310. What happens in an Inductor when a continuous 28VDC supply is abruptly switched off? (a) 28VDC can be measured with inverse polarity on the inductor. (b) the voltage drops to zero immediately. (c) A very high reverse polarity voltage can be measured on the inductor.

(c) A very high reverse polarity voltage can be measured on the inductor. Power supply is removed. High reverse polarity voltage can be measured on the inductor

313. When inductors are connected in series, with no element of mutual coupling (where magnetic fields from each inductor do not affect the others) in a circuit the total inductance is: (a) less than the inductance of the lowest inductor (b) equal to the the inductance of the biggest inductor (c) equal to the sum of all the inductances

(c) equal to the sum of all the inductances Inductors in series and parallel behave like reisistors in series and parallel

314. The effect of inductance occurs in an electrical circuit when: (a) the current is steady (b) the resistance is steady (c) the current is changing.

(c) the current is changing. Inductance occurs when current is changing

27. Which of the following will increase the inductance of an Inductor? (a) reduce the number of turns (b) change its iron to an air core (c) decrease the coil’s length

(c) decrease the coil’s length

3.11: Inductance/Inductor Flashcards by Joshua McDonnell

The current in a coil changes from 10A to 2A in 0.1 Seconds. If the inductance of
the coil is 1.5H, what is the induced emf?
(a) 1.2 kV
(b) 12 V
(c) 120 V

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Which of the followigf will increase the inductance of an Inductor?
(a) reduce the number of turns
(b) change its iron to an air core
(c) decrease its length

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Five 10 mH inductors are connected parallel the total inductance is;
(a) 2mH
(b) 5mH
(c) 10mH

(a) 2mH

Inductors in parallel behave like resistors in parallel
1/Lt = 1/L1 + 1/L2 +….

However, if same value inductors, we can use simple formula 1/5 * 10 = 2

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Inductors in series add like:
(a) Resistors in parallel
(b) Capacitors in parallel
(c) Capacitors in Series

(b) Capacitors in parallel

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When current through an inductor doubles the energy stored is:
(a) Halved,
(b) Doubled.
(c) Quadruples.

Energy in an inductor:
Vstored = 1/2 * L * I^2
Where L is inductance in Henrys and I is current flowing through inductor

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According to Lenz’s Law, the induced emf produced by a change in current in an
inductive circuit tends to have what effect on the current?
(a) It aids a rise in current and opposes fall in current
(b) It aids a fall in current and opposes a rise in current
(c) It opposes either a rise or a fall in current

Lenz’s law = Induced EMF by a change in Current. It opposes either a rise or a fall in current
or another way
‘‘The induced EMF in any circuit is always in a direction to oppose the effect that produced it’’

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If 9 volts are induced in a conductor when the current changes by 4.5 amperes in
one second, what is the total inductance of the circuit?
(a) 2.0 henries
(b) 13.5 henries
(c) 40.0 henries

(a) 2.0 henries

Induced voltage = L x change in current

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An AC electrical current varies 1.5 amperes in one second and is applied to a 10-
henry coil. What is the value of the emf induced across the coil?
(a) 1.0 volt
(b) 1.5 volts
(c) 15.0 volts

Induced voltage = L x change in current

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What happens in an Inductor when a continuous 28VDC supply is abruptly
switched off?
(a) 28VDC can be measured with inverse polarity on the inductor.
(b) the voltage drops to zero immediately.
(c) A very high reverse polarity voltage can be measured on the inductor.

Power supply is removed. High reverse polarity voltage can be measured on the inductor

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A soft iron core will increase inductance because it has which of the following
characteristics?
(a) High permeability and low reluctance
(b) Low permeability and low reluctance
(c) High permeability and high reluctance

(a) High permeability and low reluctance

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The power consumed in a conductor in realigning the atoms which set up the
magnetic field is known as what type of loss?
(a) Hysteresis loss
(b) Magnetic loss
(c) Heat loss

(a) Hysteresis loss

Hysteresis = power consumed realigning the atoms

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When inductors are connected in series, with no element of mutual coupling
(where magnetic fields from each inductor do not affect the others) in a circuit the
total inductance is:
(a) less than the inductance of the lowest inductor
(b) equal to the the inductance of the biggest inductor
(c) equal to the sum of all the inductances

Inductors in series and parallel behave like reisistors in series and parallel

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The effect of inductance occurs in an electrical circuit when:
(a) the current is steady
(b) the resistance is steady
(c) the current is changing.

Inductance occurs when current is changing

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When inductors are connected in parallel, with no element of mutual coupling
(where magnetic fields from each inductor do not affect the others) in a circuit the
total inductance is:
(a) less than the inductance of the lowest inductor
(b) equal to the the inductance of the biggest inductor
(c) equal to the sum of all the inductances

(a) less than the inductance of the lowest inductor

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Five 10mH inductors are connected in series the total inductance is:
(a) 20mH
(b) 30 MH
(c) 50mH.

