Physics Unit 3



Ultrasound are high-frequency waves that have a frequency above 20,000Hz

They are used:

  • Scanning to diagnose disease e.g. pre-natal scans
  • Medical Treatment e.g. treatment of kidney stones

Ultrasound directed at the body will be partly reflected at each interface between the different tissues

This principle is used in ultrasound scans. These are widely used in pre-natal scanning to check that a foetus is developing normally and to take measurements of its growth. Computers combine these reflections to produce a detailed image.

Travel at different speeds through different media

If a question involves a reflected ray remember to half the answer to calculate the distance as that is an answer of the distance to the object there and back

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X-rays are part of the electromagnetic spectrum. They have a very short wavelength and cause ionisation.

More dense areas in our bodies, such as bone, allow fewer X-rays through and so appear paler on X-ray images. Less dense areas, such as skin and many tissues, allow more X-rays though. These appear black on X-ray images. X-rays give more reliable results than ultrasound when checking for broken bones and so X-rays are used more often to check for breaks. 

CCDs forms an image electronically allowing them to be recorded and stored for later examination. 

Properties of X-rays include:

  • They affect a photographic film in the same way as light
  • They are absorbed by metal and bone
  • They are transmitted by healthy tissue
  • Their wavelength is of the same order of magnitude as the diameter of an atom

CT Scans

  • They are much more detailed than standard scans and give a 3D image
  • They expose you to a larger amount of radiation
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Refraction is the change of direction of light as it passes from one medium to another. The two media must have different densities such as air and glass. When light passes into a denser medium, it slows down. The rays bend towards the normal. This means that the angle of refraction is smaller than the angle of incidence. When light passes into a less dense medium, it speeds up. The rays bend away from the normal. This means that the angle of refraction is greater than the angle of incidence. However, if the light hits the boundary at 90 degrees, in this case, it goes in a  straight line.

The amount of refraction depends on;

  • The angle the light hits the boundary
  • The difference in relative densities
  • The degree to which a material slows the speed of light is its refractive index.

Refractive index  is the ratio of the velocity of light in a vacuum to its velocity in a specified medium

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Converging and Diverging Lenses

These lenses focus the rays of light on producing a real image- an image that can be projected onto a screen. The focal length is the distance between the centre of the lens and the image.

A converging lens is used in a refracting telescope to focus the image.

A diverging lens will go outwards and a converging lens will focus light inwards

Ray diagrams

  • Ray 1 is parallel to the axis and is refracted through F
  • ray 2 passes straight through the centre of the lens
  • Ray 3 passes through F and is refracted parallel to the axis
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Long and Short Sight

Long Sight

Occurs when the eye cannot focus on near objects- the uncorrected image forms behind the retina

Caused by

  • The eyeball being too short
  • The eye lens being unable to focus

Converging lens used to correct it.

Short Sight

Occurs when the eye cannot focus on distant objects- the uncorrected image forms in front of the retina

Caused by

  • The eyeball being too long
  • The eyes lens being unable to focus

Diverging lens used to correct it.

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The Eye

The ciliary muscles alter the thickness of the lens. They are attached to the edge of the lens by the suspensory ligaments. The fibres of the ciliary muscles are parallel to the circular edge of the eye lens. When they contract they shorten and squeeze the eye lens, making it thicker.

  • Iris- coloured ring of muscle that controls the amount of light entering the eye
  • Cornea- transparent layer that protects the eye and helps to focus light onto the retina
  • Ciliary Muscles- change the thickness of the lens, attached by the suspensory ligaments
  • Retina- the light-sensitive cells around the inside of the eye
  • Eye Lens- focuses light onto the retina

The normal eye has a range of vision from 25cm to infinity. This means it can clearly see any object that is 25cm or more from the eye.

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The Eye compared to a camera

 The Eye               The Camera

Lens- Converging, variable focus Converging fixed focus

Adjustment- Ciliary-muscle alters the thickness Adjustment of lens position

Image- Real, inverted, magnification less than one

Detection- Light sensitive cells on the retina   Photographic film(CCD sensors in a digital camera)

Brightness Control-Iris controls the width of the pupil Adjustment of aperture

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Total Internal Reflection

The incident ray is at just the right angle for the refracted ray to travel along the boundary- the angle of refraction is 90 degrees. 

The angle of refraction is now called the critical angle

If the angle of incidence is increased beyond the critical angle, the refracted ray is totally internally refracted. We call this total internal refraction.

Optical Fibres

  • Visible light is sent down an optical fibre
  • View internal organs without surgery
  • Surgery carries a risk of infection


  • Has two bundles of optical fibres as one takes light and one is the image coming back
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Centre of Mass

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

To find the centre of mass of a symmetrical shape, you put  lines through the lines of symmetry and where they intersect is the centre of mass

For a non-symmetrical shape, pick a location that is near to the edge and drop a plumb line from that point and mark on the object where the plum line goes. Suspend the shape from another location not too close to the first. Drop a plumb line again and mark. The intersection is where the centre of mass is.

If suspended, a body will come to rest with its centre of mass directly below the point of suspension.

