Physics MCQs for NEET — Practice Questions with Answers

The NCERT states, 'The magnetic flux through a surface of area A placed in a uniform magnetic field B is defined as, $\Phi_B = B \cdot A = BA \cos \theta$', where $\theta$ is the angle between B and A. The dot product signifies a scalar quantity.

From the example provided in the NCERT text, 'Initial flux through the coil, $\Phi_B$ (initial) = BA cos $\theta$ = $3.0 \times 10^{-5} \times (\pi \times 10^{-2}) \times \cos 0° = 3\pi \times 10^{-7}$ Wb'. Since the plane is perpendicular to the field, the angle $\theta$ between the area vector and B is 0°.

The NCERT states, 'Faraday’s laws of induction imply that the emf induced in a coil of N turns is directly related to the rate of change of flux through it'. Also, 'Faraday arrived at a conclusion that an emf is induced in a coil when magnetic flux through the coil changes with time.'

The NCERT states, 'Lenz’s law states that the polarity of the induced emf is such that it tends to produce a current which opposes the change in magnetic flux that produces it. The negative sign in the expression for Faraday’s law indicates this fact.'

According to Faraday's laws of induction given in the summary, 'The emf induced in a coil of N turns is directly related to the rate of change of flux through it, $\varepsilon = -Nd\Phi_B/dt$'. Here $\Phi_B$ is the flux linked with one turn of the coil.

The NCERT example 6.2 states, 'Note that the earth’s magnetic field also produces a flux through the loop. But it is a steady field (which does not change within the time span of the experiment) and hence does not induce any emf.' An emf is only induced when the magnetic flux changes with time.

The NCERT explains, 'If the magnetic field has different magnitudes and directions at various parts of a surface as shown in Fig. 6.5, then the magnetic flux through the surface is given by $\Phi_B = B_1 \cdot dA_1 + B_2 \cdot dA_2 + ... = \sum B_i \cdot dA_i$ where ‘all’ stands for summation over all the area elements $dA_i$ comprising the surface and $B_i$ is the magnetic field at the area element $dA_i$.'

The introduction states, '...electric currents were induced in closed coils when subjected to changing magnetic fields. ... The phenomenon in which electric current is generated by varying magnetic fields is appropriately called electromagnetic induction.'

Example 6.4 Solution notes, 'Note that there are no induced current as long as the loops are completely inside or outside the region of the magnetic field.' This is because for an induced current, there must be a change in magnetic flux, which doesn't occur when the loop is fully within or outside a uniform, constant field.

The NCERT states, 'Magnetic flux through a plane of area A placed in a uniform magnetic field B (Fig. 6.4) can be written as $\Phi_B = B \cdot A = BA \cos \theta$' and also, 'Magnetic flux is a scalar quantity.' This quantity quantifies the total 'flow' or number of magnetic field lines through an area.