Science (English Medium) Class 12 - CBSE Question Bank Solutions

Relation between ray and wavefront is ______.

In which case is electron emission from a metal not known?

Alpha particles used in Geiger-Marsden experiment were obtained from ______.

In Geiger-Marsden experiment prediction was that ______.

The basic force acting on the alpha particles using Coulomb's law is ______.

In Geiger-Marsden experiment, actual results were ______.

The model that best explains the results of Geiger-Marsden experiment is ______.

In the case of metals, the valence and conduction bands ______.

In the case of semiconductors, the valence and conduction bands ______.

Doping of semiconductor is the process of ______.

An A.C. source is connected to a resistive circuit. Which of the following is true?

The focal length of a convex lens made of glass of refractive index (1.5) is 20 cm.

What will be its new focal length when placed in a medium of refractive index 1.25?

Is focal length positive or negative? What does it signify?

How does the sign of the phase angle `phi`, by which the supply voltage leads the current in an LCR series circuit, change as the supply frequency is gradually increased from very low to very high values.

An electromagnetic wave travelling along z-axis is given as: E = E₀ cos (kz – ωt.). Choose the correct options from the following;

1. The associated magnetic field is given as `B = 1/c hatk xx E = 1/ω (hatk xx E)`.
2. The electromagnetic field can be written in terms of the associated magnetic field as `E = c(B xx hatk)`.
3. `hatk.E = 0, hatk.B` = 0.
4. `hatk xx E = 0, hatk xx B` = 0.

The charge on a parallel plate capacitor varies as q = q₀ cos 2πνt. The plates are very large and close together (area = A, separation = d). Neglecting the edge effects, find the displacement current through the capacitor?

A variable frequency a.c source is connected to a capacitor. How will the displacement current change with decrease in frequency?

Show that the magnetic field B at a point in between the plates of a parallel-plate capacitor during charging is `(ε_0mu_r)/2 (dE)/(dt)` (symbols having usual meaning).