The line of action is a straight line that goes through the centre of mass, if the line of action goes outside of the base it will fall over 

Explain why when the basket is displaced to one side it returns to a stable equilibrium

When it is displaced at the instant of release, the line of action of the weight no longer passes through the point of support, so the weight returns the object to equilibrium. The weight exerts a turning effect causing the object to return to an equilibrium position.

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The turning effect of a force is called the moment. The size of the moment is given by:

Moment= Force x Distance from the pivot

The size of the turning force of moment depends upon:

  • The distance of the force from the pivot
  • The size of the force

The principle of moments is the sum of all the clockwise movements about any point = the sum of all the anti-clockwise movement.

Why does a tractor not tip?

The line of action is inside the wheelbase so there will be no turning effect so the resultant moment is 0.

Increasing the turning effect

  • Increase the force
  • Increase the length
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Liquids are virtually incompressible. The pressure is equally transmitted in all direction. This means a force exerted in one part of the liquid will be transmitted to all parts of a liquid.

As liquids are virtually incompressible, the volume doesn't change, the oil is pumped into the upper or lower part of the cylinder to make the piston move in and out of the cylinder. The force of a hydraulic system is much greater than the force applied to it.

In hydraulic machines, exerting a small force over a small cross-sectional area can lead to pressure being transmitted, creating a large force over a large cross-sectional area. This ability to multiply the size of forces allows hydraulics to be used in many applications such as car braking systems

It takes a large force to slow down or to stop a car that is travelling at speed. Hydraulics are used in the braking system of a car. They cause a relatively small force from the driver’s foot to be multiplied to produce a greater force, which acts equally on all four brake pads.

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Circular Motion

For an object moving in a circle or at a constant speed, at any instant:

  • The objects velocity is directed along a tangent to the circle;
  • The velcoity is constantly changes direction as the object moves around the circle
  • The chnage in velocity is towards the centre of the circle

The obejct therefore always accelerates towards the centre of the circle. This acceleration chnages direction but not the speed. We call this acceleration centripetal aceleration. The resultant force on the moving object acts towards tthe centre of the circle. We call this resultant force centripetal force.

Centripetal force needed to make an object an object perofrm circular motion increases as:

  • The mass of the object increases
  • The speed of the object increases
  • The radius of the object decreases

The centripetal force on a vehicle moving round a roundabout is due to friciton between the tyres and the road

On a gravity wheel the centripetal force is fue to the weight of the rider and the downward push from the wheel on the rider.

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  • Insulated wire, wrapped around an iron core
  • When a current passes through the wire, a magnetic field is created, the wire magnetises the iron core

The interaction between two magnetic fields will cause an interaction between them.

Once the steel is magnetised it cannot be switched off, unlike iron which can be switched on and off.

Electromagnetic crane

  • The current is turned on which attracts the metal, the current is turned off to drop the metal into a container

Circuit breaker

  • When too much much current passes through the electromagnetic, the switch will be pulled downwards and then the switch will open, this breaks the circuit and will stop the flow of current.

Electric Bell

  • When the bell is connected to the battery, the iron armature is pulled down to the electromagnet. This opens the make and break switch and the electromagnet switches off. As a result the armature springs back, this then cycles.
  • The electromagnetic field attracts the iron armature towards the iron core.
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The Motor Effect

The electric motors work because a force can act on a wire in a magnetic field when we pass a current through the wire, we call this the motor effect. 

The size of the force can be increased by:

  • increasing the current
  • using a stronger magnet.

The size of the force depends on the angle between the wire and the magnetic field lines.

  • The force is greater when the wire is perpendicular to the magnetic field 
  • Zero when the wire is parallel to the magnetic field lines

The direction of the force is always at right angles to the wire and the field lines. The direction is also reversed if the direction of the current or magnetic field is reversed.

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The Motor Effect Examples

Electric Motor

  • There is a rectangular coil of insulated wire called the armature, this is forced to rotate.
  • The coil us connected to the circuit by graphite brushes when they are in contact current will flow, when they are not in contact the current will not flow and the cycle continues.
  • A force acts on each side of the coil, one is forced up and one is forced down.
  • Force on one side is the opposite to the other, a split ring commutator reverses the current every half term this is what causes the motor to spin

The Loudspeaker

  • When a current passes through the coil, a force due to the motor effect causes the coil to move.
  • Each time the current changes its direction, the force reverses direction.
  • So the coil is repeatedly forced backwards and forwards, the motion makes the diaphragm vibrate.
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A generator contains coils of wire that spin in a magnetic field. When they do this a potential difference or voltage is induced across the ends of the wire when they cut across the magnetic field lines.

Electromagnetic induction is the process of creating a potential difference suing a magnetic field.

If the wire is part of a complete circuit, the induced p.d makes an electrical current flow around the circuit

Transformers are used to change the size of the alternating potential difference. A transformer has 2 coils of insulated wire, both wound around the same iron core. When an alternating current passes through the primary coil an alternating p.d is induced in the secondary coil.

An alternating current passing through the primary coil produces an alternating magnetic field. The lines of the alternating magnetic field pass through the secondary coil, which induces a current. The magnetic field is changing.

Step up- makes the p.d across the secondary coil greater than the p.d across the primary coil. Its secondary coil has more coils than the primary coil.

Step down- makes the p.d across the secondary coil is less than the primary coil, the primary coil has more coils than the secondary.

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