1. The property of coil by which a counter e.m.f. is induced in it when the current through the coil changes is known as
A. self-inductance
B. mutual inductance
C. series aiding inductance
D. capacitance
Answer: A. self-inductance
Explanation:
Self-induction:
- Self-inductance is the property of the current-carrying coil that resists or opposes the change of current flowing through it.
- This occurs mainly due to the self-induced emf produced in the coil itself.
Mutual-inductance:
- When two coils are brought in proximity with each other the magnetic field in one of the coils tends to link with the other. If this magnetic field of the first coil is changed then the magnetic flux associated with the second coil changes and this leads to the generation of voltage in the second coil.
- This property of a coil that affects or changes the current and voltage in a secondary coil is called mutual inductance.
- The SI unit of the mutual inductance is Henry.
Important Information:
- Self inductance is the property of a component that opposes the change of current flowing through it. Even straight wire has some inductance. Therefore option 1 is correct.
- When the emf is induced in the same circuit in which the current is changing, this effect is known as self-inductance.
2. As per Faraday's laws of electromagnetic induction, an e.m.f. is induced in a conductor whenever it
A. lies perpendicular to the magnetic flux
B. lies in a magnetic field
C. cuts magnetic flux
D. moves parallel to the direction of the magnetic field
Answer: C. cuts magnetic flux
Explanation:
- Faraday’s first law of electromagnetic induction states that whenever a conductor is placed in a varying magnetic field, emf is induced which is called induced emf.
- If the conductor circuit is closed, the current will also circulate through the circuit and this current is called induced current.
- Faraday's second law of electromagnetic induction states that the magnitude of emf induced in the coil is equal to the rate of change of flux that linkages with the coil.
- The flux linkage of the coil is the product of a number of turns in the coil and flux associated with the coil.
3. Which of the following circuit element stores energy in the electromagnetic field ?
A. Inductance
B. Condenser
C. Variable resistor
D. Resistance
Answer: A. Inductance
Explanation:
- An element which stores energy in the form of a magnetic field is Carbon Inductor.
- Circuits containing only resistive element has no transients because resistors don’t store energy in any form.
- Resistor dissipates energy in form of heat coming from I2R loss.
- Inductor stores energy in the form of the magnetic field.
- Capacitor stores energy in the form of the electric field.
- So that transients are present in the inductor & capacitor.
4. The inductance of a coil will increase under all the following conditions except
A. when more length for the same number of turns is provided
B. when the number of turns of the coil increase
C. when more area for each turn is provided
A. when permeability of the core in-creases
Answer: A. when more length for the same number of turns is provided
Explanation:
- The inductances of a coil will increase under all the conditions except when more length for the same number of turns is provided.
Number of Wire Turns in the Coil
- The greater the number of turns of wire in the coil, the greater the inductance.
- Fewer turns of wire in the coil results in lesser inductance.
- More coils of wires indicate a greater amount of magnetic field force for a given amount of coil current.
Coil Area
- The greater the coil area, the greater the inductance. Lesser coil area results in less inductance.
- Greater coil area presents less opposition to the formation of magnetic field flux, for a given amount of field force
Core Material
- The greater the magnetic permeability of the core to which the coil is wrapped around, the greater the inductance; the lesser the permeability of the core, the lesser the inductance.
Coil Length
- The longer the coil’s length, the lesser the inductance. The shorter the coil’s length, the greater the inductance.
5. Higher the self-inductance of a coil,
A. lesser its weber-turns
B. lower the e.m.f. induced
C. greater the flux produced by it
D. longer the delay in establishing steady current through it
Answer: D. longer the delay in establishing steady current through it
Explanation:
- Self-inductance of a coil is defined as the ratio of self-induced emf to the rate of change of current in the coil.
- Higher the self-inductance of a coil, longer is the delay in establishing a steady current through it.
Important Points:
- Self induction is the phenomenon on production of induced emf in a coil when changing current passes through it.
- The self-inductance of a coil is said to be one Henry if an induced emf of one volt is set up in it when the current in it changes at the rate of one amphere per second.
