 University Physics Volume 2

58 .

Calculate the magnetic force on a hypothetical particle of charge $1.0×10−19C1.0×10−19C$ moving with a velocity of $6.0×104i^m/s6.0×104i^m/s$ in a magnetic field of $1.2k^T.1.2k^T.$

59 .

Repeat the previous problem with a new magnetic field of $(0.4i^+1.2k^)T.(0.4i^+1.2k^)T.$

60 .

An electron is projected into a uniform magnetic field $(0.5i^+0.8k^)T(0.5i^+0.8k^)T$ with a velocity of $(3.0i^+4.0j^)×106m/s.(3.0i^+4.0j^)×106m/s.$ What is the magnetic force on the electron?

61 .

The mass and charge of a water droplet are $1.0×10−4g1.0×10−4g$ and $2.0×10−8C,2.0×10−8C,$ respectively. If the droplet is given an initial horizontal velocity of $5.0×105i^m/s,5.0×105i^m/s,$ what magnetic field will keep it moving in this direction? Why must gravity be considered here?

62 .

Four different proton velocities are given. For each case, determine the magnetic force on the proton in terms of e, $v0,v0,$ and $B0.B0.$ 63 .

An electron of kinetic energy 2000 eV passes between parallel plates that are 1.0 cm apart and kept at a potential difference of 300 V. What is the strength of the uniform magnetic field B that will allow the electron to travel undeflected through the plates? Assume E and B are perpendicular.

64 .

An alpha-particle $(m=6.64×10−27kg,(m=6.64×10−27kg,$ $q=3.2×10−19C)q=3.2×10−19C)$ moving with a velocity $v→=(2.0i^−4.0k^)×106m/sv→=(2.0i^−4.0k^)×106m/s$ enters a region where $E→=(5.0i^−2.0j^)×104V/mE→=(5.0i^−2.0j^)×104V/m$ and $B→=(1.0i^+4.0k^)×10−2T.B→=(1.0i^+4.0k^)×10−2T.$ What is the initial force on it?

65 .

An electron moving with a velocity $v→=(4.0i^+3.0j^+2.0k^)×106m/sv→=(4.0i^+3.0j^+2.0k^)×106m/s$ enters a region where there is a uniform electric field and a uniform magnetic field. The magnetic field is given by $B→=(1.0i^−2.0j^+4.0k^)×10−2T.B→=(1.0i^−2.0j^+4.0k^)×10−2T.$ If the electron travels through a region without being deflected, what is the electric field?

66 .

At a particular instant, an electron is traveling west to east with a kinetic energy of 10 keV. Earth’s magnetic field has a horizontal component of $1.8×10−5T1.8×10−5T$ north and a vertical component of $5.0×10−5T5.0×10−5T$ down. (a) What is the path of the electron? (b) What is the radius of curvature of the path?

67 .

What is the (a) path of a proton and (b) the magnetic force on the proton that is traveling west to east with a kinetic energy of 10 keV in Earth’s magnetic field that has a horizontal component of 1.8 x 10–5 T north and a vertical component of 5.0 x 10–5 T down?

68 .

What magnetic field is required in order to confine a proton moving with a speed of $4.0×106m/s4.0×106m/s$ to a circular orbit of radius 10 cm?

69 .

An electron and a proton move with the same speed in a plane perpendicular to a uniform magnetic field. Compare the radii and periods of their orbits.

70 .

A proton and an alpha-particle have the same kinetic energy and both move in a plane perpendicular to a uniform magnetic field. Compare the periods of their orbits.

71 .

A singly charged ion takes $2.0×10−3s2.0×10−3s$ to complete eight revolutions in a uniform magnetic field of magnitude $2.0×10−2T.2.0×10−2T.$ What is the mass of the ion?

72 .

A particle moving downward at a speed of $6.0×106m/s6.0×106m/s$ enters a uniform magnetic field that is horizontal and directed from east to west. (a) If the particle is deflected initially to the north in a circular arc, is its charge positive or negative? (b) If B = 0.25 T and the charge-to-mass ratio (q/m) of the particle is $4.0×107C/kg,4.0×107C/kg,$ what is the radius of the path? (c) What is the speed of the particle after it has moved in the field for $1.0×10−5s?1.0×10−5s?$ for 2.0 s?

