 # CBSE Delhi-Set-1-2011

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• Q1

A point charge Q is placed at point O as shown in the figure. Is the potential difference VA – VB positive, negative or zero, if Q is (i) positive (ii) negative? Marks:1 • Q2

A plane electromagnetic wave travels in vacuum along Z-direction. What can you say about the direction of electric and magnetic field vectors?

Marks:1

The electric and magnetic field vectors will be in the X-Y plane. They will be mutually perpendicular.

• Q3

A resistance R is connected across a cell of emf ∈ and internal resistance r. A potentiometer now measures the potential difference between the terminals of the cell as V. Write the expression for ‘r’ in terms ∈, V and R.

Marks:1 • Q4

The permeability of a magnetic material is 0.9983. Name the type of magnetic material it represents.

Marks:1

The given permeability is less than 1. So, the material is diamagnetic.

• Q5

Show graphically, the variation of the de-Broglie wavelength (l) with the potential (V) through which an electron is accelerated from rest.

Marks:1 • Q6

In a transistor, doping level in base is increased slightly. How will it affect (i) collector current and (ii) base current?

Marks:1

On increasing the doping level in base –
(i) collector current will decrease.
(ii) base current will increase.

• Q7

Define the term ‘wattless current’.

Marks:1

The current through special electrical device like pure capacitive or pure inductive circuit which consumes no power for its maintenance is called wattless current.

• Q8

When monochromatic light travels from one medium to another, its wavelength changes but frequency remains the same. Explain.

Marks:1

The light wave penetrates deeply into the material, which results in small vibrations in the electrons. The electrons forward these vibrations to the atoms in the material and they send out light waves of the same frequency as the incident wave, but the speed of light changes. Therefore, wavelength also changes keeping frequency fixed.

• Q9

Two uniformly large parallel thin plates having charge densities + and - are kept in the x-Z plane at a distance ‘d’ apart. Sketch an equipotential surface due to electric field between the plates. If a particles of mass m and charge ‘-q’ remains stationary between the plates. What is the magnitude and direction of this field?

Marks:2 Gravitational force on charge = mg, downward
Electrostatic force on charge = qE, upward
mg = qE
So, E = mg/q; direction of electric field must be in downward direction (towards negative y-axis).

Two small identical electrical dipoles AB and CD, each of dipole moment ‘p’ are kept at an angle of 120as shown in the figure. What is the resultant dipole moment of this combination? If this system is subjected to electric field directed along +X direction, what will be the magnitude and direction of the torque acting on this?  