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# Multiple choice question for engineering

## Set 1

1. A parallel plate capacitor has a capacitance of 10μF. If the linear dimensions of the plates are doubled and distance between them is also doubled, the new value of capacitance would be __________
a) 10μF
b) 20μF
c) 5μF
d) 40μF

Answer: b [Reason:] C1= 10μF = ε0A1/x1 C2= ε0A2/x2 , A2=4A1(as the linear dimensions are doubled, area increases by 4 times) and x2=2x1 ⇒ C2= ε04A1/2x1 = 2∗10μF =20μF

2. A parallel plate capacitor is changed and then the DC supply is disconnected. Now plate separation is allowed to decrease due to force of attraction between the two plates. As a consequence __________
(i) charge on the plate increases
(ii) charge on the plates remain constant
(iii) capacitance C increases
(iv) capacitance C remains constant
(v) potential difference increases
(vi) potential difference decreases
(vii) energy stored decreases
(viii) energy stored increases

Which of the above statements are correct?
a) (i),(iii),(vi)
b) (ii),(iv),(viii)
c) (ii),(iii),(vi),(vii)
d) (ii),(iv),(v),(viii)

Answer: c [Reason:] C= εA/x and as separation is decreased, capacitance increases. Also, C=q/v and as charge remains constant, voltage decreases. Similarly, energy stored, Wfld=1/2q2/c(x) and it decreases.

3. A parallel plate capacitor is charged and then the DC supply is disconnected. The plate separation is then increased. Between the plates,
(i) electric field intensity is unchanged
(ii) flux density decreases
(iii) potential difference decreases
(iv) energy stored increases

Which of the above statements are correct?
a) (i),(iv)
b) (ii),(iv)
c) (ii),(iii),(iv)
d) (i),(iii),(iv)

Answer: a [Reason:] We know C=ε0A/x, as x increases, C decreases. Charge(q) on plate remains constant and q=Cv implies v increases. Energy stored, Wfld= 1/2q2/C(x), as C decreases, Wfld increases. Similarly as q=DA and q,A remains constant, D doesn’t change and as electric field intensity is given by, E=D/ε0, E also doesn’t change.

4. The area of two parallel plates is doubled and the distance between these plates is also doubled. The capacitor voltage is kept constant. Under these conditions, force between the plates of this capacitor __________
a) decreases
b) increases
c) reduce to half
d) gets doubled

Answer: c [Reason:] A2=2A1, x2=2x1, v2=v1, fe2=1/2v2dC(x)/dx2, C20A2/x2 ⇒ dC2/dx2= -ε0A2/x22 dC2/dx2= -2ε0A1/4x12 = -ε0A1/2x12 ⇒ f&e2= -1/2v12ε0A1/2x12= -1/2(fe1) ⇒ force reduces to half.

5. A parallel plate capacitor has an electrode area of 1000 mm2, with a spacing of 0.1 mm between the electrodes. The dielectric between the plates is air with a permittivity of 8.85∗10-12 F/m. The charge on the capacitor is 100 v. The stored energy in the capacitor is ____________
a) 44.3 J
b) 444.3 nJ
c) 88.6 nJ
d) 44.3 nJ

Answer: d [Reason:] Wfld = 1/2 q2/C(x) C(x) = ε0A/x = 8.85∗10-12∗100∗(10-3)2/0.1∗10-3 = 8.85∗10-12 F Wfld = 1/2∗Cv = 1/2∗1002∗8.85∗10-12 = 44.3nJ

6. A parallel plate capacitor is connected to a DC source. Now the plates are allowed to move a small displacement under the influence of force of attraction between the two plates. As a result ____________
(i) charge on the plates increases
(ii) charge on plates remains constant
(iii) energy stored increases
(iv) energy stored remains constant
(v) electric field intensity is unchanged
(vi) flux density increases

Which of the above statements are correct?
a) (ii),(iv),(v)
b) (ii),(iii),(vi)
c) (i),(iii),(v)
d) (i),(iii),(vi)

Answer: d [Reason:] C = ε0A/x ⇒ C ∝ 1/x ⇒ If the force if of attraction type, then x decreases and C increases. q=Cv, v constant⇒ as C increases, q also increases. We also know that q=DA and if q increaes, D increases. Finally, Wfld= 1/2 D20 and as D is increasing, Wfld increases.

