An Overview of Ncert Solutions Class 12 Micro Economics Chapter 1
FAQs on Ncert Solutions Class 12 Micro Economics Chapter 1
1. Which topics from Chapter 6, Electromagnetic Induction, are most important for the CBSE Class 12 Physics board exam 2025-26?
For the CBSE 2025-26 board exams, the most important topics in Electromagnetic Induction are:
- Faraday's Laws of Induction and Lenz's Law (conceptual and numerical questions).
- Derivation of motional EMF (E = Blv).
- Concepts of self-inductance and mutual inductance, including derivations for a solenoid.
- Eddy currents, their disadvantages, and applications.
- The principle, construction, and working of an AC generator (often a 5-mark question).
Students should focus on both the theoretical derivations and the application-based numerical problems for these topics.
2. State Faraday's laws of electromagnetic induction. What is a typical 2-mark question based on this?
Faraday's laws of electromagnetic induction are:
- First Law: Whenever the magnetic flux linked with a closed circuit changes, an electromotive force (EMF), and hence a current, is induced in it.
- Second Law: The magnitude of the induced EMF is directly proportional to the rate of change of the magnetic flux linked with the circuit. Mathematically, ε ∝ dΦ/dt.
A typical 2-mark question would be: "A square loop of side 10 cm is placed in a magnetic field of 0.1 T. If this field is reduced to zero in 0.5 s, calculate the magnitude of the induced EMF."
3. Explain why Lenz's Law is a direct consequence of the law of conservation of energy.
Lenz's Law states that the direction of the induced current is such that it opposes the change in magnetic flux that produced it. This is a manifestation of the law of conservation of energy. To move a magnet towards or away from a coil (to change the flux), mechanical work must be done against the opposing magnetic force created by the induced current. This mechanical work is then converted into electrical energy in the circuit, which appears as the induced current. If the induced current were to aid the change, it would lead to a perpetual increase in energy without any external work, violating the law of conservation of energy.
4. What is the physical significance of the negative sign in the expression for Faraday's law of induction, ε = -dΦB/dt?
The negative sign in the equation ε = -dΦB/dt is crucial as it signifies the direction of the induced EMF and represents Lenz's Law. It indicates that the induced EMF (and the resulting current) is always in a direction that opposes the change in magnetic flux (dΦB/dt) that causes it. If the flux is increasing, the induced EMF will create a current and a magnetic field to counteract this increase. If the flux is decreasing, it will create a field to support it. Thus, the negative sign embodies the principle of opposition inherent in electromagnetic induction.
5. Derive the expression for the motional EMF induced in a straight conductor moving in a uniform magnetic field. What is the expected marks for this question?
This is a frequently asked 3-mark question. Consider a straight conductor PQ of length 'l' moving with a velocity 'v' perpendicular to a uniform magnetic field 'B', which is directed into the plane of the paper.
- The Lorentz force on a free charge 'q' in the conductor is given by F = q(v × B).
- The magnitude of this force is F = qvB, acting from Q to P.
- Work done in moving the charge 'q' from Q to P is W = F × l = qvBl.
- By definition, the induced EMF (ε) is the work done per unit charge: ε = W/q.
- Therefore, ε = (qvBl)/q = Blv.
This is the expression for the motional EMF induced in the conductor.
6. What are eddy currents? How are they reduced in devices like transformers?
Eddy currents are loops of electrical current induced within conductors by a changing magnetic field in the conductor, according to Faraday's law of induction. They flow in closed loops within the bulk of the material, in planes perpendicular to the magnetic field.
These currents cause significant energy loss in the form of heat (I²R loss). To reduce eddy currents in transformers, the solid iron core is replaced with a laminated core. The core is made of thin, insulated sheets of soft iron stacked together. This lamination increases the overall resistance in the path of the eddy currents, thereby reducing their magnitude and minimising heat loss.
7. Define self-inductance and mutual inductance. Which factors do they depend on?
Self-Inductance (L): It is the property of a coil by virtue of which it opposes any change in the strength of the current flowing through it by inducing an EMF in itself. The self-inductance of a coil depends on its geometry (number of turns, area, length) and the permeability of the medium within the coil.
Mutual-Inductance (M): It is the phenomenon where a changing current in one coil induces an EMF in a neighbouring coil. The mutual inductance of a pair of coils depends on their geometry, the number of turns in both coils, and their relative separation and orientation.
8. What are some common mistakes or conceptual traps students face in important questions from Electromagnetic Induction?
Students often make the following mistakes in board exam questions for this chapter:
- Direction of Current: Incorrectly applying Lenz's Law or Fleming's Right-Hand Rule to determine the direction of induced current. It's crucial to first identify if the flux is increasing or decreasing.
- Motional EMF Formula: Forgetting that the formula ε = Blv is only valid when the velocity, length, and magnetic field are mutually perpendicular. Problems with angles are often solved incorrectly.
- Sign Conventions: Ignoring the negative sign in Faraday's Law, which can lead to errors in questions that ask for both magnitude and direction.
- Calculations: Errors in calculating the rate of change of flux (dΦ/dt), especially when the magnetic field or area is a function of time.
9. Explain the principle, construction, and working of an AC generator. Why is this considered a 5-mark important question?
This is a classic 5-mark question because it tests multiple concepts: principle, diagram-based construction, working, and derivation of the induced EMF.
- Principle: An AC generator works on the principle of electromagnetic induction. When a coil is rotated in a uniform magnetic field, the magnetic flux linked with it changes, and an alternating EMF is induced.
- Construction: It consists of an armature coil (ABCD), strong field magnets (N and S poles), slip rings (S1, S2), and brushes (B1, B2). A labelled diagram is essential.
- Working: As the armature coil rotates, the arms AB and CD move up and down, cutting the magnetic field lines. According to Fleming's Right-Hand Rule, an induced current flows. After half a rotation, the direction of motion of the arms reverses, reversing the direction of the current. The slip rings ensure that the current in the external circuit is always alternating, producing an AC output. The induced EMF is given by ε = NBAω sin(ωt).
