Electromagnetism

Question 1

1. An electric field is most directly
   related to:
   A. the momentum of a test charge
   B. the kinetic energy of a test charge
   C. the potential energy of a test
   charge
   D. the force acting on a test charge

Answer 1

1. D. the force acting on a test charge

Question 2

2. An electron is accelerated from rest
   through a potential difference V. Its
   final speed is proportional to:
   A. V
   C. V»
   B. V«
   D. 1/V

Answer 2

B. V«

Question 3

3. The magnetic force on a charged
   particle is in the direction of its
   velocity if:
   A. it is moving in the direction of the
   field
   B. it is moving opposite to the
   direction of the field
   C. it is moving perpendicular to the
   field
   D. never

Answer 3

D. never

Question 4

4. Charge Q is distributed uniformly
   throughout an insulating sphere of
   radius R. The magnitude of the
   electric field at a point R/2 from the
   center is:
   A. Q/4 πε0R»
   B. Q/πε0R»
   C. 3Q/4ε0R»
   D. Q/8πε0R»

Answer 4

D. Q/8πε0R»

Question 5

6. A cyclotron operates with a given
   magnetic field and at a given
   frequency. If R denotes the radius of
   the final orbit, the final particle
   energy is proportional to:
   A. 1/R
   C. R
   B. R»
   D. R»>

Answer 5

B. R»

Question 6

7. Choose the correct statement
   concerning electric field lines:
   A. field lines may cross
   B. field lines are close together
   where the field is large
   C. field lines point away from a
   negatively charged particle
   D. a charged pint particle released
   from rest moves along a field line

Answer 6

B. field lines are close together
where the field is large

Question 7

8. The force acting on a pole of 3 Wb is
   12 N. The magnetic intensity of the
   magnetic field is
   A. 4 N/Wb C. 12 N/Wb
   B. 36 N/Wb D. 8 N/Wb

Answer 7

A. 4 N/Wb

Question 8

9. A particle with charge Q is placed
   outside a large neutral conducting
   sheet. At any point in the interior of
   the sheet the electric field produced
   by chares on the surface is directed:
   A. toward the surface
   B. away from the surface
   C. toward Q
   D. away from Q

Answer 8

C. toward Q

Question 9

10. An electron volt is:
    A. the force acting on an electron in
    a field of 1 N/C
    B. the force required to move an
    electron 1 meter
    C. the energy gained by an electron
    in moving through a potential
    difference of 1 volt
    D. the energy needed to move an
    electron through 1 meter in any
    electric field

Answer 9

. C. the energy gained by an
electron in moving through a
potential difference of 1 volt

Question 10

11. An EMF of one volt:
    A. Cannot drive much current
    through a circuit.
    B. Represents a low resistance.
    C. Can sometimes produce a large
    current.
    D. Drops to zero in a short time.

Answer 10

11. C. Can sometimes produce a large
    current.

Question 11

12. At any point the magnetic field lines
    are in the direction of:
    A. the magnetic force on a moving
    positive charge
    B. the magnetic force on a moving
    negative charge
    C. the velocity of a moving positive
    charge
    D. None of the above

Answer 11

12. D. None of the above

Question 12

14. A positive electric pole:
    A. Has a deficiency of electrons.
    B. Has fewer electrons than the
    negative pole.
    C. Has an excess of electrons.
    D. Has more electrons than the
    negative pole.

Answer 12

14. B. Has fewer electrons than the negative pole.

Question 13

15. Two-point particles, with charges of
    q1 and q2, are placed a distance r
    apart. The electric field is zero at a
    point P between the particles on the
    line segment connecting them. We
    conclude that:
    A. q1 and q2 must have the same
    magnitude and sign
    B. P must be midway between the
    particles
    C. q1 and q2 must have the same sign
    but may have different
    magnitudes
    D. q1 and q2 must have equal
    magnitudes and opposite signs

Answer 13

15. C. q1 and q2 must have the same
    sign but may have different
    magnitudes

Question 14

16. A small single-phase transformer has
    10.2 watts at no load. The core has a
    volume of 750 cubic cm. The
    maximum flux density is 10,000
    gauss and the hysteresis constant of
    the core is 5 x 10-4
    . Using the
    Steinmetz law to find the hysteresis,
    determine the eddy current loss.
    A. 4.55 W C. 5.55 W
    B. 3.55 W D. 2.55 W

Answer 14

16. A. 4.55 W

Question 15

17. A hollow conductor is positively
    charged. A small uncharged metal
    ball is lowered by a silk thread
    through a small opening in the top of
    the conductor and allowed to touch
    surface. After the ball is removed, it
    will have:
    A. a positive charge
    B. a negative charge
    C. no appreciable charge
    D. a charge whose sign depends on
    what part of the inner surface it
    touched