7. The force produced by electric field in a singly excited energy conversion device, using electric field as coupling medium can be obtained by ___________
a) use of field energy function only
b) use of coenergy function only
c) any of the mentioned
d) none of the mentioned

Answer: c [Reason:] fe = -∂Wfld(q,x)/∂x fe = -∂Wfld1(q,x)/∂x

8. Charge and voltage associated with electric field are analogous, respectively to __________ and ___________ in magnetic field.
b) flux density and current
d) MMF and current

Answer: a [Reason:] In magnetic field, the basic terms are flux linkages and current, which are responsible for energy conversion and in electric field, charge and voltage are basically responsible for energy conversion. dWelec = Ψi, for magnetic field dWelec = vidt=vdq, for electric field.

9. Two parallel plates, each of area A = 1m2 are separated by a distance g. The electric field intensity between the plates is 3∗106 v/m. What is the force between the two plates?
a) 1/2π∗103 N
b) 1/8π∗103 N
c) 8π∗103 N
d) 0 N

Answer: b [Reason:] When electric field is applied, the plates move towards each other because of force of attraction by a distance x ⇒ C= ε0A/(g-x) Wfld(q,x) = 1/2 q2/C = 1/2 q2(g-x)/Aε0 fe= -∂Wfld(q,x)/∂x= 1/2q2x/Aε0 We know, q=DA=ε0EA and ε0 = 10-9/36Π fe= 1/2 E2ε0A= 1/2∗(3∗106)2∗10-9/36Π∗1 = 1/8π∗103 N

## Set 2

1. For a toroid to extract the energy from the supply system, the flux linkages of the magnetic field must be ________
a) zero
b) changing or varying
c) constant
d) any of the mentioned

Answer: b [Reason:] dWelec=idφ=eidt, where dWelec= differential electrical energy to coupling field, and if the flux linkages are either constant or zero, i.e, dφ=0, then dWelec=0.

2. Magnetic stored energy density for iron is given by ______
a) 1/2 B/μ
b) 1/2 B2 μ
c) 1/2 ∅2 Rl
d) 1/2 B2

Answer: d [Reason:] Magnetic stored energy density for iron is given as wfld=Wfld/((Length of the magnetic path through Iron)*(Iron area normal to the magnetic flux))=1/2 (F∅)/(length*Area)=1/2 F/length ∅/area=1/2 H*B Also, H= B/μ,thus wfld=1/2 B2/μ.

3. The energy stored in a magnetic field is given by ____________ where L=self-inductance and Rl=reluctance.
a) 1/2 Li2
b) 1/2 (mmf*Rl)2
c) 1/2∅Rl
d) 1/2 φ2i

Answer: a [Reason:] We know that Wfld=1/2 φi and L=φ/i , thus Wfld=1/2 Li2.

4. When a current of 5A flows through a coil of linear magnetic circuit, it has flux linkages of 2.4 wb-turns. What is the energy stored in the magnetic field of this coil in Joules?
a) 6
b) 12
c) 1.2
d) 2.4

Answer: a [Reason:] Wfld=1/2 φ*i= 1/2*2.4*5=6 Joules.

5. For a linear electromagnetic circuit, which of the following statement is true?
a) Field energy is less than the Co-energy
b) Field energy is equal to the Co-energy
c) Field energy is greater than the Co-energy
d) Co-energy is zero

Answer: b [Reason:] Wfld=Wfld1=1/2 φ*i=1/2 F*∅.

6. The electromagnetic force and/or torque, developed in any physical system, acts in such a direction as to tend to ____________
a) decrease the magnetic stored energy at constant mmf
b) decrease the magnetic stored energy at constant flux
c) increase the magnetic stored energy at constant flux
d) increase the magnetic stored energy at constant current

Answer: b [Reason:] fe=-(∂Wfld (φ,x))/∂x= -(∂Wfld (∅,x))/∂x and Te=-(∂Wfld(φ,θ))/∂θ=-(∂Wfld (∅,θ))/∂θ The negative sign before ∂Wfld indicates that fe acts in a direction as to tend to decrease the stored energy at constant mmf.

7. The electromagnetic force developed in any physical system acts in such a direction as to tend to _____________
a) decrease the co-energy at constant mmf
b) increase the co-energy at constant flux
c) decrease the co-energy at constant flux
d) increase the co-energy at constant mmf

Answer: d [Reason:] fe=(∂Wfld1 (i,x))/∂x=(∂Wfld1 (F,x))/∂x, the positive sign before ∂Wfld1 indicates that force fe acts in a direction as to tend to increase the co-energy at constant mmf.

