ACES EE EE Terms Refresher - 9

Which statement about a charge placed on a dielectric material is true?

• The charge diffuses across the materials surface

• The charge diffuses through the interior of the material

• The charge is confined to the region in which the charge was placed

• the charge increases the conductivity of the material

The coulomb force, F acts on two charges a distance, r apart. What is F proportional to?

• r

• r²

• 1/r²

• 1/r³

Which of the following is NOT a property of magnetic field lines?

• the field is stronger where the lines are closer together

• the lines intersect surfaces of equal intensity at right angles

• magnetic field lines have no beginnings and no ends

• the lines cross themselves only at right angles

The tesla us a unit of

• permittivity

• capacitance

• inductance

• magnetic flux

Two identical coils of radius r are placed at a distance r apart as shown. Such a configuration is called a Helmholtz coil. Which of the following describes the magnetic field created by passing a uniform current through the assembly?

• The magnetic field is negligible regardless of the magnitude of I

• the magnetic field is zero midway between the two coils

• the magnetic field is fairly uniform between the two coils

• the magnetic field is zero at the centers of the coils

Which statement is true?

• magnetic flux lines have sources only

• magnetic flux lines have sinks only

• magnetic flux lines have both sources and sinks

• magnetic flux lines do not have sources or sinks

Which of the following statements regarding the motion of a conductor through a changing magnetic field is FALSE?

• the lines of magnetic flux pass from the north pole to the south pole of the magnet

• when a conductor is "open circuited", no current flows despite its motion through the field

• the conductor must move at constant velocity in order to generate a current

• a current flowing counter to the direction of the conductor's motion will create a torque

Modern locomotive are powered by DC motors driven by DC generators. These are in turn driven by diesel engines. Which of the following is the reason for using such a configuration instead of AC generator-motor sets?

• the DC configuration provides high torque and good incremental power at low speeds, and performs equally well at high speeds

• the DC equipment is significantly less expensive

• historically, the DC engine configuration has been used. There is no reason to change this

• by using the DC equipment the power factor problems associated with AC equipment are avoided

In a DC motor, what is the definition of "field resistance"?

• it is the load resistance seen by a generator without considering inductance

• it is the resistance of the excitation circuit

• it is the resistance of the armature windings plus the load resistance

• it is the static resistance of the motor

In a DC motor, which of the following does NOT cause sparking at a commutator?

• no load on the output leads

• frozen armature

• high brush contact resistance

• graphite brushes with good contact pressure