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`CBSE`

Physics

CBSE Class 12

1.

An electric dipole is held in a uniform electric field.

(i) Show that the net force acting on it is zero.

(ii) The dipole is aligned parallel to the field. Find the work done in rotating it through the angle of 180°.

i) Dipole moment of dipole is given by,

i)

Force on –q at A = -qE

Force on +q at B = + qE

So,

Net force = qE – qE = 0

ii)

Work done on dipole is given by,

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2.

A charge ‘q’ is placed at the centre of a cube of side l. What is the electric flux passing through each face of the cube?

Using Gauss theorem,

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3.

A bar magnet is moved in the direction indicated by the arrow between two coils PQ and CD.

Predict the directions of induced current in each coil.In the figure, N pole is receding away coil (1), so in coil (1), the nearer faces will act as S-pole and in coil (2) the nearer face will also act as S-pole to oppose the approach of magnet towards coil (2), so current in coils will flow clockwise as seen from the side of magnet. The direction of current will be from P to Q in coil (1) and from C to D in coil (2).

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4.

A light bulb is rated 100 W for 220 V ac supply of 50 Hz. Calculate

(a) the resistance of the bulb;

(b) the rms current through the bulb

OR

An alternating voltage given by V = 140 sin 314 t is connected across a pure resistor of 50 W. Find

(i) the frequency of the source.

(ii) the rms current through the resistor.OR

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5.

Two wires of equal length, one of copper and the other of manganin have the same resistance. Which wire is thicker?

Resistance of a material is given by,

Resistance for copper wire and manganin wire is given by,

That is, manganin wire is thicker.

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6.

State the underlying principle of a transformer. How is the large scale transmission of electric energy over long distances done with the use of transformers?

The principle of transformer is based upon the principle of mutual induction which states that due to continuous change in the current in the primary coil an emf gets induced across the secondary coil. At the power generating station, the step up transformers step up the output voltage which reduces the current through the cables and hence reduce resistive power loss. Then, at the consumer end, a step down transformer steps down the voltage. Therefore, the large scale transmission of electric energy over long distances is done with the uses of transformers.

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7.

What are the directions of electric and magnetic field vectors relative to each other and relative to the direction of propagation of electromagnetic waves?

Both electric and magnetic field vectors are perpendicular to each other and perpendicular to the direction of propagation.

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8.

A test charge ‘q’ is moved without acceleration from A to C along the path from A to B and then from B to C in electric field E as shown in the figure. (i) Calculate the potential difference between A and C. (ii) At which point (of the two) is the electric potential more and why?

i) Potential difference between A and C

ii) Direction of electric field is in higher to lower potential. So, V_{c} > V_{A}

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9.

A capacitor of capacitance of ‘C’ is being charged by connecting it across a dc source along with an ammeter. Will the ammeter show a momentary deflection during the process of charging? If so, how would you explain this momentary deflection and the resulting continuity of current in the circuit? Write the expression for the current inside the capacitor.

When capacitor is charged by a dc source, ammeter will show a momentary deflection because of the presence of displacement current.

Conduction current is equal to the displacement current.

i.e.,

Resulting continuity of current becomes zero because, is maximum when, capacitor is fully charged.

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10.

Mention the two characteristic properties of the material suitable for making core of a transformer.

Two characteristic properties are:

i) Low hysteresis loss

ii) Low coercivity.

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