8. Consider a magnetic relay with linear magnetization curve in both of its open and closed position. What happens to the electrical energy input to the relay, when the armature moves slowly from open position to closed position ?
a) Welec=Wfld
b) Welec=Wmech
c) Welec=Wmech/2+Wfld/2
d) Welec=0

Answer: c [Reason:] For the above mentioned case, Wfld=Wmech and Wfld=Welec/2 hence, option “c” is the correct answer.

9. The electromagnetic torque developed in any physical system, and with magnetic saturation neglected, acts in such a direction as to tend to ____________
a) decrease both the reluctance and inductance
b) increase both the reluctance and inductance
c) decrease the reluctance and increase the inductance
d) increase the reluctance and decrease the inductance

Answer: c [Reason:] fe=1/2 ∅2 dRl/dx , Te=-1/2 ∅2 dRl/dθ= 1/2 i2 dL/dθ.

10. Electromagnetic force and/or torque developed in any physical system, acts in such a direction as to tend to ____________
a) increase both the field energy and co-energy at constant current
b) increase the field energy and decrease the co-energy at constant current
c) decrease both the field energy and co-energy at constant current
d) decrease the field energy and increase the co-energy at constant current

## Set 3

1. For a P-pole machine, the relation between electrical and mechanical degrees is given by _____________
a) θelec = 2/P θmech
b) θelec = 4/P θmech
c) θmech = P/2 θelec
d) θelec = P/2 θmech

Answer: d [Reason:] For a P-pole machine, P/2 cycles of EMF will be generated in one revolution, and thus θelec = P/2 θmech.

2. What is the relation between ‘ω’, angular speed in electrical radians per second and ‘ωm‘, angular speed in mechanical radians per second, for a P-pole machine?
a) ω = 2/Pωm
b) ω = 4/Pωm
c) ωm = P/2ω
d) ω = P/2ωm

Answer: d [Reason:] We know θelec = P/2θmech ⇒ d/dt(θelec) = P/2d/dt(θmech) ⇒ ω = P/2ωm

3. A pole pitch in electrical machine _____________
a) = 180° electrical
b) = 180° mechanical
c) > 180° electrical
d) < 180° electrical

Answer: a [Reason:] Pole pitch is always expressed in electrical degrees, rather than in mechanical degrees. pole pitch = 180 electrical degrees or π electrical radians

4. The part of the coil in which EMF is generated is known as _____________
a) end connection
b) coil sides
c) coil span
d) none of the mentioned

Answer: b [Reason:] The EMF is generated in the active lengths only, and these active lengths are called the coil-sides of a coil.

5. A coil consists of _____________
a) two conductors
b) two coil sides
c) two turns
d) four turns

Answer: b [Reason:] One coil is made up of two coil sides.

6. One turn consists of _____________
a) two coil sides
b) two conductors
c) four conductors
d) four coil sides

Answer: b [Reason:] One turn consists of two conductors, and one coil is made up of two coil-sides.

7. In a full pitch coil, the two coil sides are how many electrical space degrees apart?
a) 180 electrical degrees
b) 90 electrical degrees
c) 45 electrical degrees
d) none of the mentioned

Answer: a [Reason:] A coil, with two coil-sides 180 electrical space degrees apart (or one pole-pitch apart), is called a full pitch coil.

8. A chording angle ε is defined as the angle by which coil span departs from __________
a) 90° electrical space degrees
b) 180° electrical space degrees
c) 360° electrical space degrees
d) any of the mentioned

Answer: b [Reason:] If chording angle is ε°, chorded coil has a coil span = 180°-ε.

9. If Bp= peak value of sinusoidal flux density wave, L = armature core length, D = armature diameter, P = number of poles, then
(i) flux per pole, Φ=2/PBpLD
(ii) Average flux density, Bav=2/πBp
(iii) Φ=4/PBpLD
(iv) Bav=3/πBp
(v) Bav=PΦ/πDL
(vi) Bav=PΦ/πrL
Which of the above statements are correct ?
a) (i),(ii),(v)
b) (ii),(iii),(v)
c) (ii),(iii),(vi)
d) (iii),(iv),(v)

Answer: a [Reason:] Total flux per pole= 4/PBprL we know, r=D/2 ⇒ Φ=2/PBpLD and Bav=2/πBp. As Φ = (Average value of constant amplitude flux density wave under one pole)∗(Area pertaining to one pole of the flux density wave), Φ = (Bav)(2πrL/P)and average value of the positive or negative half cycle of a sine wave = 2/π(peak value of sine wave).

