Science (English Medium) Class 12 - CBSE Important Questions for Physics

Define a wavefront.

Write two important advantages of reflecting telescope over a refracting telescope.

Why are infra-red waves often called heat waves? Explain.

Draw the circuit diagram of a full wave rectifier. Explain its working showing its input and output waveforms.

Write the S.I. unit of mutual inductance.

State the two Kirchhoff’s rules used in electric networks. How are there rules justified?

In a single slit diffraction experiment, the width of the slit is made double the original width. How does this affect the size and intensity of the central diffraction band?

Give the truth table and circuit symbol for NAND gate

Define electric flux.

Four point charges Q, q, Q and q are placed at the corners of a square of side 'a' as shown in the figure.

Find the

1) resultant electric force on a charge Q, and

2) potential energy of this system.

A 10 V cell of negligible internal resistance is connected in parallel across a battery of emf 200 V and internal resistance 38 Ω as shown in the figure. Find the value of current in the circuit.

Define the term 'conductivity' of a metallic wire. Write its SI unit.

State Ampere’s circuital law.

A proton and an electron travelling along parallel paths enter a region of uniform magnetic field, acting perpendicular to their paths. Which of them will move in a circular path with higher frequency?

A bar magnet of magnetic moment 6 J/T is aligned at 60° with a uniform external magnetic field of 0·44 T Calculate  

a) the work done in turning the magnet to align its magnetic moment

i) normal to the magnetic field, 

ii) opposite to the magnetic field, and

b)  the torque on the magnet in the final orientation in case (ii).

An iron ring of relative permeability µ_r has windings of insulated copper wire of n turns per meter. When the current in the windings is I, find the expression for the magnetic field in the ring.

The susceptibility of a magnetic material is 0·9853. Identify the type of magnetic material. Draw the modification of the field pattern on keeping a piece of this material in a uniform magnetic field.

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.