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Units of Chapter 19 Magnets, Magnetic Poles, and Magnetic Field Direction Magnetic Field Strength and Magnetic Force Applications: Charged Particles in Magnetic Fields Magnetic Forces on Current-Carrying Wires Applications: Current-Carrying Wires in Magnetic Fields Electromagnetism: The Source of Magnetic Fields Units of Chapter 19 Magnetic Materials Geomagnetism: The Earth’s Magnetic Field 19.1 Magnets, Magnetic Poles, and Magnetic Field Direction Magnets have two distinct types of poles; we refer to them as north and south. 19.1 Magnets, Magnetic Poles, and Magnetic Field Direction Two magnetic poles of opposite kind form a magnetic dipole. All known magnets are dipoles (or higher poles); magnetic monopoles could exist but have never been observed. A magnet creates a magnetic field: The direction of a magnetic field (B) at any location is the direction that the north pole of a compass would point if placed at that location. 19.1 Magnets, Magnetic Poles, and Magnetic Field Direction North magnetic poles are attracted by south magnetic poles, so the magnetic field points from north poles to south poles. The magnetic field may be represented by magnetic field lines. The closer together (that is, the denser) the B field lines, the stronger the magnetic field. At any location, the direction of the magnetic field is tangent to the field line, or equivalently, the way the north end of a compass points. 19.2 Magnetic Field Strength and Magnetic Force A magnetic field can exert a force on a moving charged particle. 19.2 Magnetic Field Strength and Magnetic Force The magnitude of the force is proportional to the charge and to the speed: SI unit of magnetic field: the tesla, T 19.2 Magnetic Field Strength and Magnetic Force In general, if the particle is moving at an angle to the field, The force is perpendicular to both the velocity and to the field. 19.2 Magnetic Field Strength and Magnetic Force A right-hand rule gives the direction of the force. 19.3 Applications: Charged Particles in Magnetic Fields A cathode-ray tube, such as a television or computer monitor, uses a magnet to direct a beam of electrons to different spots on a fluorescent screen, creating an image. 19.3 Applications: Charged Particles in Magnetic Fields A velocity selector consists of an electric and magnetic field at right angles to each other. Ions entering the selector will experience an electric force: and a magnetic force: These two forces will be perpendicular to each other. 19.3 Applications: Charged Particles in Magnetic Fields Given the values of the electric and magnetic fields, there will be a single velocity that will allow ions to pass through the selector undeflected. 19.3 Applications: Charged Particles in Magnetic Fields A mass spectrometer can be used to measure the masses of ions with equal charges and velocities. After passing through a velocity selector, ions enter a magnetic field. They will move in a circle of radius: 19.3 Applications: Charged Particles in Magnetic Fields Then the mass of the ion is: 19.4 Magnetic Forces on CurrentCarrying Wires The magnetic force on a current-carrying wire is a consequence of the forces on the charges. The force on an infinitely long wire would be infinite; the force on a length L of wire is: θ is the angle between I and B. 19.4 Magnetic Forces on CurrentCarrying Wires The direction of the force is given by a righthand rule: When the fingers of the right hand are pointed in the direction of the conventional current I and then curled toward the vector B, the extended thumb points in the direction of the magnetic force on the wire. 19.4 Magnetic Forces on CurrentCarrying Wires A current loop in a magnetic field will experience a torque: If there are multiple loops in a coil, The magnetic moment: 19.5 Applications: Current-Carrying Wires in Magnetic Fields A galvanometer has a coil in a magnetic field. When current flows in the coil, the deflection is proportional to the current. 19.5 Applications: Current-Carrying Wires in Magnetic Fields An electric motor converts electric energy into mechanical energy, using the torque on a current loop. 19.5 Applications: Current-Carrying Wires in Magnetic Fields An electronic balance uses magnetic force to balance an unknown mass. The amount of current required is proportional to the mass. 19.6 Electromagnetism: The Source of Magnetic Fields Experimentally, we observe that a current-carrying wire creates a magnetic field. 19.6 Electromagnetism: The Source of Magnetic Fields The magnitude of the field is given by: The constant μ0 is called the permeability of free space. 19.6 Electromagnetism: The Source of Magnetic Fields The field lines form circles around the wire; the direction is given by a right-hand rule. 19.6 Electromagnetism: The Source of Magnetic Fields The magnetic field at the center of a current loop: 19.6 Electromagnetism: The Source of Magnetic Fields A solenoid is a wire coiled into a long cylinder. The magnetic field inside is given by: 19.7 Magnetic Materials Atomic electrons have a property called “spin” that gives them a small magnetic moment. In multielectron atoms, the electrons are usually paired with an electron of the opposite spin, leaving no net magnetic moment. However, this is not always the case, and some atoms do have a permanent magnetic moment. They will experience a torque in a magnetic field, and will tend to align with it. 19.7 Magnetic Materials In ferromagnetic materials, the forces between neighboring atoms are strong enough that they tend to align in clusters called domains. These domains are macroscopic in size. 19.7 Magnetic Materials When a ferromagnet is placed in a magnetic field, the domains tend to align with it. 19.7 Magnetic Materials When the external magnetic field is removed, the domains tend to stay aligned, creating a permanent magnet. The most common ferromagnetic materials are iron, nickel, and cobalt. Some rare earth alloys are also ferromagnetic. 19.7 Magnetic Materials Ferromagnetic materials can be used to form electromagnets. Putting this material within a solenoid greatly enhances the magnetic field: Here, κm is the magnetic permeability of the material; for ferromagnets, κm is typically several thousand. 19.7 Magnetic Materials For commercially useful ferromagnets, a type of iron is used that does not retain its magnetization when the current is turned off (why?). 19.7 Magnetic Materials A “permanent” magnet can lose its magnetization through impact or heating. Every ferromagnetic material has a Curie temperature, above which the thermal motion immediately destroys any magnetic alignment. Lava flows “freeze” a record of the Earth’s magnetic field at the point where they cooled below the Curie temperature. In this way, historical values of the Earth’s field may be determined. 19.8 Geomagnetism: The Earth’s Magnetic Field The Earth’s magnetic field is similar to that of a bar magnet, although its origin must be in the currents of molten rock at its core. Its magnitude is approximately 10–5 to 10–4 T. 19.8 Geomagnetism: The Earth’s Magnetic Field The magnetic poles are not in exactly the same place as the geographic poles; when navigating with a compass, you need to know the angle between them, called the declination, at your position. 19.8 Geomagnetism: The Earth’s Magnetic Field Charged particles can become trapped around magnetic field lines. Such trapping of solar wind particles has resulted in bands of charged particles around the Earth called Van Allen belts. Review of Chapter 19 Opposite magnetic poles attract; like poles repel. Magnetic force on a charged particle: Magnetic force on a current-carrying wire: Force directions are determined using a right-hand rule. Review of Chapter 19 Torque on a current loop: Magnetic field produced by a long straight wire: The field forms circles around the wire. Review of Chapter 19 Magnetic field at the center of a current loop: Magnetic field at the center of a solenoid: Right-hand rules determine the directions of the fields. Review of Chapter 19 Ferromagnetic materials spontaneously align into domains. The domains then align with an external magnetic field. When the external field is removed, the ferromagnet may retain its magnetism.