# Viewing underneath the ground

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## Free-fall gravimeter

• Object falls in a vacuum.
• Laser beam hits a partially reflecting mirror, so that half of the beam travels to a reference mirror and half travels to the falling object.
• The two beams of reflected light interfere with each other and are detected by a light sensor.
• This allows the distance travelled by the falling object to be calculated.
• s=0.5at^2 --> a=g.
• Can be done many times with an average calculated to improve the accuracy of the results.
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## Producing strong magnetic fields

To increase the magnetic flux density for a solenoid:

• Increase the number of turns in the coil.
• Increase the current flowing through the centre of the solenoid,
• Decrease the radius of the solenoid.
• Use an iron core rather than an air core (this produces a field strength which is many hundreds of times greater).
• Using a superconductor can allow very high currents (and therefore greater magnetic field strengths) without melting the wires due to resistance (P=I^2.R)
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## Measuring magnetic fields

• 1 Tesla is the magnetic flux density when it causes a force of 1N on a wire of length 1m carrying a current of 1A.
• 1 gamma = 10^-9 Tesla- commonly used unit by geophysicists and archaeologists,

Hall Probes:

• A small slice of semiconductor carries a steady current.
• When a magnetic field is perpendicular to the probe, the charge carriers are deflected to one side. This creates a potential difference which is perpendicular to the original current.
• The induced emf is known as the Hall Voltage and can be measured. If calibrated, the measurements can be used to measure the actual value of B to a precision of +- 1x10^-6 T.
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## Proton Magnetometer

• Proton rich liquid such as paraffin used,
• When a current is added, the protons align themselves with the magnetic field.
• When the power is switched off, the protons precess around the direction of the Earth's magnetic field.
• The frequency of the precession (the Lamor Frequency) is proportional to the Earth's magnetic flux density at that point.
• The precession of the protons induces an emf in the coil (the precession is measured by the nuumber of oscillations in the induced emf in a given time).
• f=0.04258 x B (in nT).
• B = 23.49 f.
• Clocks using the oscillations of quartz crystals are used for accurate timing.
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1:

When a conductor cuts magnetix 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 magnitude of the induced emf depends on the rate at which flux is cut.

or

The magnitude of the induced emf depends on the rate of change of flux linkage.

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## Lenz's Law

When a current is induced by electromagnetic induction, the direction of the induced current is always such as to oppose the change that caused the current.

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## Geophone

The magnet has a high inertia so remains stationary at first. The coil moves relative to the magnet so an emf is induced. The magnitude of the emf measures the magnitude of the vibrations in the Earth's surface. The voltage pulse can be used to start or stop a timer.

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