voltage across the secondary coil.

* ... and so an alternating current in the seconary coil .

The Potential Difference across the primary and secondary coils of a transformer are related by the equation :

   Vp = Np  (V = voltage)

   Vs    Ns  (N = Number of turns)

* If transformers are assumed to be 100% efficient, the elctrical power output would equal the electrical power input.

  Vp x Ip = Vs x Is   (V = P.D. / I = Current, A)

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Switch Mode Transformers & The National Grid

* Switch mode transformers operate at high frequency, often between 50KHz and 200KHz.

* They are much lighter and smaller than traditional transformers working from a 50Hz mains supply (UK mains frequency).

* They use very little power when they are switched on but no load is applied.

* They are often found in laptop and mobile phone chargers.


The National Grid

* Transformers are an essential part of the National Grid.

* They help to increase the voltage after the power station and help to decrease it again when it gets to your house.

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