The Metal Detector Metaldetectors use induced currents to sense the presence of anymetal-not just magnetic materials such as iron. A metal detector,shown in the figure, consists of two coils: a transmitter coilgenerates an oscillating magnetic field along the axis and anoscillating induced current in the receiver coil.
If a piece of metal is placed between the transmitter and thereceiver, the oscillating magnetic field in the metal induces eddycurrents in a plane parallel to the transmitter and receiver coils.The receiver coil then responds to the superposition of thetransmitter's magnetic field and the magnetic field of the eddycurrents. Because the eddy currents attempt to prevent the fluxfrom changing, in accordance with Lenz's law, the net field at thereceiver decreases when a piece of metal is inserted between thecoils. Electronic circuits detect the current decrease in thereceiver coil and set off an alarm.
1. Suppose the magnetic field from the transmitter coil in thefigure 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 receivercoil. Counterclockwise in the metal, clockwise in the receivercoil. Counterclockwise in the metal, counterclockwise in the receivercoil.
2. Which of the following changes would not produce a larger eddycurrent in the metal? Increasing the frequency of the oscillating current in thetransmitter coil. Increasing the magnitude of the oscillating current in thetransmitter coil. Increasing the resistivity of the metal. Decreasing the distance between the metal and the transmitter
