Magnetism: Understanding Magnetic Fields, Forces, and Currents, Slides of Physics

Download Magnetism: Understanding Magnetic Fields, Forces, and Currents and more Slides Physics in PDF only on Docsity! Chapter 22 Magnetism | Docsity.com Units of Chapter 22 • The Magnetic Field • The Magnetic Force on Moving Charges • The Motion of Charged Particles in a Magnetic Field • The Magnetic Force Exerted on a Current-Carrying Wire • Loops of Current and Magnetic Torque Docsity.com 22-1 The Magnetic Field The magnetic field can be visualized using magnetic field lines, similar to the electric field. If iron filings are allowed to orient themselves around a magnet, they follow the field lines. Docsity.com 22-1 The Magnetic Field By definition, magnetic field lines exit from the north pole of a magnet and enter at the south pole. Magnetic field lines cannot cross, just as electric field lines cannot. Docsity.com 22-1 The Magnetic Field The Earth’s magnetic field resembles that of a bar magnet. However, since the north poles of compass needles point towards the north, the magnetic pole there is actually a south pole. Docsity.com 22-2 The Magnetic Force on Moving Charges In order to figure out which direction the force is on a moving charge, you can use a right-hand rule. This gives the direction of the force on a positive charge; the force on a negative charge would be in the opposite direction. Docsity.com 22-2 The Magnetic Force on Moving Charges This relationship between the three vectors – magnetic field, velocity, and force – can also be written as a vector cross product: Docsity.com 22-3 The Motion of Charged Particles in a Magnetic Field A positively charged particle in an electric field experiences a force in the direction of the field; in a magnetic field the force is perpendicular to the field. This leads to very different motions: Docsity.com 22-3 The Motion of Charged Particles in a Magnetic Field In a mass spectrometer, ions of different mass and charge move in circles of different radii, allowing separation of different isotopes of the same element. Docsity.com 22-3 The Motion of Charged Particles in a Magnetic Field If a particle’s velocity makes an angle with the magnetic field, the component of the velocity along the magnetic field will not change; a particle with an initial velocity at an angle to the field will move in a helical path. Docsity.com 22-4 The Magnetic Force Exerted on a Current-Carrying Wire The force on a segment of a current-carrying wire in a magnetic field is given by: Docsity.com 22-5 Loops of Current and Magnetic Torque If the plane of the loop is at an angle to the magnetic field, Docsity.com 22-5 Loops of Current and Magnetic Torque To increase the torque, a long wire may be wrapped in a loop many times, or “turns.” If the number of turns is N, we have Docsity.com 22-5 Loops of Current and Magnetic Torque The torque on a current loop is proportional to the current in it, which forms the basis of a variety of useful electrical instruments. Here is a galvanometer: Docsity.com 22-6 Electric Currents, Magnetic Fields, and Ampère’s Law The magnetic field is inversely proportional to the distance from the wire: Docsity.com 22-6 Electric Currents, Magnetic Fields, and Ampère’s Law Ampère’s Law relates the current through a surface defined by a closed path to the magnetic field along the path: Docsity.com 22-6 Electric Currents, Magnetic Fields, and Ampère’s Law We can use Ampère’s Law to find the magnetic field around a long, straight wire: Docsity.com 22-7 Current Loops and Solenoids A solenoid is a series of current loops formed into the shape of a cylinder: Docsity.com 22-7 Current Loops and Solenoids We can use Ampère’s Law to find the field inside the solenoid: Docsity.com 22-8 Magnetism in Matter The electrons surrounding an atom create magnetic fields through their motion. Usually these fields are in random directions and have no net effect, but in some atoms there is a net magnetic field. If the atoms have a strong tendency to align with each other, creating a net magnetic field, the material is called ferromagnetic. Docsity.com 22-8 Magnetism in Matter Many materials that are not ferromagnetic are paramagnetic – they will partially align in a strong magnetic field, but the alignment disappears when the external field is gone. Docsity.com Finally, all materials exhibit diamagnetism – an applied magnetic field induces a small magnetic field in the opposite direction in the material. 22-8 Magnetism in Matter Docsity.com Summary of Chapter 22 • All magnets have two poles, north and south. • Magnetic fields can be visualized using magnetic field lines. These lines point away from north poles and toward south poles. • The Earth produces its own magnetic field. • A magnetic field exerts a force on an electric charge only if it is moving: • A right-hand rule gives the direction of the magnetic force on a positive charge. Docsity.com Summary of Chapter 22 • Electric currents create magnetic fields; the direction can be determined using a right-hand rule. • Ampère’s law: • Magnetic field of a long, straight wire: • Force between current-carrying wires: Docsity.com Summary of Chapter 22 • The magnetic field of a current loop is similar to that of a bar magnet. • The field in the center of the loop is: • Magnetic field of a solenoid: Docsity.com Summary of Chapter 22 • A paramagnetic material has no magnetic field, but will develop a small magnetic field in the direction of an applied field. • A ferromagnetic material may have a permanent magnetic field; when placed in an applied field it develops permanent magnetization. • All materials have a small diamagnetic effect; when placed in an external magnetic field they develop a small magnetic field opposite to the applied field. Docsity.com