The Metal Detector Metal detectors use induced currents to sense the presence of
ID: 1703976 • Letter: T
Question
The Metal Detector Metal detectors use induced currents to sense the presence of any metal-not just magnetic materials such as iron. A metal detector, shown in the figure, consists of two coils: a transmitter coil generates an oscillating magnetic field along the axis and an oscillating induced current in the receiver coil.If a piece of metal is placed between the transmitter and the receiver, the oscillating magnetic field in the metal induces eddy currents in a plane parallel to the transmitter and receiver coils. The receiver coil then responds to the superposition of the transmitter's magnetic field and the magnetic field of the eddy currents. Because the eddy currents attempt to prevent the flux from changing, in accordance with Lenz's law, the net field at the receiver decreases when a piece of metal is inserted between the coils. Electronic circuits detect the current decrease in the receiver coil and set off an alarm.
Why won't the metal detector detect insulators?
Insulators block magnetic fields.
No eddy current can be produced in an insulator.
No emf can be produced in an insulator.
An insulator will increase the field at the receiver.
Part B -
A metal detector can detect the presence of metal screws used to repair a broken bone inside the body. What does this tell us?
The screws are made of magnetic materials.
The tissues of the body are conducting.
The magnetic fields of the device can penetrate the tissues of the body.
The screws must be perfectly aligned with the axis of the device.
Part C -
Suppose the magnetic field from the transmitter coil in the figure points toward the receiver coil and is increasing with time. As viewed along this axis, what are the induced currents?
Clockwise in the metal, clockwise in the receiver coil.
Clockwise in the metal, counterclockwise in the receiver coil.
Counterclockwise in the metal, clockwise in the receiver coil.
Counterclockwise in the metal, counterclockwise in the receiver coil.
Part D -
Which of the following changes would not produce a larger eddy current in the metal?
Increasing the frequency of the oscillating current in the transmitter coil.
Increasing the magnitude of the oscillating current in the transmitter coil.
Increasing the resistivity of the metal.
Decreasing the distance between the metal and the transmitter
Explanation / Answer
Part A: no eddy current can be produced in an insulator Part B: the magnetic fields of the device can penetrate the tissues of the body Part C: Counterclockwise in the metal, counterclockwise in the receiver coil Part D: Increasing the resistivity of the metal