Answer 15

17. C. no appreciable charge

Question 16

18. The direction of the magnetic field in
    a certain region of space is
    determined by firing a test charge
    into the region with its velocity in
    various directions in different trials.
    The field direction is:
    A. one of the directions of the
    velocity when the magnetic force
    is zero
    B. the direction of the velocity when
    the magnetic force is a maximum
    C. the direction of the magnetic
    force
    D. perpendicular to the velocity
    when the magnetic force is zero

Answer 16

18. A. one of the directions of the
    velocity when the magnetic
    force is zero

Question 17

20. The reluctance of a non-magnetic
    circuit is 12 units. How much flux
    will be set up if surrounded by a coil
    600 turns carrying a current of 3A?
    A. 150 Wb C. 16.67 Wb
    B. 50 Wb D. 200 Wb

Answer 17

20. A. 150 Wb

Question 18

21.If R is the distance from a magnetic
dipole, then the magnetic field it
produces is proportional to:
A. 1/R
C. R»
B. 1/R»
D. 1/R»>

Answer 18

D. 1/R»>

Question 19

22. A toroid with a square cross section
    carries current i. The magnetic field
    has its largest magnitude:
    A. at the center of the hole
    B. just inside the toroid at its inner
    surface
    C. just inside the toroid a its outer
    surface
    D. at any point inside (the field is
    uniform)

Answer 19

22. B. just inside the toroid at its inner
    surface

Question 20

23. The equipotential surfaces associated
    with a charged point particle are:
    A. radially outward form the particle
    B. vertical planes
    C. horizontal planes
    D. concentric spheres centered at the
    particle

Answer 20

23. D. concentric spheres centered at
    the particle

Question 21

24. The outer surface of the cardboard
    center of a paper towel roll:
    A. is a possible Gaussian surface
    B. cannot be a Gaussian surface
    because it encloses no charge
    C. cannot be a Gaussian surface
    since it is an insulator
    D. cannot be a Gaussian surface
    because it is not a closed surface

Answer 21

24. D. cannot be a Gaussian surface
    because it is not a closed surface

Question 22

25. A magnetic circuit consists of silicon
    steel of 3000 permeability and an air
    gap. The length of the steel core is
    10cm and the air gap is 2 cm. Both have the same cross section of 1.5 sq.
    cm. A current of 1/2 ampere flows
    through the windings to produce
    2351 maxwells flux. How many turns
    are there in the coil?
    A. 4120 turns
    B. 5000 turns
    C. 2500 turns
    D. 1250 turns

Answer 22

25. B. 5000 turns

Question 23

26. A magnetic torque exerted on a flat
    current-carrying loop of wire by a
    uniform magnetic field
    B
    is:
    A. maximum when the plane of the
    loop is perpendicular to
    B
    B. maximum when the plane of the
    loop is parallel to
    B
    C. dependent on the shape of the
    loop for a fixed loop area
    D. independent of the orientation of
    the loop

Answer 23

26. B. maximum when the plane of the
    loop is parallel to
    B

Question 24

27. A square loop of current-carrying
    wire with edge length a is in the xy
    plane, the origin being at its center.
    Along which of the following lines
    can be charge move without
    experiencing a magnetic force?
    A. x = 0, y = a/2
    B. x = a/2, y = a/2
    C. x = a/2, y = 0
    D. x = 0, y = 0

Answer 24

27. D. x = 0, y = 0

Question 25

28. A magnetic field exerts a force on a charged particle: A. always B. never C. if the particle is moving across the field lines D. if the particle is moving along the field lines

Answer 25

28. C. if the particle is moving across the field lines

Question 26

29. Helmholtz coils are commonly used in the laboratory because the magnetic field between them: A. can be varied more easily than fields of other current arrangements B. is especially strong C. nearly cancels Earth’s magnetic field D. is nearly uniform

Answer 26

29. D. is nearly uniform

Question 27

31. Two spheres separated from each other by 10m have charges of 0.001 coulomb and 0.003 coulomb respectively. In between the two spheres is a point of zero electric field. What is the distance from the 0.001 coulomb sphere? A. 6.34 m C. 7.78 m B. 2.24 m D. 3.66 m

Answer 27

31. D. 3.66 m

Question 28

32. An electron traveling north enters a region where the electric field is uniform and points north. The electron: A. speed up B. slow down C. veers east D. veers west

Answer 28

32. B. slow down

Question 29

33. Lines of the magnetic field produced by a long straight wire carrying a current are: A. in the direction of the current B. radially outward form the wire C. radially inward toward the wire D. circles that are concentric with the wire

Answer 29

33. D. circles that are concentric with the wire

Question 30

35. An electron is moving north in a region where the magnetic field is south. The magnetic force exerted on the electron is: A. zero C. up B. down D. east

