AQA A2 Physics - Imaging the Invisible Revision Notes

Notes for 'Imaging the Invisible' chapter of the Unit 4 aqa A2 exam for Physics B: physics in context

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  • Created on: 27-04-11 15:50
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Imaging the Invisible
Beneath the Earth:
Physics can be used to investigate things that cannot be seen directly with the eye. There are a
range of techniques available to geophysicists and archaeologists in the search for new minerals and
buried remains. It is very important for us to be able to know what lies underneath the ground so
that we can look for raw materials in an environmentally friendly and cost-effective way, make sure
we are building on stable ground, rediscover the past by digging up artefacts.
There are a range of possible techniques that can be used in order to find out what is beneath the
earth:
Electrical resistivity
Velocity of sound waves transmitted through the ground
Reflection of radio waves, allowing distance measurement
Gravitational field strength
Magnetic field strength
Radioactivity
Instruments can be carried in aircraft and can be made sensitive enough t make readings from a
distance above the surface, and can look for any anomalies over a large area of land. If earth's crust
was made of only one material any physical quantity measurements would not vary from place to
place, however even in places where the crust is reasonably homogenous slight differences occur.
This causes some properties such as gravity and magnetism vary.
To find these differences, geophysical surveys are conducted in a structured way. Measurements are
made at regular intervals using a grid system.
Anomalies arise due to many reasons
o Could be a denser body containing useful ores
o Could be a less dense body which may contain lighter materials or hydrocarbons
o Indication of a cave
o Indication of previous excavation

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Gravity Surveying:
Newton's Gravitational Law
Force between two objects of mass "m" and "M" by a distance "r" is given by
GMm
F= r2
The Gravitational Field Strength at any point is the gravitational force on a mass of 1kg at any point,
F
g= m
Variations in the field strength (g) are due to the Earth not being a perfect sphere and due to
the rotation, resulting in values that vary from 9.78 N kg-1 at the equator to 9.…read more

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The Gal is the unit used for gravitational field strength. One gal is equal to 0.01Nkg . It is equivalent
to an acceleration of free fall of 1cm s-2 , meaning normal gravitational field strength is 981gals
Gravimeters
A gravimeter is any instrument that can be used for investigating variations in g
Modern gravimeters are capable of measuring of measuring g or changes in g to 1 part in
1000 million (equivalent to precision of 0.…read more

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Magnetic Surveys:
Magnetic Field Surveying relies on differences in measurements of magnetic field strength
A magnetic field is a region in which a magnetic effect may be observed. The direction of the
magnetic field is the direction of the force on a magnetic north pole due to the field.
Cause of Magnetic Fields
Magnetic Fields are produced by the movement of charge. They are easily produced by passing a
current through a wire as this results in a movement of electrons.…read more

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Producing Strong Magnetic Fields
There are two ways of producing a magnetic field in lab; a solenoid and an electro-magnet. The
two are very similar. A solenoid is a wire carrying a current (usually coiled) and an electro-magnet is a
solenoid with an iron core (to increase the strength of the field).…read more

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Fleming's Left Hand Rule shows the direction of the force on a conductor carrying a current in
a magnetic field:
The magnitude of Force increases when
o Current increases
o Length of wire increases
o Magnetic flux density increases
F = BIL
Proton Magnetometer ­ measures flux density (B)
Protons have charge and they spin, meaning they behave like magnets. Hydrogen rich liquids
(Paraffin) protons are normally randomly aligned, producing no overall magnetism.…read more

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Faraday's Laws
1. When a conductor cuts magnetic flux an EMF is induced in the conductor
Or when the magnetic flux linking a coil changes an EMF is induced in the coil
2.…read more

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BA)
= Nt = N t
Direction of the Induced EMF: Lenz's Law
Lenz proposed that induced currents are in a direction that produces a force to oppose the change
that is producing the current
When a magnet moves into the coil the charge in the coil always flows to produce a
decelerating force on the magnet, as the end of the coil acts like a north pole, therefore
repelling the magnet.…read more

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Eddy Currents
Induced currents that circulate in closed loops within electrical conductors when subjected
to a changing magnetic field
Set up by induced EMF in metallic conductors
Used in metal detectors
Flux change without movement:
Induced EMF's can be produced without moving a coil using a set up where there is one coil
connected to a battery/power supply and another separate coil wrapped around the same iron core
When switch is open no magnetic flux is produced by the first coil, therefore there is no…read more

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A metal detector produces an alternating current, which in turn produces an alternating magnetic
field. This will induce eddy currents in any metallic objects nearby, which can be detected with a
magnetometer.
Measuring Soil Conductivity Using Magnetic Fields
The electrical resistance of the ground varies according to the number of ions present in it.…read more

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