Metal Detectors

Short Description of Topic

Introduction to the invention of metal detectors and the physics behind the tool.

The Main Idea

Metal detectors are electromagnetic devices using Faraday's Law used to sense the presence of metal within its reach.

How They Work

Simple metal detectors have two types of coils: transmitter coils, coils of wire wrapped around the typically circular head at the end of the device's handle and receiver coils. According to Maxwell's equations, as the electricity flows through the transmitter coil and creates an electric field, so too does a magnetic field occur all around it as well. It is necessary for the user to move the detector across the ground so that the magnetic field will move around too. By moving the detector over a metal object, the moving magnetic field around the detector affects the atoms inside the metal, causing the electrons to move in a different manner. Simply put, the detector's moving magnetic field induces electrical activity in the metal object, in turn, now causing another magnetic field to appear around the metal object.

The metal detector is able to detect this new magnetic field because of the receiver coil that is connected to a circuit with a loudspeaker. When the detector moves around the metal object, the induced magnetic field on the object cuts through the coil, causing electricity to flow through the receiver coil, which makes the loudspeaker beep or click. The closer the transmitter coil is to the object, the louder the noise will be from the receiver coil.

A Mathematical Model

The primary physics equation used when analyzing spark plugs is Faraday's Law, which states [math]\displaystyle{ \mathcal{E} = -{{d\Phi_B} \over dt} \ }[/math], where [math]\displaystyle{ \mathcal{E} }[/math] is the emf produced from the time-varying magnetic flux Φ_B. The flux is given by [math]\displaystyle{ \int_{\Sigma} \mathbf{B} \cdot d\mathbf{A}. }[/math]

The spark plug essentially consists of 2 coils, a primary coil and a much larger secondary coil. As current from the car battery runs through the primary coil, it is interrupted by cam action, which varies the electric current running through the circuit. Because the primary coil is is wrapped around the secondary coil, a varying magnetic field induces an electric current to be run through the secondary coil, which is much larger, which in turn produces a much larger EMF due to the number of loops in the coil.

[math]\displaystyle{ {\frac{d\vec{p}}{dt}}_{system} = \vec{F}_{net} }[/math] where p is the momentum of the system and F is the net force from the surroundings.

A Computational Model

How do we visualize or predict using this topic. Consider embedding some vpython code here Teach hands-on with GlowScript

Examples

Anatomy of a Typical Spark Plug

Connectedness

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History

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See also

Magnetic Field of a Solenoid

Curly Electric Fields

Faraday's Law

Inductance

Transformers

Further reading

Books, Articles or other print media on this topic

External links

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References

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