1. Number of turns :
- Larger the number of turns in the solenoid, largerf is its self- inductance.
2. Area of cross-section :
- Larger the area of cross-section of the solenoid. larger is self -inductance.
(ii) The mutal inductance of two solenoids depends on their geometry and the magnetic permeability of the core material.
1. Number of turns :
- Larger the number of turns in the two solenoids, larger will be their inductance.
2. Relative separation :
- Larger the distance between two solenoids, smaller will be the magnetic flux linked with the secondary coil due to current in the primary coil. Hence, smaller will be the value of M.
6. In an iron cored coil the iron core is removed so that the coil becomes an air cored coil. The inductance of the coil will
A. increase
B. decrease
C. remain the same
D. initially increase and then decrease
Answer: B. decrease
Explanation:
- If the core were removed, the magnetic flux within the coils will diminish causing a loss of e.m.f. according to Faradays laws.
- In other words removing the core has the same effect as greatly reducing the number of turns in the coils or reducing the magnetizing flux.
- If the iron core is removed, it will reduce the magnetic field around and hence, the inductance will decrease.
The inductance is directly proportional to
- The square of the number of turns
- The area i.e. square of the diameter
- The ratio of coil radius to coil length
7. An open coil has
A. zero resistance and inductance
B. infinite resistance and zero inductance
C. infinite resistance and normal inductance
D. zero resistance and high inductance
Answer: B. infinite resistance and zero inductance
Explanation:
- Here Coil is open coil I=0 so R =V/I = V/0 = Infinite and Inductance L = di/dt = 0/dt = 0. so Inductance Zero and Resistance is infinite.
- If an induction coil is open, then it can be assumed as a short circuit to the direct current.
- When the storage phase is completed, the inductor current that flows through the inductor will be stable.
- Also, there will be no self-induced electro-motive force and the value of potential difference across the inductor is zero as it is a short circuit.
- Hence, the inductor will behave like a connecting wire. We know that the resistance of the inductor is equal to the ratio of voltage and current through it.
- Hence, the value of resistance is zero and inductance L will become infinite.
- Therefore, if an induction coil is open then its L and R become infinite and zero respectively.
8. Both the number of turns and the core length of an inductive coil are doubled. Its self-inductance will be
A. unaffected
B. doubled
C. halved
D. quadrupled
Answer: B. doubled
Explanation:
- When both the number of turns and the core length of an inductive coil are doubled, then the self-inductance of an inductive coil will get doubled.
9. If current in a conductor increases then according to Lenz's law self-induced voltage will
A. aid the increasing current
B. tend to decrease the amount of current
C. produce current opposite to the increasing current
D. aid the applied voltage .
Answer: C. produce current opposite to the increasing current
Explanation:
- This self-induced emf opposes the change that is causing it and the faster the rate of change of current the greater is the opposing emf.
- This self-induced emf will, by Lenz's law oppose the change in current in the coil and because of its direction this self-induced emf is generally called a back-emf.
- The amount of voltage induced depends on the rate of change of the magnetic field flux and the number of turns of wire (if coiled) exposed to the change in flux.
10. The direction of induced e.m.f. can be found by
A. Laplace's law
B. Lenz's law
C. Fleming's right hand rule
D. Kirchhoff's voltage law
Answer: B. Lenz's law
Explanation:
Lenz's Law:
- According to this law, the direction of induced emf or current in a circuit is such as to oppose the cause that produces it.
- This law gives the direction of induced emf/induced current.
- This law is based upon the law of conservation of energy.
Important Points
- Laplace's law indicates that the tension on the wall of a sphere is the product of the pressure times the radius of the chamber and the tension is inversely related to the thickness of the wall.
- According to Lenz's law, the direction of induced emf or current in a circuit is such as to oppose the cause that produces it.
- Fleming's right-hand rule shows the direction of induced current but it gives no relation between the direction of induced emf or current in a circuit is such as to oppose the cause that produces it.
- This law is also known as loop rule or voltage law (KVL) and according to it “the algebraic sum of the changes in potential in a complete traversal of a mesh (closed-loop) is zero”, i.e. Σ V = 0.