73 .

A proton, deuteron, and an alpha-particle are all accelerated from rest through the same potential difference. They then enter the same magnetic field, moving perpendicular to it. Compute the ratios of the radii of their circular paths. Assume that $md=2mpmd=2mp$ and $mα=4mp.mα=4mp.$

74 .

A singly charged ion is moving in a uniform magnetic field of $7.5×10−2T7.5×10−2T$ completes 10 revolutions in $3.47×10−4s.3.47×10−4s.$ Identify the ion.

75 .

Two particles have the same linear momentum, but particle A has four times the charge of particle B. If both particles move in a plane perpendicular to a uniform magnetic field, what is the ratio $RA/RBRA/RB$ of the radii of their circular orbits?

76 .

A uniform magnetic field of magnitude $BB$ is directed parallel to the z-axis. A proton enters the field with a velocity $v→=(4j^+3k^)×106m/sv→=(4j^+3k^)×106m/s$ and travels in a helical path with a radius of 5.0 cm. (a) What is the value of $BB$? (b) What is the time required for one trip around the helix? (c) Where is the proton $5.0×10−7s5.0×10−7s$ after entering the field?

77 .

An electron moving along the +x -axis at $5.0×106m/s5.0×106m/s$ enters a magnetic field that makes a $75o75o$ angle with the x-axis of magnitude 0.20 T. Calculate the (a) pitch and (b) radius of the trajectory.

78 .

(a) A 0.750-m-long section of cable carrying current to a car starter motor makes an angle of 60º with Earth’s $5.5×10−5T5.5×10−5T$ field. What is the current when the wire experiences a force of $7.0×10−3N?7.0×10−3N?$ (b) If you run the wire between the poles of a strong horseshoe magnet, subjecting 5.00 cm of it to a 1.75-T field, what force is exerted on this segment of wire?

79 .

(a) What is the angle between a wire carrying an 8.00-A current and the 1.20-T field it is in if 50.0 cm of the wire experiences a magnetic force of 2.40 N? (b) What is the force on the wire if it is rotated to make an angle of 90º with the field?

80 .

A 1.0-m-long segment of wire lies along the x-axis and carries a current of 2.0 A in the positive x-direction. Around the wire is the magnetic field of $(3.0i^×4.0k^)×10−3T.(3.0i^×4.0k^)×10−3T.$ Find the magnetic force on this segment.

81 .

A 5.0-m section of a long, straight wire carries a current of 10 A while in a uniform magnetic field of magnitude $8.0×10−3T.8.0×10−3T.$ Calculate the magnitude of the force on the section if the angle between the field and the direction of the current is (a) 45°; (b) 90°; (c) 0°; or (d) 180°.

82 .

An electromagnet produces a magnetic field of magnitude 1.5 T throughout a cylindrical region of radius 6.0 cm. A straight wire carrying a current of 25 A passes through the field as shown in the accompanying figure. What is the magnetic force on the wire? 83 .

The current loop shown in the accompanying figure lies in the plane of the page, as does the magnetic field. Determine the net force and the net torque on the loop if I = 10 A and B = 1.5 T. 84 .

A circular coil of radius 5.0 cm is wound with five turns and carries a current of 5.0 A. If the coil is placed in a uniform magnetic field of strength 5.0 T, what is the maximum torque on it?

85 .

A circular coil of wire of radius 5.0 cm has 20 turns and carries a current of 2.0 A. The coil lies in a magnetic field of magnitude 0.50 T that is directed parallel to the plane of the coil. (a) What is the magnetic dipole moment of the coil? (b) What is the torque on the coil?

86 .

A current-carrying coil in a magnetic field experiences a torque that is 75% of the maximum possible torque. What is the angle between the magnetic field and the normal to the plane of the coil?

87 .

A 4.0-cm by 6.0-cm rectangular current loop carries a current of 10 A. What is the magnetic dipole moment of the loop?

88 .

A circular coil with 200 turns has a radius of 2.0 cm. (a) What current through the coil results in a magnetic dipole moment of 3.0 Am2? (b) What is the maximum torque that the coil will experience in a uniform field of strength $5.0×10−2T?5.0×10−2T?$ (c) If the angle between μ and B is 45°, what is the magnitude of the torque on the coil? (d) What is the magnetic potential energy of coil for this orientation?

