Applications: Motors Loudspeakers Galvanometers Chapter 27 opener. Magnets produce magnetic fields, but so do electric currents. An electric current flowing in this straight wire produces a magnetic field which causes the tiny pieces of iron (iron “filings”) to align in the field. We shall see in this Chapter how magnetic field is defined, and that the magnetic field direction is along the iron filings. The magnetic field lines due to the electric current in this long wire are in the shape of circles around the wire. We also discuss how magnetic fields exert forces on electric currents and on charged particles, as well as useful applications of the interaction between magnetic fields and electric currents and moving electric charges.

An electric motor uses the torque on a current loop in a magnetic field to turn magnetic energy into kinetic energy. Figure 27-23. Diagram of a simple dc motor. Figure 27-24. The commutator-brush arrangement in a dc motor ensures alternation of the current in the armature to keep rotation continuous. The commutators are attached to the motor shaft and turn with it, whereas the brushes remain stationary.

Loudspeakers use the principle that a magnet exerts a force on a current-carrying wire to convert electrical signals into mechanical vibrations, producing sound. Figure 27-26. Loudspeaker.

A galvanometer takes advantage of the torque on a current loop to measure current; the spring constant is calibrated so the scale reads in amperes. Figure 27-27. Galvanometer.

Discovery & Properties of the Electron Electrons were first observed in “cathode ray tubes”. These tubes had a very small amount of gas inside, & when a high voltage was applied to the cathode, some “cathode rays” appeared to travel from the cathode to the anode. Figure 27-29. Discharge tube. In some models, one of the screens is the anode (positive plate).

The value of e/m for the “cathode rays” was measured in 1897 using the apparatus below; it was then that the “rays” began to be called electrons. Figure 27-30. Cathode rays deflected by electric and magnetic fields.

the electron is a constituent of the atom. Robert A. Millikan measured the electron charge e directly shortly thereafter, using the oil-drop apparatus diagrammed below, & showed that the electron is a constituent of the atom. (& not an atom itself, as its mass is far too small). Currently accepted values of the electron mass & charge are m = 9.1  10-31 kg e = 1.6  10-19 C Figure 27-31. Millikan’s oil-drop experiment.

The Hall Effect When a current-carrying wire is placed in a magnetic field, there is a sideways force (due to v  B) on the electrons in the wire. This tends to push them to one side & results in a potential difference from one side of the wire to the other; this is called the Hall Effect. The emf differs in sign depending on the sign of the charge carriers; this is how it was first determined that the charge carriers in ordinary conductors are negatively charged. Figure 27-32. The Hall effect. (a) Negative charges moving to the right as the current. (b) Positive charges moving to the left as the current.

Drift velocity using the Hall effect A long copper strip 1.8 cm wide and 1.0 mm thick is placed in a 1.2-T magnetic field. When a steady current of 15 A passes through it, the Hall emf is measured to be 1.02 μV. Calculate the drift velocity of the electrons & the density of free (conducting) electrons (number per unit volume) in the copper. Solution: The drift velocity is EH/Bd = 4.7 x 10-5 m/s. Since I = nevdA, n = I/evdA = 11 x 1028 m-3.

Mass Spectrometer* A mass spectrometer measures the masses of atoms. If a charged particle is moving through perpendicular electric and magnetic fields, there is a particular speed at which it will not be deflected, which then allows the measurement of its mass:

All the atoms reaching the second magnetic field will have the same speed; their radius of curvature will depend on their mass. Figure 27-33. Bainbridge-type mass spectrometer. The magnetic fields B and B’ point out of the paper (indicated by the dots), for positive ions.

Example Mass spectrometry. Carbon atoms of atomic mass 12.0 u are found to be mixed with another, unknown, element. In a mass spectrometer with fixed B′, the carbon traverses a path of radius 22.4 cm and the unknown’s path has a 26.2-cm radius. What is the unknown element? Assume the ions of both elements have the same charge. Solution: The ratio of masses is equal to the ratio of the radii, or 1.17. Therefore the unknown element has a mass of 14.0 u, and is probably nitrogen.

Summary Magnets have north & south poles. Like poles repel, unlike attract. Unit of magnetic field: tesla. Electric currents produce magnetic fields. A magnetic field exerts a force on an electric current: A magnetic field exerts a force on a moving charge: Torque on a current loop: Magnetic dipole moment: