Torque and Electromagnetism

It’s possible to use a magnetic field to create a force on a length of wire.

If we place a current carrying wire perpendicular to the magnetic field, the wire will feel a force perpendicular to both.

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x x x x x xx x x x x xx x x

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This would be much more useful and practical if we could create a spinning motion.

We could apply this spinning motion to wheels or motors; very useful devices!

Is there a way to create this spinning motion with magnetic fields?

Yes! It requires a coil of wire.

Torque

If a wire loop is placed in a uniform magnetic field,

the loop experiences a torque (turning force), which causes the loop to rotate.

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F

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The loop will only rotate 90°, then stop. This is the basis for electric meters and motors.

Electric MetersGalvanometers:

Measure weak electric currents, and thus measure magnetic fields.

A galvanometer is made by placing a coil of wire in a magnetic field. The coil is attached to a spring. The more current that flows through the coil, the more the needle moves.

Ammeters:A modified galvanometer that measures larger currents.

A low resistance shunt is put in parallel with the galvanometer so that most of the current flows through the shut. The needle moves less since less current is flowing through the galvanometer, to ensure bigger currents.

The lower the shunt’s resistance, the bigger a current it can measure.

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Voltmeter:

A galvanometer modified to measure potential difference.

To do this, a high resistance is linked to the galvanometer in series.

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