Marist Brothers First Trinal Examinations Notre Dame of Kidapawan College July 14-16, 2015 First Semester, SY 2015-2016 ECE414 Energy Conversion Kidapawan City Mar Lou P. Galinato, ECE, MEP Name: ___________________________________________ Program/Year: _______________________

TEST I. MULTIPLE CHOICE [50 PTS] Shade only the letter of your choice to each item. Erasures will be considered wrong. [1] When the north poles of two bar magnets are

brought close together, there will be [A] no force [B] a downward force [C] a force of attraction [D] a force of repulsion

[2] The direction of a magnetic field within a

magnet is [A] from south to north [B] from north to south [C] back to front [D] front to back

[3] If the cross-sectional area of a magnetic field

increases, but the flux remains the same, the flux density [A] increases [B] decreases [C] remains the same [D] doubles

[4] What is the magnetomotive force in a 75-turn

coil of wire when there are 4 A of current through it? [A] 18.75 At [B] 30 At [C] 300 At [D] 187 At

[5] When a solenoid is activated, the force that

moves the plunger is [A] an electromagnetic field [B] a permanent magnetic field [C] varying voltage [D] a steady current

[6] What is the flux density in a magnetic field in

which the flux in 0.1 2 is 600 ? [A] 6,000 [B] 600 [C] 600 [D] 6

[7] What is the flux density when the flux is

5.5 and the cross-sectional area is 6 103 2? [A] 91.7 T [B] 917 T [C] 91 T [D] 9.7 T

[8] What is the reluctance of a material that has a

length of 0.07 , a cross-sectional area of

0.014 2, and a permeability of 4,500

?

[A] 1111 At/Wb [B] 111 At/Wb [C] 11 At/Wb [D] 1 At/Wb

[9] What is the magnetomotive force (mmf) of a

wire with 8 turns carrying three amperes of current? [A] 2,400 At [B] 240 At [C] 24 At [D] 2.4 At

[10] What is the reluctance of the circuit in problem

9 if the flux is 275 Wb? [A] 0.0066 At/Wb [B] 0.066 At/Wb [C] 8,727 At/Wb [D] 872 At/Wb

[11] When the current through the coil of an

electromagnet reverses, the [A] direction of the magnetic field reverses [B] direction of the magnetic field remains

unchanged [C] magnetic field expands [D] magnetic field collapses

[12] An electromagnetic field exists only when there is [A] an increasing current [B] decreasing current [C] voltage [D] current

[13] For a given wirewound core, an increase in

current through the coil [A] reverses the flux lines [B] decreases the flux density [C] increases the flux density [D] causes no change in flux density

[14] The ability of a material to remain magnetized after removal of the magnetizing force is known as [A] permeability [B] reluctance [C] hysteresis [D] retentivity

[15] The induced voltage across a stationary

conductor in a stationary magnetic field is [A] reversed in polarity [B] increased [C] decreased [D] zero

[16] When the speed at which a conductor is moved through a magnetic field is increased, the induced voltage [A] increases [B] decreases [C] remains constant [D] reaches zero

[17] If the steel disk in a crankshaft position sensor has stopped with the tab in the magnet's air gap, the induced voltage [A] increases [B] decreases [C] is zero [D] will remain constant

[18] A coil of wire is placed in a changing magnetic field. If the number of turns in the coil is decreased, the voltage induced across the coil will [A] increase [B] decrease [C] remain constant [D] be excessive

[19] A basic one-loop dc generator is rotated at 90

rev/s. How many times each second does the dc output voltage peak (reach maximum)? [A] 90 [B] 180 [C] 270 [D] 360

[20] The voltage induced across a certain coil is 200

mV. A 120 resistor is connected to the coil terminals. The induced current is [A] 1.7 mA [B] 16 mA [C] 12 mA [D] 120 mA

[21] There is 900 mA of current through a wire

with 40 turns. What is the reluctance of the circuit if the flux is 400 Wb? [A] 14,400 At/Wb [B] 1,440 At/Wb [C] 9,000 At/Wb [D] 90,000 At/Wb

[22] The induced voltage across a coil with 250

turns that is located in a magnetic field that is changing at a rate of 8 Wb/s is [A] 1,000 V [B] 2,000 V [C] 31.25 V [D] 3,125 V

[23] If a loop in a basic dc generator suddenly begins rotating at a faster speed, the induced voltage [A] remains unchanged [B] reverses polarity [C] increases [D] decreases

[24] The unit for reluctance is [A] Tesla [B] At/Wb [C] At/m [D] Wb

[25] The unit for permeability is [A] Wb/Atm [B] At/m [C] At/Wb [D] Wb

TEST II. TRUE OR FALSE [20 PTS] Write T is statement is TRUE or F if the statement is FALSE on the space provided before each item.

____ [1] Electromagnetic induction is the force that produces a magnetic field.

____ [2] The tesla is the unit of magnetic flux.

____ [3] A relay is an electromagnetically controlled mechanical device in which electrical contacts are opened or closed by a magnetizing current.

____ [4] A magnetic field is made up of flux lines.

____ [5] Reluctance in a magnetic circuit is analogous to current in an electrical circuit.

____ [6] If a conductor is moved back and forth at a constant rate in a constant magnetic field, the voltage in the conductor will reverse polarity.

____ [7] Lenz's law defines the polarity of the induced voltage.

____ [8] Hysteresis is the number of lines of force per unit area in a magnetic field.

____ [9] Materials with a low retentivity do not retain a magnetic field very well.

____ [10] Ampere-turn is the unit of magnetomotive force (mmf).

TEST III. ESSAY/EXPLANATION [60 PTS] Write your answer clearly on provided newsprints. [1] Explain the difference between magnetism and

electromagnetism.

[2] Explain a magnetic disk and tape Read/Write operation.

[3] Why is low retentivity required for tape recorder read/write heads but high retentivity is required for magnetic tape?

[4] When there is current through a conductor in a magnetic field, what happens?

[5] Explain how a crankshaft position sensor works.

[6] Explain how a dc generator works.

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