20-11 The Hall Effect
• When a current-carrying conductor is held
firmly in the magnetic field, the field exerts a
sideways force on the charges moving in the
conductor. This results in a potential
difference in the conductor. When the
potential difference builds up and produces
electric field EH it produces and exerts a force
eEH , that is equal and opposite to the
magnetic force…this is called the Hall effect.
• The potential difference produced is called
the Hall emf.
20-12 Mass Spectrometer
• Mass spectrometers were designed to measure
the mass of atoms. Ions are produced by heating
substances to high temperatures…
• If the electric force qE up on + ions is just
balanced by magnetic force qvB (down on
positive ions) is balanced, then the ion will pass
through a selector slit. ( qE=qvB or v=E/B
• Isotopes were found in this way, because only
ions or atoms of the same mass pass through the
slit. (see p607 and p608)
• See Example 20-14 p608
20-13 Ferromagnetism; Domains
*Some materials can be made into ‘permanent’
magnets.
•These materials are said to be ferromagnetic.
•Small, often microscopic regions, called
domains, (at most 1mm) are aligned in ferromagnetic substances. They are random in nonferromagnetic substances.
•Above a certain temperature, called the Curie
Temperature, (1043 K for iron) no magnetism can
be produced at all
20-14 Electromagnets and
Solenoids
*When a coil is wrapped around iron, or other
ferromagnetic substance, an electromagnet can be
produced.
*Electromagnets have many uses. (see p 611)
20-15 Magnetic Fields in
Magnetic Materials ; Hysteresis
*The magnetic field inside a solenoid can be found
using B0 = u0 nI
•When a ferromagnetic material is inside a solenoid
we use the formula:
B = u nI where u is the magnetic permeability of the
material in the center of the solenoid or electromagnet.
*Measurements of magnetic materials are often made
using a torus, which is an iron ring solenoid.
* Hysteresis is a curve resulting from these
measurements, using a torus, where the curves do not
retrace their data points of the previous curve.
Homework ch20TXTD
• P618-619 prob. 47,50-51,56