Answer 30

35. A. zero

Question 31

36. A uniform magnetic field is the positive z direction. A positively charged particle is moving in the positive x direction through the field. The net force on the particle can be made zero by applying an electric field in what direction? A. Positive y B. Negative y C. Positive x D. Negative x

Answer 31

36. B. Negative y

Question 32

38. The purpose of Milliken’s oil drop experiment was to determine: A. the mass of an electron B. the charge of an electron C. the ratio of charge to mass of an electron D. the sign of the charge on an electron

Answer 32

38. B. the charge of an electron

Question 33

39. Two electrons in a vacuum experience a force of 2 x 10-15 N. How far apart are the electrons? A. 0.48 x 10-7 m B. 2.84 x 10-12 m C. 3.39 x 10-7 m D. 5.05 x 10-12 m

Answer 33

39. C. 3.39 x 10-7 m

Question 34

40. An electron is moving north in a region where the magnetic field is south. The magnetic force exerted on the electron is: A. zero C. up B. down D. east

Answer 34

40. A. zero

Question 35

41.In an overhead straight wire, the current is north. The magnetic field due to this current, at our point of observation, is: A. east C. north B. down D. west

Answer 35

41. D. west

Question 36

42. An electron travels due north through a vacuum in a region of uniform magnetic field B^-> that is also directed due north. It will: A. be unaffected by the field B. speed up C. slow down D. follow a right-handed corkscrew path

Answer 36

42. A. be unaffected by the field

Question 37

43. The relative permeability of a certain silicon steel is 4500. A certain magnetic loop consists of a silicon steel of 10 cm square, 20 cm long and an air gap of 1/4 cm. What is the reluctance of the magnetic circuit? A. 2.54 x 10-2 Gb/Mx B. 6.44 x 10-3 Gb/Mx C. 6.44 x 10-4 Gb/Mx D. 2.54 x 10-3 Gb/Mx

Answer 37

43. D. 2.54 x 10-3 Gb/Mx

Question 38

44. The torque exerted by an electric field on a dipole is: A. parallel to the field and perpendicular to the dipole moment B. parallel to both the field and dipole moment C. perpendicular to both the field and dipole moment D. parallel to the dipole moment and perpendicular to the field

Answer 38

44. C. perpendicular to both the field and dipole moment

Question 39

45. A certain laminated steel core has a permeability of 3000. The length is 5 cm and the cross section is 2 sq. cm. What is its reluctance? A. 7500 C. 1200 B. 8.33 x 10-4 D. 2.5

Answer 39

45. B. 8.33 x 10-4

Question 40

46. The force exerted by a uniform electric field on a dipole is: A. parallel to the dipole moment B. perpendicular to the dipole moment C. perpendicular to the electric field D. None of the above

Answer 40

46. D. None of the above

Question 41

47.If a current of 5A flows through a long wire of radius 0.004 meter, what is the flux density (H) produced 0.02 meter away from the surface of the wire. A. 19.70 AT/m B. 39.80 AT/m C. 66.34 AT/m D. 33.17 AT/m

Answer 41

47. D. 33.17 AT/m

Question 42

48. A charged particle is projected into a region into a region of uniform parallel, E and B fields. The force on the particle is: A. zero B. at some angle < 90 with the field lines C. along the field lines D. perpendicular to the field lines

Answer 42

48. B. at some angle < 90 with the field lines

Question 43

49. A magnetomotive force is supplied by a current of one ampere through 100 turns. The magnetic circuit consists of a steel core of 1000 permeability, 10 cm long and 4 sq. cm area and an air gap one cm long. What is the field intensity at the air gap? A. 12.25 Oe C. 497.66 Oe B. 100.53 Oe D. 124.4 Oe

Answer 43

49. D. 124.4 Oe

Question 44

50. An electric field exerts a torque on a dipole only if: A. the field is parallel to the dipole moment B. the field is not parallel to the dipole moment C. the field is perpendicular to the dipole moment D. the field is not perpendicular to the dipole moment

Answer 44

50. B. the field is not parallel to the dipole moment

Question 45

37. Two point charges are the same value, what is the value of each charge in stat coulomb? A. 8.90 stat coulomb B. 9.90 stat coulomb C. 6.90 stat coulomb D. 7.90 stat coulomb

Answer 45

37. B. 9.90 stat coulomb

Question 46

34. Three equal positive charges of 10 statcoulomb each are located at the vertices of an equilateral triangle of 2 cm leg. What is the magnitude of the force acting on each charge? A. 12.5 dynes C. 25 dynes B. 43.3 dynes D. 50 dynes

