AQA GCSE Physics Unit 3

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• Created by: Georgia
• Created on: 18-05-13 17:56

X-rays

X-rays are part of the electro-magnetic spectrum. They have a high frequency and a short wave length. They are used in hospitals to make images and CT scans and to destroys tumours at or near the body surface. X-rays and gamma radiations is dangerous as it ionises everything it passes through so it can kill living cells.

To make a radiograph, X-rays are directed at the patient and a flat-panel detector is placed on the other side of them. The flat panel dectector is a small screen that contains a CCD which has sensors covered in a substance that converts X-rays to light enabling a digital image to be displayed. They are transmitted by soft tissue but are absorbed by bones teeth and metal objects.

An organ that consistes of soft tissue can be filled with a CONTRAST MEDIUM that absorbs X-rays easily. Barium is a good absorber of x-rays. Lead is also a good absorber and is used to stop x-rays reaching other parts of the body.

CT scans can distinguish between different types of soft tissue unlike X-rays by creating a 3D image. However it gives a much higher radiation dose than X-rays and the equipment cost is much more.

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Ultrasound

Ultrasound waves are sound waves of frequency above 2000Hz.
The human ear can detect sound waves from 20Hz to 20000Hz.

Ultrasound waves are used in medicine for ultrasonic scanning and the destruction of kidney stones. It consists of a transducer placed on the body surface displaying the image. Each pulse from the transducer is partly reflected at a boundary between two different types of body tissue, arriving back at different times.

Pros: - Non- ionising so safer than x-rays
- Can be used to scan organs and other soft tissues in the body.

An OSCILLOSCOPE measures the 'transit time' of each pulse. This is the time taken for the reflections to reach a detector which can be used to determine how far away such a boundary is.

didtance travelled by pulse = speed of ultrasound waves x transit time.

the depth of the boundary below the surface = 0.5 x speed of waves x transit time.

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Refractive Index

(n) A measure of how much a substance can refract a light ray.

For a light ray travelling into glass from air...
... the angle of refraction is always less than the angle of incidence.
... the greater the angle of incidence, the greater the angle of refraction.

Snells Law

For a light ray travelling from air into a transparent substance sin i/sin r always has the same value regardless of the angle of incidence.

n = sin(i)/sin(r) OR sin(i) = n x sin(r) OR sin(r) = sin(i)/n

When a light ray travels fom a transparent substance into air...
... the light ray is refracted AWAY from the normal.
... the larger the angle of incidence, the larger the angle of refraction.

the sine of the angle in air = n x the sine of the angle in glass

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The Endoscope

Endoscopes contain optical fibres. Light rays in an optical fibre are totaly internally reflected each time it reaches the fibres boundary. Laser light can be used as the source of energy in an endoscope and can be used to see inside the body directly.

Total internal reflection only takes place for a ray travelling from glass into air when the angle of incidence of a light ray is greater than the critical angle.

Critical Angle (c) = The angle of incidence of a light ray in a transparent substance that produces refraction along the boundary.

n = 1/sin(c) OR sin(c) = 1/n

Refractive index = 1/sin(c)

If a light ray was reversed, the direction of the arrows would reverse but the path would stay the same.

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Lenses

Coverging (conVEX) lens:
- Parallel rays of light a refracted so that they converge to a point (the PRINCIPAL FOCUS).
- The distance from the centre of the lens to the principal focus is the focal length.
- If the object is further away from the lens than the principal focus, an inverted (upside down) real image is formed.
- If the object is nearer to the lens than the principal focus, an upright virtual image is formed larger than the object. The image is magnified - the lens acts as a magnifying glass. The imade is formed on the same side of the lens as the object.
- The size of the image depends on the position of the object. The nearer the object is to the lens, the larger the image.

Diverging (conCAVE) lens:
- Parallel rays of light that are refracted so that they diverge away from the principal focus.
- The image is always virtual, upright and smaller than the object.

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Using Lenses

Magnification:
- image height/object height.

A ray diagram can be drawn to find the position and nature of an image formed by a lens.

The principal focus is the line through the centre of your lens.

Include three construction rays from a single point:
1. Parallel to the axis and refracted through the principal focus.
2. Passes through the centre of the lens.
3. Passes through the principal focus and is refracted parallel to the axis.

Camera = A converging lens that is used to form a real image of an object.

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The Eye

Retina = The light sensitive cells around the inside of the eye.
Lens = Focuses light onto the retina.
Cornea = transparent layer that protects the eye and helps focus light onto the retina.
Pupil = Light enters through this central hole formed by the iris.
Iris = Coloured ring of muscle that controls the amount of light entering the eye.
Ciliary muscle = Changes the thickness of the eye lens. When they contract, they shorten and squeeze the eye lens making it thicker. This allows light to be focused at varying distances. To see a nearby object clearly, the lens has the be thicker than if the object is far away.
Suspensory Ligaments = Attaches the ciliary muscle to the lens.

Power of the lens (dioptres) = 1/ focal length in metres.
Converging lens is positive and a diverging lens is negative.

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The Eye continued

A short-sighted eye (caused by the eyeball being too long) can be corrected by using a diverging lens. The diverging lens counteracts some of the 'excess' focusing power of the eye lens.

A long-sighted eye (caused by the eyeball being too short) is corrected using a converging lens. The correcting lens makes the rays from the object diverge less and adds focusing power to the eye lens.