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Two inductors of 3.6 µH and 7.3 µH are wired together in series and they aid each
other. The mutual inductance for the circuit is 3.6µH. What is the total inductance of
the circuit?
(a) 34.4 µH
(b) 18.1 µH
(c) 17.5 µH

(b) 18.1 µH

Series inductors with Magnetic Coupling:
Lt = L1 + L2 +- 2M
where Lt is the total inductance
L1 + L2 is the inductance of L1 and L2
M is the Mutual inductance between the two inductors

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The characteristic of mutual inductance is used in which devices?
(a) DC Generators
(b) Transformers
(c) Moving-coil Instruments

(b) Transformers

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Which of the following will increase the inductance of an Inductor?
(a) reduce the number of turns
(b) change its iron to an air core
(c) decrease the coil’s length

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Question Number. 1. Two inductive coils are placed in close proximity with each other at 90 degrees. The number of flux linkages is.
Option A. 0.
Option B. maximum negative.
Option C. maximum positive.

Option A. 0.

'’The coils must be parallel for maximum flux linkages (coupling factor). Coupling factor reduces as angular difference increases’’

Question Number. 2. What is the mutual inductance if two coils of 10mH and 500mH have 90% of the flux transferred from one to the other?.
Option A. 459mH.
Option B. 4.5mH.
Option C. 63mH.

Option C. 63mH.

Total inductance = 0.9* sq root(10*500)

Question Number. 3. An inductor has 1,000 turns of wire and a cross sectional area of 0.001m2. If the core has a permeability of 0.000,001 and the coil is 0.1m. What is the value of the inductor?.
Option A. 100microH.
Option B. 100mH.
Option C. 10mH.

Option C. 10mH.

Question Number. 4. Three inductors 10 mH, 5 mH and 20 mH are connected in parallel. What is the total inductance?.
Option A. Without knowing the coupling factor the total inductance cannot be found.
Option B. 2.86mH.
Option C. 35mH.

Option B. 2.86mH.

Note: In parallel, the total induction is lower than the lowest single inductance - only 1 answer
- Inductors in series and parallel behave the exact same as resistors

Question Number. 5. If the current reaches a maximum through an inductor of 2A in 15 seconds, what is the current after 3 seconds?.
Option A. 1.26A.
Option B. 0.63A.
Option C. 1A.

Option A. 1.26A.

'’1 Time constant = 15/5 = 3s. 100% charge = 2A, so 1 time constant (63.2%) = 1.26A

Question Number. 6. The time constant of an inductor is.
Option A. L/R.
Option B. L2 /R.
Option C. LR.

Option A. L/R.

Question Number. 7. If the rate of change of current is halved, mutual inductance will. Option A. stay the same. Option B. halve. Option C. double.

Option A. stay the same. Mutual Inductance formula = k * sq root(L1 * L2) Note: Rate of change of current (i.e frequency) is not in it

Question Number. 8. Why are the iron cores of most induction coils laminated?. Option A. To reduce the effects of eddy currents. Option B. To reduce the core reluctance. Option C. To increase the core permeability.

Option A. To reduce the effects of eddy currents.

Question Number. 9. When more than two inductors of different inductance are connected in parallel in a circuit, the total inductance is. Option A. equal to the sum of the individual inductance. Option B. equal to the inductance of the highest rated inductor. Option C. less than the inductance of the lowest rated inductor.

Option C. less than the inductance of the lowest rated inductor. Inductors behave like resistors (in series and parallel) - For parallel resistors/inductors - total value is less than the lowest individual value

Question Number. 10. The time constant in an inductive circuit is the time required for the current to reach. Option A. 70.7% of maximum value. Option B. 63.2% of maximum value. Option C. 63.7% of maximum value.

Option B. 63.2% of maximum value.

Question Number. 11. An AC inductive circuit has an inductance 0f 10 mH and a frequency input of 1000Hz. The opposition to current flow is approximately. Option A. 63 ohms. Option B. 63,000 ohms. Option C. 630 ohms.

Option A. 63 ohms. Note the mH - m for milli (10^-3) Formula for inductive impedance = 2*pi*f*inductance

Question Number. 13. An inductor of resistance 5 kΩ and inductance 2mH is connected to a DC circuit. The time constant is. Option A. 400 ns. Option B. 10 seconds. Option C. 40 ms.

Option A. 400 ns. Time constant = L/R = 2mH/5kohms =2(10^-3) / 5(10^3) = 400ns

Question Number. 14. When switching off the supply, the back EMF in a collapsing field in an inductor. Option A. can be multiple times bigger than forward EMF. Option B. never exceeds forward EMF. Option C. is equal to forward EMF.

Option A. can be multiple times bigger than forward EMF. The size of the back-emf depends upon how fast the magnetic field is collapsing. That is how your car ignition turns 12V into several thousand volts

Question Number. 15. An induced current in a coil. Option A. opposes the EMF producing it. Option B. does not affect the EMF producing it. Option C. aids the EMF producing it.

Option A. opposes the EMF producing it.

Question Number. 16. 1 Volt is produced when a current of 1 Amp per second is varied in a coil'. This is a definition of. Option A. 1 Ampere per metre. Option B. 1 Henry. Option C. 1 Coulomb per second.

Option B. 1 Henry.

Question Number. 17. Two inductor coils in very close proximity; if one of the coils is rotated 90 degrees slowly away from the other the mutual inductance. Option A. decreases. Option B. remains the same. Option C. increases.

Option A. decreases. Coupling factor (mutual inductance) decreases to zero at 90 degrees

Question Number. 18. An inductor is said to be saturated when. Option A. the current is at a maximum. Option B. the current through it is zero. Option C. it will no longer accept lines of flux.

Option C. it will no longer accept lines of flux.

What is the formula for inducted EMF?