Answer 46

B. 43.3 dynes

Question 47

31. Two spheres separated from each other by 10m have charges of 0.001 coulomb and 0.003 coulomb respectively. In between the two spheres is a point of zero electric field. What is the distance from the 0.001 coulomb sphere? A. 6.34 m C. 7.78 m B. 2.24 m D. 3.66 m

Answer 47

D. 3.66 m

Question 48

1 The electric field due to a uniform distribution of charge on a spherical shell is zero:

Answer 48

only inside the shell

Question 49

2 An electron traveling north enters a region where the electric field is uniform and points north. The electron:

Answer 49

slow down

Question 50

3 Calculate the magnitude of the electric field at a point that is 30 cm from a point charge Q = -3.2 x 10ˉ⁶ C.

Answer 50

3.0 x 10⁵ N/C

Question 51

4 Two previously uncharged capacitors are connected in series and then charged with a 12 V source. One capacitor is 30 µF and the other is unknown. If the voltage across the first capacitor is 8 V, find the capacitance of the unknown capacitor.

Answer 51

60 µF

Question 52

5 An electric field is most directly related to:

Answer 52

the force acting on a test charge

Question 53

6 Three condensers of 6, 10 and 15 µF are connected in series across a 200 V supply. If the condensers are disconnected, how much energy can the charged condensers provide if connected across each other with like polarities together?

Answer 53

0.052 J

Question 54

7 Choose the INCORRECT statement:

Answer 54

According to Gauss` law. if a closed surface encloses no charge, then the electric field must vanish everywhere on the surface

Question 55

8 A 20 µF capacitor is charged by a 12-V battery. What is the stored energy in the capacitor?

Answer 55

1.44 x 10ˉ³ J

Question 56

9 A 40 µF capacitor is charged to store 0.2 J of energy. An uncharged 60 µF capacitor is connected in parallel with the first one through a perfectly conducting leads. What is the energy lost due to the work done by the charges in spreading out over two capacitors?

Answer 56

0.12 J

Question 57

10 A charged particle is placed is placed in an electric field that varies with location. No force is exerted on this charge:

Answer 57

at locations where the electric field is zero

Question 58

11 Find the connection of capacitors 1 µF, 3 µF and 4 µF needed to have a total of 2 µF.

Answer 58

1 µF and 3 µF in parallel, series with 4 µF.

Question 59

12 When the dipole moment of a dipole in a uniform electric field rotates to become more nearly aligned with the field:

Answer 59

the field doe positive work and the potential energy decreases

Question 60

13 A charged point is particle is placed at the center of a spherical Gaussian surface. The electric flux ɸ E is changed if:

Answer 60

the point charge is moved to just outside the sphere

Question 61

14 Three capacitors C₁ = 3 µF, C₂ = 6 µF and C₃ = 9 µF are connected in parallel. Find the equivalent capacitance in µF.

Answer 61

18

Question 62

15 Two electrons in vacuum experience a force of 2 x 10ˉ¹⁵ N. How far apart are the electrons?

Answer 62

3.39 x 10ˉ⁷ m

Question 63

16 Find the capacitance of a two-plate capacitor of plate area 12 in² dielectric constant of 6 and dielectric thickness of 0.01 in.

Answer 63

1.6 nF

Question 64

17 A hollow conductor is positively charged. A small uncharged metal ball is lowered by a sild thread through a small opening in the top of the conductor and allowed to touch its inner surface. After the ball is removed, it will have:

Answer 64

no appreciable charge

Question 65

19 Three equal positive charges of 10 statC each are located at the vertices of an equilateral triangle of 2 cm leg. What is the magnitude of the force acting on each charge?

Answer 65

43.3 dynes

Question 66

20 Three capacitors having capacitances of 5, 10 and 12 µF are connected in parallel across 600 V mains. Determine the single capacitor in µF that can replace the combination.

Answer 66

27

Question 67

21 Two-point particles, with charges of q₁ and q₂, are placed a distance r apart. The electric field is zero at a point P between the particles on the line segment connecting them. We conclude that:

Answer 67

q₁ and q₂ must have the same sign but may have different magnitudes

Question 68

22 A capacitor whose plates is 20 cm x 3 cm and is separated by a 1.0-mm air gap is connected across a 12-V battery. Determine the charge accumulated on each plate after a long time.

Answer 68

6.4 x 10ˉ¹⁰ C

Question 69

23 A particle with charge Q is placed outside a large neutral conducting sheet. At any point in the interior of the sheet the electric field produced by chares on the surface is directed:

Answer 69

toward Q

Question 70

24 Two square parallel plates (0.04 m x 0.04 m) are separated by a 0.1 cm thick insulator with dielectric constant of 3.4. The plates are connected across 200 V. What is the charge on the plates?

Answer 70

9.63 x 10ˉ⁹ C

Question 71

25 An electric field exerts a torque on a dipole only if:

Answer 71

the field is not parallel to the dipole moment