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mass spectrometer

electron microscopes

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1. two parallel charged plates, providing constant electric field strength E, are placed in a vacuum

2. a magnetic field of flux density B is applied to the region at right angles

3. charged particles travel through this region with different speeds

4. the magnetic force FB is greater in particles with greater speeds since FB = Bev

5. for particles with the desired speed v, FE and FB cancel out, so they move past undeflected

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'holes' are electric charge carriers with a positive charge

when holes are used in the Hall probe instead of electrons, the polarity of VH does not change

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to get a good VH reading:

't' needs to be small, i.e probe should be very thin

'n' needs to be small, i.e probe should be made out of a semiconductor

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1. a current is supplied through the sides of the probe

2. the probe is placed on a magnetic field such that its large face is perpendicular to the field lines

3. this results in the electrons being pushed of to one side of the probe

4. negative charge accumulates on one side, which causes an electric field to be formed

5. equilibrium is established between FB and FE

6. a constant potential difference, called the Hall voltage, forms between the two sides of the probe

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probe is made from a semiconductor material

semiconductors have a smaller number of electrons, so they have a larger drift velocity compared to metals, for the same value of current [ I=nAvq ]

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The Hall effect is applied to a simple device, known as Hall probe, which is used to get an instant measure of the magnetic flux density of a magnetic field.

It is so sensitive and accurate that it can be used to measure the magnetic field of the Earth as well

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American physicist who discovered the Hall effect in 1879