89 .

The current through a circular wire loop of radius 10 cm is 5.0 A. (a) Calculate the magnetic dipole moment of the loop. (b) What is the torque on the loop if it is in a uniform 0.20-T magnetic field such that $μμ$ and B are directed at $30°30°$ to each other? (c) For this position, what is the potential energy of the dipole?

90 .

A wire of length 1.0 m is wound into a single-turn planar loop. The loop carries a current of 5.0 A, and it is placed in a uniform magnetic field of strength 0.25 T. (a) What is the maximum torque that the loop will experience if it is square? (b) If it is circular? (c) At what angle relative to B would the normal to the circular coil have to be oriented so that the torque on it would be the same as the maximum torque on the square coil?

91 .

Consider an electron rotating in a circular orbit of radius r. Show that the magnitudes of the magnetic dipole moment μ and the angular momentum L of the electron are related by:

$μ L = e 2 m . μ L = e 2 m .$
92 .

The Hall effect is to be used to find the sign of charge carriers in a semiconductor sample. The probe is placed between the poles of a magnet so that magnetic field is pointed up. A current is passed through a rectangular sample placed horizontally. As current is passed through the sample in the east direction, the north side of the sample is found to be at a higher potential than the south side. Decide if the number density of charge carriers is positively or negatively charged.

93 .

The density of charge carriers for copper is $8.47×10288.47×1028$ electrons per cubic meter. What will be the Hall voltage reading from a probe made up of $3cm×2 cm×1 cm(L×W×T)3cm×2 cm×1 cm(L×W×T)$ copper plate when a current of 1.5 A is passed through it in a magnetic field of 2.5 T perpendicular to the $3cm×2 cm.3cm×2 cm.$

94 .

The Hall effect is to be used to find the density of charge carriers in an unknown material. A Hall voltage 40 $μVμV$ for 3-A current is observed in a 3-T magnetic field for a rectangular sample with length 2 cm, width 1.5 cm, and height 0.4 cm. Determine the density of the charge carriers.

95 .

Show that the Hall voltage across wires made of the same material, carrying identical currents, and subjected to the same magnetic field is inversely proportional to their diameters. (Hint: Consider how drift velocity depends on wire diameter.)

96 .

A velocity selector in a mass spectrometer uses a 0.100-T magnetic field. (a) What electric field strength is needed to select a speed of $4.0×106m/s?4.0×106m/s?$ (b) What is the voltage between the plates if they are separated by 1.00 cm?

97 .

Find the radius of curvature of the path of a 25.0-MeV proton moving perpendicularly to the 1.20-T field of a cyclotron.

98 .

Unreasonable results To construct a non-mechanical water meter, a 0.500-T magnetic field is placed across the supply water pipe to a home and the Hall voltage is recorded. (a) Find the flow rate through a 3.00-cm-diameter pipe if the Hall voltage is 60.0 mV. (b) What would the Hall voltage be for the same flow rate through a 10.0-cm-diameter pipe with the same field applied?

99 .

Unreasonable results A charged particle having mass $6.64×10−27kg6.64×10−27kg$ (that of a helium atom) moving at $8.70×105m/s8.70×105m/s$ perpendicular to a 1.50-T magnetic field travels in a circular path of radius 16.0 mm. (a) What is the charge of the particle? (b) What is unreasonable about this result? (c) Which assumptions are responsible?

100 .

Unreasonable results An inventor wants to generate 120-V power by moving a 1.00-m-long wire perpendicular to Earth’s $5.00×10−5T5.00×10−5T$ field. (a) Find the speed with which the wire must move. (b) What is unreasonable about this result? (c) Which assumption is responsible?

101 .

Unreasonable results Frustrated by the small Hall voltage obtained in blood flow measurements, a medical physicist decides to increase the applied magnetic field strength to get a 0.500-V output for blood moving at 30.0 cm/s in a 1.50-cm-diameter vessel. (a) What magnetic field strength is needed? (b) What is unreasonable about this result? (c) Which premise is responsible?

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