10. In a 4 pole machine, what is the flux per pole produced, if the armature length is l and radius is r and Bp is peak value of sinusoidal flux?
a) 4Bplr
b) 2Bplr
c) Bplr
d) Bplr/2

Answer: c [Reason:] Total flux per pole=4/PBplr, since P=4, Φ=Bplr.

11. The equation for flux per pole Φ=4/PBplr, is valid for _____________
a) for salient pole rotor only
b) for cylindrical pole rotor only
c) for both cylindrical and salient pole rotor
d) none of the mentioned

Answer: c [Reason:] It does not make any difference whether the field winding is on cylindrical pole structure or salient pole structure.

## Set 4

1. The magnitude of electromagnetic or interaction torque, in all rotating machines, is given by ____________
a) Te ∝(stator field strength)sinδ
b) Te ∝(rotor field strength)sinδ
c) Te ∝(stator field strength)(rotor field strength)sinδ
d) Te ∝ sinδ

Answer: c [Reason:] The tendency of two fields (stator field and rotor field), to align themselves in the same direction, is called interaction torque, and it also depends on the torque angle & delta;.

2. In a rotating electrical machine with 2 poles on the stator and 4 poles on the rotor, spaced equally, the net electromagnetic torque developed is ____________
a) maximum
b) zero or no torque is developed
c) minimum
d) none of the mentioned

Answer: b [Reason:] In a machine with 2 stator poles and 4 rotor poles spaced equally, the force of attraction will be cancelled due to the force of repulsion, and hence the net electromagnetic torque will be zero.

3. For the development of electromagnetic torque in a rotating electrical machine, the number of rotor poles must be ____________
a) greater than the stator poles
b) less than the stator poles
c) equal to the stator poles
d) either greater or lesser than the stator poles

Answer: c [Reason:] If the rotor poles are not equal to the stator poles, the net electromagnetic torque will be zero as the force of attraction will be cancelled by the force of repulsion.

4. Torque angle ‘δ’ is the angle between ____________
a) rotor field axis and resultant field axis
b) stator field axis and rotor field axis
c) stator field axis and mutual field axis
d) stator field axis and resultant field axis

Answer: b [Reason:] The angle between the stator field axis and rotor field axis is called as torque angle.

5. The interaction torque in a rotating electrical machine depends on which of the following components?
(i) sinδ
(ii) cosδ
(iii) torque angle δ
(iv) stator field strength
(v) rotor field strength
a) (i),(iv),(v)
b) (ii),(iii),(v)
c) (i),(iii),(v)
d) (ii),(iv),(v)

Answer: a [Reason:] Interaction torque Te∝(stator field strength)(rotor field stregth)sinδ

6. The basic torque and EMF expression of rotating electrical machines are ____________
a) applicable to DC machines only
b) applicable to AC machines only
c) applicable to both AC and DC machines
d) none of the mentioned

Answer: c [Reason:] Basic torque and EMF expressions are same for both AC and DC machines as the fundamental principles underlying the operation of AC and DC machines are same.

7. The final forms of the expression for generated EMF and torque,for AC and DC machines differ, because ____________
a) the fundamental principles underlying their operation are same
b) the fundamental principles underlying their operation are different
c) their construction details are same
d) their construction details are different

Answer: d [Reason:] Though AC and DC machine have same operating fundamental principles, their construction details differ.

8. The voltage equation for the electrical system, in an electromechanical energy conversion device is as follows:
vt=ir+Ldi/dt+i(dL/dθrr = (i)+(ii)+(iii)
What does the (ii) and (iii) terms represent respectively?
a) transformer voltage term and speed/rotational voltage term
b) speed/rotational voltage term and transformer voltage term
c) force voltage term and transformer voltage term
d) none of the mentioned

Answer: a [Reason:] (ii) term = Ldi/dt is known as transformer voltage because it involves the time derivative of current. (iii) term = i(dL/dθrr is the speed/rotational voltage term because of the presence of speed ωr in it.

9. For an inductor made from magnetic core, with two air gaps of equal length g, exciting coil with 1000 turns, A=5cm∗5cm and g=1cm, what will be the coil inductance?
a) 0.314H
b) 0.157H
c) 0.078H
d) 0.628H

Answer: b [Reason:] Reluctance Rl=2g/μ0A and coil Inductance L=N2/Rl = N2μ0A/2g =(1000)2(4π∗10-7)(0.05)2/(2∗0.01) = 0.15708H.

