SL Arora 11th Physics

Explanation:
A dynamo delivers a current of 4 A and dissipates 20 W internally while maintaining a terminal voltage of 220 V. The goal is to determine the total EMF it generates.

A dynamo converts mechanical energy into electrical energy, producing an EMF that is partially lost as Heat in its internal resistance. The terminal voltage is the usable voltage, which is lower than the EMF due to internal dissipation. Internal resistance can be calculated from the power dissipated:
P = I² R, giving R = P / I².

Next, the EMF can be related to terminal voltage and internal resistance using E = V + I R. Substituting the known values of current, terminal voltage, and internal resistance allows understanding of the total voltage generated by the dynamo.

Imagine a water pump where some water leaks through a small pipe—the output pressure is less than the total generated pressure. Similarly, the terminal voltage is reduced by internal resistance.

This approach shows how EMF includes both the energy delivered externally and the energy lost internally, reflecting the difference between generated and usable voltage.

Explanation:
This question asks which conservation principle is demonstrated by Lenz’s law, where induced currents oppose the change in magnetic flux.

Lenz’s law states that the direction of induced current always opposes the change in magnetic flux that produces it. This ensures that energy is not spontaneously created or destroyed, maintaining overall energy conservation in the system.

When a conductor experiences a changing magnetic field, an induced current arises. Lenz’s law ensures the induced current opposes the flux change. The work done by the induced current comes from the external source that causes the flux change. energy is transferred from the source to maintain this opposition, preventing spontaneous energy generation.

Think of pushing a magnet into a coil like pushing a heavy cart uphill—the resistance of the induced current requires input energy from the source of motion.

Overall, Lenz’s law enforces energy conservation by ensuring induced currents oppose changes, maintaining the system’s energy balance without creating extra energy.

Explanation:
A straight wire has negligible geometry to create significant magnetic flux linking with itself. Self-inductance depends on the ability of a conductor to generate magnetic flux that links with its own length.

For a straight wire, the magnetic field lines produced by current spread outward and do not loop back along the wire in a way that can induce EMF in the same wire. Self-inductance arises from the flux linkage per unit current. Since a straight wire provides almost no closed-loop flux linkage, its self-inductance is extremely small, approaching zero.

An analogy is a straight river channel with no loops—the water flow cannot create circulating eddies around itself.

This explains why self-inductance of a straight wire is effectively zero or negligible.

Explanation:
Mutual induction occurs when a change in current in one coil induces EMF in another nearby coil. Devices like transformers, Tesla coils, and induction coils rely on this principle to transfer energy between circuits without direct electrical contact.

A motor, however, converts electrical energy directly into mechanical motion. While magnetic fields are involved in motors, the primary mechanism is Lorentz force acting on conductors in a magnetic field, not induction between two separate coils. Therefore, motors do not operate based on mutual induction principles.

Think of a motor as a paddlewheel turned by Electricity, while a transformer is like a pair of linked water wheels transferring flow without touching.

Mutual induction is central to transformers and coils but not to motors, which work by direct electromagnetic forces.

Explanation:
Self-inductance of a coil depends on the square of the number of turns (N²), the coil radius, and the coil length. Reducing the number of turns reduces the flux linkage and thus the inductance.

For two similar coils, the ratio of inductances is proportional to the square of the ratio of their turns: L₂ / L₁ = (N₂ / N₁)². Using this relationship, one can determine how the inductance changes when the number of turns is decreased while other factors remain constant.

Analogous to stacking multiple loops of string to trap more energy, fewer loops trap less energy.

The self-inductance decreases proportionally to the square of the reduction in turns for a similar coil.