Induced current due to eddy currents Receiver coil Eddy currents in the Metal metal reduce the induced current in the receiver coil. Induced current due Transmitter coil to the transmitter coil FIGURE P25.78
Metal detectors use induced currents to sense the presence of any metal—not just magnetic materials such as iron. A metal detector, shown, consists of two coils: a transmitter coil and a receiver coil. A high-frequency oscillating current in the transmitter coil generates an oscillating magnetic field along the axis and a changing flux through the receiver coil. Consequently, there is 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.
Which of the following changes would not produce a larger eddy current in the metal?
A. Increasing the frequency of the oscillating current in the transmitter coil
B. Increasing the magnitude of the oscillating current in the transmitter coil
C. Increasing the resistivity of the metal
D. Decreasing the distance between the metal and the transmitter
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