## Set 5

1. Statement 1: Cabinet tray dyers can be used for heat sensitive materials.
Statement 2: Bin dryer is used for materials which follow the falling rate drying curve.
a) True, False
b) True, True
c) False, False
d) False, True

Answer: d [Reason:] Cabinet tray dyers cannot be used for heat sensitive materials as materials are exposed to the drying medium for several hours. Bin dryer is used for materials which follow the falling rate drying curve as the materials in the bin follow a gradient of moisture content from bottom to top of the bin.

2. Advantage of turbo dryer over cabinet tray dryer is _____
a) It is more effective
b) The feed is uniformly heated at the edges
c) None of the mentioned
d) Both of the mentioned

Answer: b [Reason:] Advantage of turbo dryer over cabinet tray dryer is that the feed is uniformly heated at the edges.

3. Hannah ordered one ton of mushrooms. But, the entire lot was brown. Which dryer could it possibly have been?
a) Bin dryer
b) Tray dryer
c) Tower Dryer
d) Vacuum dryer

Answer: b [Reason:] Browning of heat sensitive food items may take place due excess exposure to hot air. Tray dryer has a high residence time. Hence, it is the most possible guess out of the four.

4. Statement 1: Which tray dryer has indirect heating?
Statement 2: Which tray dryer is good for making powders?
a) Bin dryer, vacuum dryer
b) Tray dryer, vacuum dryer
c) Tower Dryer, vacuum dryer
d) Vacuum dryer, vacuum dryer

Answer: d [Reason:] Vacuum dryer has indirect heating as hot air is generally not blown through it. Vacuum dryer is good for making powders.

5. Which of the following dryers requires non-sticky granular feed?
a) Rotary dryer
b) Fluidized bed dryer
c) Both of the mentioned
d) None of the mentioned

Answer: c [Reason:] Both rotary dryer and fluidized bed dryer require non-sticky granular feed.

6. After incipient fluidization, the pressure briefly _____
a) Increases
b) Decreases
c) Stays constant
d) Varies exponentially

Answer: b [Reason:] After incipient fluidization, the pressure briefly decreases. This is because at incipient fluidization, the particles only become loose. They aren’t in contact with each other. They don’t start rising up. Hence, the pressure briefly decreases. Once they start moving up, the pressure increases.

7. Statement 1: Vibro-fluidized bed dryer can reduce the air flow rate consumptions as air is only used for drying and not for fluidizing.
Statement 2:_____ are often called desolventizers.
a) True, screw conveyor
b) True, pneumatic conveyor
c) False, screw conveyor
d) False, pneumatic conveyor

Answer: a [Reason:] Vibro-fluidized bed dryer can reduce the air flow rate consumptions as air is only used for drying and not for fluidizing. Mechanical vibratory motion is used for fluidization. Screw conveyors are often called desolventizers because the main purpose is to remove the solvent from the pastes and not to recover it.

8. Which of the following dryers cannot take feed in the form of pastes/slurries/sticky items?
a) Drum dryer
b) Screw conveyor dryer
c) Pneumatic dryer
d) Spray dryer

Answer: c [Reason:] In a pneumatic dryer, feed in the form of pastes/slurries/sticky items cannot be taken.

9. Which of the following is also called ring dryer?
a) Pneumatic conveyor dryer
b) Drum dryer
c) Freeze dryer
d) Spray dryer

Answer: a [Reason:] Pneumatic conveyor dryer is also called ring dryer because feed is made to pass through a loop at one point and comes back there. (Not always-depends on drying)

10. Which of the following dryers is used for making baby food?
a) Pneumatic conveyor dryer
b) Drum dryer
c) Freeze dryer
d) Spray dryer

Answer: b [Reason:] Drum dryers are used for making baby food mostly because they take feed in the form of slurries.

11. Coffee powders can be made from which of the following dryers?
a) Pneumatic conveyor dryer
b) Drum dryer
c) Freeze dryer
d) Spray dryer

Answer: d [Reason:] Coffee powder is porous, hygroscopic and low in density. These are the desired properties for a spray dryer.

12.
X-axis is temperature and Y-axis is pressure.
Which of the following curves is followed for freeze drying of prawns?
a) AA’
b) BB’
c) ABO
d) A’OB’