TRA174: Ch 2 Inductance

Q10.a. The maximum current applied to an inductor is 1.8 amperes. How much current flowed in the inductor 3 time constants after the circuit was first energized? b. What is the minimum number of time constants required for the current in an LR circuit to increase to its maximum value? c. A circuit containing only an inductor and a resistor has a maximum of 12 amperes of applied current flowing in it. After 5 L/R time constants the circuit is opened. How many time constants is required for the current to decay to 1.625 amperes?

A10.a. 1.71 amperes.b. 5 time constants.c. 2 time constants.

Q11. State three types of power loss in an inductor.

A11. Copper loss; hysteresis loss; eddy-current loss.

Q12. Define mutual inductance.

A12. Mutual inductance is the property existing between two coils so positioned that flux from one coil cuts the windings of the other coil.

Q13. When are two circuits said to be coupled?

A13. When they are arranged so that energy from one circuit is transferred to the other circuit.

Q14. What is meant by the coefficient of coupling?

A14. The ratio of the fines of force produced by one coil to the lines of force that link another coil. It is never greater than one.

Q15. Two series-connected 7-H inductors are adjacent to each other; their coefficient of coupling is 0.64. What is the value of M?

A15. 4.48H (because M=K x sqrt(7Hx7H)=0.64x7H=4.48H)

Q16. A circuit contains two series inductors aligned in such a way that their magnetic fields oppose each other. What formula should you use to compute total inductance in this circuit?

A16. LT=L1(plus)L2-2M

Q17. The magnetic fields of two coils are aiding each other. The inductance of the coils are 3 H and 5 H, respectively. The coils' mutual inductance is 5 H. What is their combined inductance?

A17. LT=18H (because LT=L1(plus)L2(plus)2M so LT=3H(plus)5H(plus)2x(5H) = 18H)

Q4. What is meant by induced emf? By counter emf?

A4. Induced emf is the emf which appears across a conductor when there is relative motion between the conductor and a magnetic field; counter emf is the emf induced in a conductor that opposes the applied voltage.

Q5. State Lenz's law.

A5. The induced emf in any circuit is in a direction to oppose the effect that produced it.

Q6. What effect does inductance have (a) on steady direct current and (b) on direct current while it is changing in amplitude?

A6.a. No effect.b. Inductance opposes any change in the amplitude of current.

Q7.a. List five factors that affect the inductance of a coil. b. Bending a straight piece of wire into a loop or coil has what effect on the inductance of the wire? c. Doubling the number of turns in a coil has what effect on the inductance of the coil? d. Decreasing the diameter of a coil has what effect on the inductance of the coil? e. Inserting a soft-iron core into a coil has what effect on the inductance of the coil? f. Increasing the number of layers of windings in a coil has what effect on the inductance of the coil?

A7.(a).1. The numbers of turns in a coil.2. The type of material used in the core.3. The diameter of the coil.4. The coil length.5. The number of layers of windings in the coil.(b). Increases inductance.(c). Increases inductance.(d). Decreases inductance.(e). Increases inductance.(f). Increases inductance.

Q8.a. When voltage is first applied to a series LR circuit, how much opposition does the inductance have to the flow of current compared to that of the circuit resistance? b. In a series circuit containing a resistor (R 1) and an inductor (L1), what voltage exists across R1 when the counter emf is at its maximum value? c. What happens to the voltage across the resistance in an LR circuit during current buildup in the circuit, and during current decay in the circuit?

A8.(a). Inductance causes a very large opposition to the flow of current when voltage is first applied to an LR circuit; resistance causes comparatively little opposition to current at that time.(b). Zero.(c). During current buildup, the voltage across the resistor gradually increases to the same voltage as the source voltage; and during current decay the voltage across the resistor gradually drops to zero

Q9. What is the formula for one L/R time constant?

A9. t=L/R

Q3. Define inductance.

Inductance is the property of a coil (or circuit) which opposes any CHANGE in current.

Q2. An emf is generated in a conductor when the conductor is cut by what type of field?

Magnetic field.

Q1. What is the basic unit of inductance and the abbreviation for this unit?

The henry, H.