Science (English Medium) कक्षा १२ - CBSE Important Questions for Physics

The teachers of Geeta’s school took the students on a study trip to a power generating station, located nearly 200 km away from the city. The teacher explained that electrical energy is transmitted over such a long distance to their city, in the form of alternating current (ac) raised to a high voltage. At the receiving end in the city, the voltage is reduced to operate the devices. As a result, the power loss is reduced. Geeta listened to the teacher and asked questions about how the ac is converted to a higher or lower voltage.

1) Name the device used to change the alternating voltage to a higher or lower value. State one cause for power dissipation in this device.

2) Explain with an example, how power loss is reduced if the energy is transmitted over long distances as an alternating current rather than a direct current.

3) Write two values each shown by the teachers and Geeta.

A device X is connected across an ac source of voltage V = V₀ sin ωt. The current through X is given as 

`I = I_0 sin (omega t + pi/2 )`

1) Identify the device X and write the expression for its reactance.

2) Draw graphs showing the variation of voltage and current with time over one cycle of ac, for X.

3) How does the reactance of the device X vary with the frequency of the ac? Show this variation graphically.

4) Draw the phasor diagram for the device X.

Name the electromagnetic radiations used for (a) water purification, and (b) eye surgery.

The figure shows a ray of light falling normally on the face AB of an equilateral glass prism having refractive index`3/2`, placed in water of refractive index `4/3`.Will this ray suffer total internal reflection on striking the face AC? Justify your answer.

When a tiny circular obstacle is placed in the path of light from a distant source, a bright spot is seen at the centre of the shadow of the obstacle. Explain why?

State Huygen’s principle.

Show using a proper diagram how unpolarised light can be linearly polarised by reflection from a transparent glass surface.

If one of two identical slits producing interference in Young’s experiment is covered with glass, so that the light intensity passing through it is reduced to 50%, find the ratio of the maximum and minimum intensity of the fringe in the interference pattern.

What kind of fringes do you expect to observe if white light is used instead of monochromatic light?

Using Huygens’ principle, verify the laws of reflection at a plane surface.

If light of wavelength 412.5 nm is incident on each of the metals given below, which ones will show photoelectric emission and why?

A hydrogen atom initially in the ground level absorbs a photon, which excites it to the n = 4 level. Determine the wavelength and frequency of the photon.

State Bohr's postulate to define stable orbits in the hydrogen atom. How does de Broglie's hypothesis explain the stability of these orbits?

A radioactive isotope has a half-life of T years. How long will it take the activity to reduce to a) 3.125%, b) 1% of its original value?

A student wants to use two p-n junction diodes to convert alternating current into direct current. Draw the labelled circuit diagram she would use and explain how it works.

Draw a typical input and output characteristics of an n-p-n transistor in CE configuration. Show how these characteristics can be used to determine (a) the input resistance (r₁), and (b) current amplification factor (β)

Give three reasons why modulation of a message signal is necessary for long-distance transmission

Show graphically an audio signal, a carrier wave and an amplitude modulated wave

A carrier wave of peak voltage 15 V is used to transmit a message signal. Find the peak voltage of the modulating signal in order to have a modulation index of 60%

Three-point charges q, – 4q and 2q are placed at the vertices of an equilateral triangle ABC of side 'l' as shown in the figure. Obtain the expression for the magnitude of the resultant electric force acting on the charge q

(b) Find out the amount of the work done to separate the charges at infinite distance.