The film in a camera = The retina in the eye

The focal length is determined by: - the refractive index from the material the les is made of.
- the curvature of the two surfaces of the lens'.

For a given focal length, the greater the refractive index, the flatter the lens. This means that the lens can be manufactured thinner.

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Moments

The TURNING effect of a force.
Can be increased by - increasing the size of the force.
- using a spanner with a longer handle.

Load = The weight of the object.
Effort = The force a person applies to the object.
Pivot = The point at which an object turns after a force is applied.
The line along which a force acts is the line of action.

moment(Nm) = force(N) x perpendicular distance from line of action of force to the pivot(m)
m = F x d

If an object is not turning, the total clockwise moment must be exactly balanced by the total
If the line of action of a weight of an object is outside the base of the object then there will be a resultant movement and the body will tend to topple e.g if the resultant movement is not zero.

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Centre Of Mass

The point at which an objects mass may be thought to be concentrated.

To find the centre of mass of an irregular shape, freely suspend the card from a rod. The centre of mass is directly below the rod and a 'plumbline' can be used to draw a vertical line on the card fom the rod downwards. This is repeated with the card suspended fom a secon pointto give another, similar line. The centre of mass of the card is where the two lines meet.

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

When a suspended object is in equilibrium, its centre of mass is directly beneath its point of suspention.

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Hydraulics

The pressure in a liquid is transmitted equally in all directions.

The unit of pressure is the pascal (pa).
pressure = force/ area

e.g The grab of a digger is operated by a hydraulic pressure system. Also a hydraulic car jack can be used to lift a car.

The force of a hydraulic system is much grater than the force applied to it. It uses the pressure in a fluid to exert a force.

Effort = The force applied to the system.
Load = The force exerted by the system.

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.

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Circular Motion

The velocity of an object moving in a circle at constant speed is continually changing as the object's direction is continually changing.

For an object moving in a circle at constant speed, at an instant:
- the object's velocity is directed along a tangent to a circle.
- its velocity changes direction as it moves round.
- the change of velocity is towards the centre of the circle.

Centripetal force = the resultant force actin o an object moving round a circle.

The centripetal force needed to make an object perform circular motions increase as:
- the mass of the object increases.
- the speed of the object increase.
- the radius of the object decreases.

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The Pendulum

The time period depends on the length of the pendulum.

To measure the time period of a pendulum, we can measure the average time for 20 oscillations and divide the timing by 20.

the time period(s) = 1/ frequency of oscillations(Hz)
OR
frequency = 1/ time

The frequency of the oscillations os the number of complete cycles of oscillation per second.

The greater the frequency, the shorter the time period.

Friction and air resistance can stop oscillation if it is not pushed repeatedly.

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Electromagnets

like poles repel; unlike poles attract

An electromagnet consists of a coil of insulated wire wrapped round and iron core. When a current flows through the wire, a magnetic field is created around the wire. The magnetic field magnetises the iron bar so when the current is switched off, the iron bar loses its magnetism easily.

Steel is unsuitable as th core because it keeps its magnetism when the current is off.
Electromagnets are used in scapyard cranes to lift iron and steel and in electric bells.

Electromagnetic induction is the process of creating potential difference using a magnetic field. When an inductor cuts the lines of a magnetic field, a potential difference is induced across the ends of the conductor.

When one end of a bar is pushed into the coil, the ammeter pointer deflects because:
- the movement of the bar magnet caused an induced pd in the coil.
- the induced pd causes a current, because the coil is part of a complete circuit.

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The Motor Effect

The electric motor works because a force can act on a wire in a magnetic field when we pass a current through the wire.

Flemmings left hand rule = First finger (field), Second finger (current), Thumb (movement).

The size of the force can be increased by increasing the current or using a stronger magnet.

The force is greatest when the wire is perpendicular to the magnetic field.

The force is zero when the wire is parallel to the magnetic field lines.

The direction of the force is always at right angles to the fire and the field lines.

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

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The Motor Effect Continued

Electric Motor - We can control the speed of the motor by changing the current.
- We can reverse the direction the motor turns by reversing the current.

When a current is passed the the coil, the coil spins because a force acts on each side of the coil due to the motor effect.
Also, the force on one side is in the opposite direction to the other.

Loudspeaker =

When a current passes through the coil, a force due to the motor effect makes the coil move.

Each time the current changes its direction, the force reverses its direction. This makes the diaphragm vibrate so soundwaves are created.

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Transformers

A typical power station generator produces an alternating potential difference of about 25000 volts. Mains electricity to homes is at 230 volts.

A transformer has two coils of insulated wire wrapped around the same iron core.
When alternating current passes through the primary coil, an alternating potential difference is induced in the secondary coil. This happens because:
- alternating current passing through the primary coil produces an alternating magnetic field.
- the lines of the alternating magnetic field pass through the secondary coil.
- the magnetic field is changing.

If a magnet is moved into a coil of wire, potential difference is induced across the ends of the coil.

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 continued

No electricity flows round the iron core.

Switch mode transformer:
- Lighter and smaller than a traditional tranformer which works at 50Hz.
- Uses very little power when there is no device connected to its output terminals.

Transformers are used to step potential differences up or down.

The higher the grid pd, the greater the efficiency of transferring electrical power through the grid.

pd across primary/ pd across secondary = no. turns primary/ no. turns secondary
Vp/ Vs = Np/ Ns

For an 100% efficient transformer:
primary pd x primary current = secondary pd x secondary current
Vp x Ip = Vs x Is

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