Ncert Books Class 12 Physics Chapter 4 Free Download

For 3-mark questions, your preparation should target specific applications and numerical problems. Key areas include:

• Calculating the Lorentz force on a charged particle moving in combined electric and magnetic fields, especially velocity selector problems.

• Finding the force between two long, straight, parallel current-carrying conductors and using it to define one Ampere.

• Solving numericals on the torque experienced by a current loop in a uniform magnetic field (τ = NIAB sinθ).

• Problems based on the conversion of a galvanometer into an ammeter and a voltmeter.

To secure full marks for a device-based question like the Moving Coil Galvanometer, structure your answer clearly. Follow these steps:

• Principle: Start by stating the underlying principle – a current-carrying coil placed in a uniform magnetic field experiences a torque.

• Construction: Draw a neat, labelled diagram showing the coil, soft iron core, permanent magnets, pointer, and spring.

• Working: Explain how current flow leads to a deflecting torque (τ = NIAB) and how the spring provides a restoring torque (τ = kφ). At equilibrium, explain that these torques are equal.

• Derivation: Show the mathematical steps where NIAB = kφ, leading to the conclusion that deflection is proportional to the current (φ ∝ I).

• Sensitivity: Briefly define current and voltage sensitivity.

The choice between these two laws depends on the symmetry of the problem. Biot-Savart Law is a universal law that calculates the magnetic field due to a small current element (dI). It is versatile and can be used for any shape of conductor, but the integration can be complex. Use it for finding the field of a finite wire or on the axis of a circular loop. In contrast, Ampere's Circuital Law is a special case that works only for highly symmetrical current distributions where you can draw an 'Amperian loop' on which the magnetic field is constant. Use it as a shortcut for an infinitely long straight wire, a solenoid, or a toroid, as it greatly simplifies the calculation.

A standard question simply asks to derive the formula for force per unit length. A HOTS version would test deeper application of this concept. For instance, you might be given a system of three or more parallel wires and asked to find the net force on one of them or to find a position where a third wire would experience zero net force. Another common HOTS approach is to combine electromagnetism with mechanics, such as asking for the current required in a lower wire to make an upper wire levitate, thereby balancing the magnetic repulsion with its gravitational force.

This is a crucial conceptual point related to the vector nature of the magnetic force. The magnetic Lorentz force is given by the formula F = q(v × B), which can be written in magnitude as F = qvB sinθ, where θ is the angle between the velocity vector (v) and the magnetic field vector (B). If a particle moves parallel (θ = 0°) or anti-parallel (θ = 180°) to the magnetic field, the value of sinθ becomes zero. Consequently, the force (F) on the particle is zero. This implies that a magnetic field can only alter the direction of a moving charge, not its speed, and it only acts on charges that cut across magnetic field lines.

To solve conversion problems correctly, remember these key distinctions:

• Ammeter Conversion: To measure larger currents, you need to lower the overall resistance. This is achieved by connecting a low-resistance wire called a shunt (S) in parallel with the galvanometer. The required shunt resistance is calculated using the formula S = (I_g * G) / (I - I_g), where I is the total current and I_g is the current for full-scale deflection.

• Voltmeter Conversion: To measure high potential differences, you need to increase the overall resistance. This is done by connecting a high resistance (R) in series with the galvanometer. The required series resistance is calculated using the formula R = (V / I_g) - G, where V is the voltage to be measured.