PHY_10_57_V1_P_Magnetic properties of a substance

Neodymium magnets.
Are the most strongest commercially available permanent magnets.
They are made from an alloy of neodymium, iron and boron.

A hard disk drive records data by magnetising and demagnetising a thin film of ferromagnetic material on a disk. Each disk is separated into many tracks and sectors and each sector has many tiny individual magnetic cells which are magnetised by the drives read/write head when data is written to the drive. Hard drive heads are made from ferrite wrapped in a fine wire coil. When writing, the coil is energised, a strong magnetic field forms, and the recording surface adjacent to the gap is magnetised. Strong magnets are also used in the actuator that moves the read/write head into position.

Permanent magnets are used in speakers alongside a current-carrying coil which converts electricity into mechanical energy that moves the speaker cone that in turn changes the pressure of the surrounding air creating sound. Microphones work in reverse; a diaphragm is attached to a coil of wire which sits within a permanent magnet, when sound moves the diaphragm the coil moves too. As the coil moves through the magnetic field created by the permanent magnet an electrical signal is produced which is characteristic of the original sound.

3. ABS (anti-lock braking) sensors

Passive ABS sensors use neodymium magnets wrapped inside copper coils. A sensor is placed close to the ABS reluctor ring and as the ring rotates a voltage is induced in the copper wire. This signal is monitored by the vehicle’s computer system and used to define wheel speed.

 Is an electronic instrument which detects the presence of a nearby metal. Metal detectors are useful for finding metal inclusions hidden within objects, or metal objects buried underground. They often consist of a handheld unit with a sensor probe which can be swept over the ground or other objects. If the sensor comes near a piece of metal this is indicated by a changing tone in earphones, or a needle moving on an indicator. Usually the device gives some indication of distance; the closer the metal is, the higher the tone in the earphone or the higher the needle goes.

Is an instrument that responds to ground motions, such as caused by earthquakes, volcanic eruptions, and explosions. Seismometers are usually combined with a timing device and a recording device to form a seismograph. The output of such a device — formerly recorded on paper (see picture) or film, now recorded and processed digitally — is a seismogram . Such data is used to locate and characterize earthquakes, and to study the earth's internal structure.

To determine the motion of the earth during an earthquake, ground motion must be measured against something that remains relatively fixed (i.e., not affected by the shaking). In a seismometer, the fixed object consists of a mass suspended on springs within a case. During an earthquake, the mass remains still while the case around it moves with the ground shaking. Most modern seismometers work electromagnetically. A large permanent magnet is used for the mass and the outside case contains numerous windings of fine wire. Movements of the case relative to the magnet generate small electric signals in the wire coil.