PHY_10_55_V2_DM_Ampere force. Lorentz force

Permanent Magnets

The Force that a Magnetic Field Exerts on a Moving Charge

1.   Which one of the following statements concerning permanent magnets is false?

      (a)  The north pole of a permanent magnet is attracted to a south pole.

      (b)  All permanent magnets are surrounded by a magnetic field.

      (c)  The direction of a magnetic field is indicated by the north pole of a compass.

      (d) Magnetic field lines outside a permanent magnet originate from the north pole and end on       the south pole.

      (e)  When a permanent magnet is cut in half, one piece will be a north pole and one piece will be

            a south pole.

2.   Which combination of units can be used to express the magnetic field?

      (a)  kg Ч m²/C                           (c)  N Ч m²/C                            (e)  kg Ч m/(C Ч s²)

      (b)  kg Ч s/C²                            (d) kg/(C Ч s) 

3.   Which one of the following statements concerning the magnetic force on a charged particle in a magnetic field is true?

      (a)  It is a maximum if the particle is stationary.

      (b)  It is zero if the particle moves perpendicular to the field.

      (c)  It is a maximum if the particle moves parallel to the field.

      (d) It acts in the direction of motion for a positively charged particle.

      (e)  It depends on the component of the particle's velocity that is perpendicular to the field.  

4.   Complete the following statement: The magnitude of the magnetic force that acts on a charged particle in a magnetic field is independent of

      (a)  the sign of the charge.                              

      (b)  the magnitude of the charge.                          

      (c)  the magnitude of the magnetic field.       

      (d) the direction of motion of the particle.

      (e)  the velocity components of the particle.

5.   A charged particle is moving in a uniform, constant magnetic field.  Which one of the following statements concerning the magnetic force exerted on the particle is false?

      (a)  It does no work on the particle.                                        (d) It can act only on a particle in motion.

      (b)  It increases the speed of the particle.        (e)  It does not change the kinetic energy of

      (c)  It changes the velocity of the particle.            the particle.

6.   A proton traveling due east in a region that contains only a magnetic field experiences a vertically upward force (away from the surface of the earth). What is the direction of the magnetic field?

      (a)  north                                  (c)  south                                 (e)  down

      (b)  east                                    (d) west  

7.   A charged particle is fired from a gun with a velocity of 5.2 ґ 10⁴ m/s at an angle of 35° with respect to a 0.0045-T magnetic field.  If the magnetic field exerts a force of  0.0026 N on the particle, determine the magnitude of the charge.

      (a)  11 µC                                (c)  19 µC                                (e)  27 µC

      (b)  15 µC                                (d) 23 µC

8.   Which one of the following statements best explains why a constant magnetic field can do no work on a moving charged particle?

      (a)  The magnetic field is conservative.

      (b)  The magnetic force is a velocity dependent force.

      (c)  The magnetic field is a vector and work is a scalar quantity.

      (d) The magnetic force is always perpendicular to the velocity of the particle.  

      (e)  The electric field associated with the particle cancels the effect of the magnetic field on the

            particle.

9.   An electron traveling due south enters a region that contains a uniform magnetic field that points due east.  In which direction will the electron be deflected?

      (a)  east                                    (c)  up                                      (e)  south

      (b)  west                                   (d) down        

10. Two electrons are located in a region of space where the magnetic field is zero.  Electron A is at rest; and electron B is moving westward with a constant velocity.  A non-zero magnetic field directed eastward is then applied to the region.  In what direction, if any, will each electron be moving after the field is applied?

            electron A                       electron B

      (a)  at rest                                westward       

      (b)  northward                         eastward

      (c)  at rest                                eastward

      (d) southward                         downward, toward the earth

      (e)  upward, away from earth  westward

11. An electron is moving with a speed of 3.5 ґ 10⁵ m/s when it encounters a magnetic field of 0.60 T.  The direction of the magnetic field makes an angle of 60.0° with respect to the velocity of the electron.  What is the magnitude of the magnetic force on the electron?

      (a)  4.9 ґ 10^–13 N                     (c)  1.7 ґ 10^–13 N                           (e)  2.9 ґ 10^–14 N

      (b)  3.2 ґ 10^–13 N                     (d) 3.4 ґ 10^–14 N

Ampere’s Law. Magnetic Materials

12. Which one of the following materials is not ferromagnetic?

      (a)  iron                                    (c)  nickel                                 (e)  cobalt

      (b)  chromium dioxide            (d) aluminum

13. Each second, 1.25 ґ 10¹⁹ electrons in a narrow beam pass through a small hole in a wall. The beam is perpendicular to the wall.  Using Ampere’s law, determine the magnitude of the magnetic field in the wall at a radius of 0.750 m from the center of the beam. 

      (a)  1.18 ґ 10^–9 T                     (c)  2.62 ґ 10^–3 T                           (e)  3.33 ґ 10¹² T

      (b)  5.33 ґ 10^–7 T                     (d) 4.14 ґ 10⁸ T

Forces on currents in magnetic fields

µ_o = 4π ґ 10^-7 N A^-2 and g = 9.81 m s^-2 

1.         (a) Write down the formula for the force on a straight wire of length L placed at right        angles to a magnetic field of flux density B.

(b)       A pivot arrangement using a wire of length 15 cm is placed in a magnetic field. When a current of 4 A is passed through the wire it is found that a mass of 2.0 g must be hung from the wire to return it to a horizontal position. g = 9.81 m s^-2

            Calculate the magnetic flux density of the magnet.

2.         A horizontal wire 6 cm long and mass 1.5 g is placed at right angles to a magnetic file of flux density 0.5 T. Calculate the current that must be passed through the wire so that it is self-supporting

3.         A wire of length 0.5 m carrying a current of 2 A is placed at right angles to a magnetic field of 0.2 T. Calculate the force on the wire.

4.         What is the force between two wires placed 0.2 m apart in a vacuum each carrying a         current of 5 A.

Questions 5 and 6:

A magnetic field exerts a force of 0.25 N on an 8.0 cm length of wire carrying a current of 3.0 A at right angles to the field.

5.         Calculate the force the same field would exert on a wire 20 cm long carrying the same current.

6.         Calculate the force the same field would exert on three insulated wires, each 20 cm long and held together parallel to each other, each carrying a current of 3.0 A in the same direction.

7. (a) Using the ideas of forces on currents in magnetic fields and the diagram explain the operation of a moving coil loudspeaker.

(b) Calculate the force on the voice coil of a loudspeaker that has a diameter of 2 cm, a 100 turns and carries a current of 20 mA if the field strength of the tubular magnet is 0.1T.