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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…

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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
F= r2

The Gravitational Field Strength at any point is the gravitational force on a mass of 1kg at any point,

g= m

Variations in the field…

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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

A gravimeter is any instrument that can be used for investigating…

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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.


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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…

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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


Proton Magnetometer ­ measures flux density (B)

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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. The induced EMF which depends on the rate at which flux is cut
Or the rate of…

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= 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…

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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…

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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

Measuring Soil Conductivity Using Magnetic Fields
The electrical resistance of the ground varies according to the number of…


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